Re: [time-nuts] Need advice for multilateration setup
Attila, From reading at the abstract, it looks interesting - bird tracking! But essentially the same problem I'm trying to solve. I was looking for a copy of the paper on the web as I'm not sure I want to purchase it. Thanks, -Bob On 04/18/2015 04:02 PM, Attila Kinali wrote: Moin, On Mon, 06 Apr 2015 07:37:53 -0500 Robert Watzlavick roc...@watzlavick.com wrote: Thank you very much for the references. I had come across [4] when searching on Kalman filters for GPS aiding of INS measurements. I didn't pay much attention to the GPS chapter at the time but I'll look at it again. I just downloaded [3] and it appears to have a good mix of practical vs. theoretical aspects. I appreciate the help! While looking for something completely different[tm] I stumbled over the paper below. It is definitly not the best paper I have seen, but it might give you some ideas. A reverse GPS architecture for tracking and location of small objects, by Andrade, Alves, Cuipdo, Santos, 2011 http://dx.doi.org/10.1109/ICL-GNSS.2011.5955273 Attila Kinali ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Moin, On Mon, 06 Apr 2015 07:37:53 -0500 Robert Watzlavick roc...@watzlavick.com wrote: Thank you very much for the references. I had come across [4] when searching on Kalman filters for GPS aiding of INS measurements. I didn't pay much attention to the GPS chapter at the time but I'll look at it again. I just downloaded [3] and it appears to have a good mix of practical vs. theoretical aspects. I appreciate the help! While looking for something completely different[tm] I stumbled over the paper below. It is definitly not the best paper I have seen, but it might give you some ideas. A reverse GPS architecture for tracking and location of small objects, by Andrade, Alves, Cuipdo, Santos, 2011 http://dx.doi.org/10.1109/ICL-GNSS.2011.5955273 Attila Kinali -- _av500_ phd is easy _av500_ getting dsl is hard ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Hi Jim, On 04/08/2015 12:46 AM, Jim Lux wrote: On 4/7/15 11:33 AM, Magnus Danielson wrote: Hi, O One might look at the available frequencies and see if there is a telemetry band available which allows wider bandwidth. For the application, I don't see that very much transmitted power is needed. If the OP is a licensed amateur radio person, then choosing one of the low microwave ham bands would be easy. Parts to generate a carrier and BPSK at 2.39-2.45,3.3-3.5, 5.6-5.8 GHz are cheap and readily available. You might be able to get away with a VCO and no crystal as the transmitter, but even if you can't, there's tons of PLLs out there that will nicely lock to a crystal and are cheap. You might want to do a link budget and see how much power you need to radiate, so that you get a decent SNR at the receiver. free space path loss between isotropic antennas (in dB) = 34 + 20 log10(freq in MHz) + 20 log10(distance in km). 1km at 3 GHz is 34+69 = 103 dB. If you radiate 1 mW (0dBm) from an omni (a piece of wire), you'll see -103 dBm at the input to your receiver, which is a fairly healthy signal. A detection bandwidth of 10 Hz would have a noise floor of -164 dBm before taking into account the receiver noise, but even if the receiver is terrible, you're still looking at tens of dB SNR with a very simple transmitter. Indeed. I realized that without doing the numbers, so I think the focus could be in how to realize a simple and light transmitter. A small FPGA will suffice for the code-generation. It will be essentially empty. Re-cycling the GPS C/A codes should be trivial. It should not be too hard to build the receiver side too. It's essentially the same as building a GPS receiver. That's a very good argument for higher chiping rates. I expect that the launch is a bit challenging for the tracking loop. If you're trying to track in real time, certainly. If you're doing post processing, less so. Fair enough. If you know you can track it in real time, then you know you can do it in post-processing. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
On Mon, 06 Apr 2015 23:02:01 +0200 Magnus Danielson mag...@rubidium.dyndns.org wrote: You want to keep your chip-rate up to make the integer ambiguity of the carrier phase simple. The carrier frequency divided by chipping rate ratio indicate how difficult problem it is to solve (GPS L1 C/A code has 1540). The 70 cm band has rather narrow allocations. The 23 cm band allow for much wide allocations. The benefit of the 70 cm band is naturally the easy of getting hardware. Yes. But I would do carrier phase tracking only after code phase tracking proved to be not accurate enough. Improving later and switching to another band is relatively easy, once you've proven that the system in principle works. Another benefit of a higher chipping rate is that it can allow for a higher bandwidth, allowing for tighter tracking of the rocket dynamics. The chipping rate at some code legnth creates the maximum tracking rate, and some fraction of that is the highest bandwidth tolerable. That's a very good argument for higher chiping rates. Attila Kinali -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Hi, On 04/07/2015 02:08 PM, Attila Kinali wrote: On Mon, 06 Apr 2015 23:02:01 +0200 Magnus Danielson mag...@rubidium.dyndns.org wrote: You want to keep your chip-rate up to make the integer ambiguity of the carrier phase simple. The carrier frequency divided by chipping rate ratio indicate how difficult problem it is to solve (GPS L1 C/A code has 1540). The 70 cm band has rather narrow allocations. The 23 cm band allow for much wide allocations. The benefit of the 70 cm band is naturally the easy of getting hardware. Yes. But I would do carrier phase tracking only after code phase tracking proved to be not accurate enough. Improving later and switching to another band is relatively easy, once you've proven that the system in principle works. One might look at the available frequencies and see if there is a telemetry band available which allows wider bandwidth. For the application, I don't see that very much transmitted power is needed. There is definitely a benefit in locking up the carrier and chipping rate, preferably so that there is an integer number of carrier cycles per chip. For those unused to the terminology, a chip is a single 0 or 1 out of the pseudo-random generator. It's encoded as +1 or -1 before being mixed with the carrier, thus forming an BPSK signal. There is a gain for the receiver if the transmitter has the carrier and code synchronized to each other like this. Another benefit of a higher chipping rate is that it can allow for a higher bandwidth, allowing for tighter tracking of the rocket dynamics. The chipping rate at some code legnth creates the maximum tracking rate, and some fraction of that is the highest bandwidth tolerable. That's a very good argument for higher chiping rates. I expect that the launch is a bit challenging for the tracking loop. Much of these challenges should be relatively easy to simulate, such that testing can be done before a the first solder-joint gets soldered. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
On 4/7/15 11:33 AM, Magnus Danielson wrote: Hi, O One might look at the available frequencies and see if there is a telemetry band available which allows wider bandwidth. For the application, I don't see that very much transmitted power is needed. If the OP is a licensed amateur radio person, then choosing one of the low microwave ham bands would be easy. Parts to generate a carrier and BPSK at 2.39-2.45,3.3-3.5, 5.6-5.8 GHz are cheap and readily available. You might be able to get away with a VCO and no crystal as the transmitter, but even if you can't, there's tons of PLLs out there that will nicely lock to a crystal and are cheap. You might want to do a link budget and see how much power you need to radiate, so that you get a decent SNR at the receiver. free space path loss between isotropic antennas (in dB) = 34 + 20 log10(freq in MHz) + 20 log10(distance in km). 1km at 3 GHz is 34+69 = 103 dB. If you radiate 1 mW (0dBm) from an omni (a piece of wire), you'll see -103 dBm at the input to your receiver, which is a fairly healthy signal. A detection bandwidth of 10 Hz would have a noise floor of -164 dBm before taking into account the receiver noise, but even if the receiver is terrible, you're still looking at tens of dB SNR with a very simple transmitter. That's a very good argument for higher chiping rates. I expect that the launch is a bit challenging for the tracking loop. If you're trying to track in real time, certainly. If you're doing post processing, less so. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Attila, On 04/06/2015 11:14 AM, Attila Kinali wrote: Moin, On Fri, 03 Apr 2015 22:51:34 -0500 Robert Watzlavick roc...@watzlavick.com wrote: On 04/03/2015 10:12 PM, Robert Watzlavick wrote: I have an amateur radio license (mostly CW/HF and some VHF/UHF experience) and I've written some driver software for an IQ demodulation board but I have to admit, I would have no idea how to begin setting up that system as initially described by Attila and expanded by you and others. I have a rudimentary understanding of the modulation schemes involved but I don't fully understand how the various codes mentioned fit in. I've poked around a bit at some articles on PN codes and I can see how data would be transmitted but I think I'm missing something key that allows you to extract positions, velocities, etc. out of the various links. I think I have some more reading to do :) The basic system is that of an DSSS modulator/demodulator. The best text on spread spectrum systems I have found sofar is [1]. I explains modulation and demodulation in a hands on fashion. But, due to the age of the book, it does not contain any of the advanced stuff done today. But I think you don't need anything more fancy than an early-prompt-late correlator architecture for tracking. For the way how GPS works and how correlation and everything is done, I would suggest [2,3,4]. [2] is a good overview of how GPS is done and contains 99% of everything you need to know (special thanks to Magnus for mentioning it). It lacks some details on how to actually implement the system though. I think that the Kaplan GPS book is better than the MisraEnge [2] in many regards. It is better at explaining the workings of a GPS receiver. [3] helps to cover some of the weaknesses of the Kaplan book. However, the MisraEnge is better at some of the more advanced topics and more thorough on details than Kaplan. So, Kaplan is better at teach how to build a normal GPS receiver, and the MisraEnge is better at teaching how to build one with advanced features. The combination kills. The Bore et. al helps to cover some details about getting that initial guess. Implementing FFT based cross-correlation phase-guessing was trivial after reading that and another book. There [3] helps a lot, as it's a book specifically on building a GPS/Galileo receiver. I only skimmed trough a digital copy of [4] yet, so I cannot say too much about it, but that it's probably the most complete book on radio and inertial navigation I have seen sofar. The level of detail seems to vary from topic to topic quite a bit, but it is a treasure trove of references for everything the book covers (which is a damn lot!) If you are tight on time I would probably recommend to start with [3] and have a look at [1] and [2] when things don't make sense. I would recommend going with Kaplan first, to get the first overview. Then, as the refreshment coarse do the MisraEnge. To head off a bunch of replies - I think I stumbled upon what is being suggested. To extract the pseudorange, you have to figure out the offset of the locally generated PN code against the one that is received. In this reverse GPS case, I assume each ground station would have to start their local PN codes at the same time? Then you would be able to get the pseudoranges at each ground station and use those values for the multilateration equations. You still would have an uncertainty of one clock cycle since the phases of the local clocks at the stations wouldn't be aligned but several folks have suggested ways around that. There are multiple things here: * PRN generation: The locally generated PRN has to be time synchronous with the one received from the rocket transmitter. If you are more than one clock period off, you will only get noise out of the demodulator. What you measure is the time difference of the locally generated PRN to your ground station system time. You might want to consider the more advanced variants of loop filters as shown in Kaplan. Works great with simple dimensioning formulas to aid the setup. * Uncertainty: The autocorrelation function of a PRN sequence has a quite steep peak at \tau=0 with width of the clock period. Yes, this does mean that you get a one clock period uncertainty, if you do a hit/miss correlation. But as the correlation function is actually triangle shaped, you can get quite a bit better than that. The limit is afaik around your sampling clock period for naive approaches, which you can further improve with some statistics (you have multiple edges to work with, ie can average over those). * Synchronisation of ground stations: There are easy and diffuclt ways to do that. Probably the easiest is to use to use an additional transmitter at the launch point on the same frequency, but with a different PRN than the rocket. This way you can do a difference of the two PRN codes in your receiver, which gets away with a lot of nasty
Re: [time-nuts] Need advice for multilateration setup
Hi Attila, On 04/06/2015 11:21 AM, Attila Kinali wrote: On Sat, 04 Apr 2015 08:49:01 +0200 Magnus Danielson mag...@rubidium.se wrote: This is on either side of the amateur 23 cm band. That's also the first band where you have bandwidth enough to fool around with stuff like this without breaking the bandplan. This shouldn't be much of a problem. Using a chiping rate of a couple of kHz should be enough for this application. The signal strength can be rather large, directive antennas can be used and the expected noise level is rather low. So there no need to use a high chipping rate to compensate for noise effects. Of course, using a higher chipping rate makes it also easier to get an higher accuracy, but I would start with something easy to do first, like a 100mW transmitter in the 70cm band with 10kHz chipping rate (or go to a sub-band, where 200kHz signals are allowed). With that kind of setup it should be possible to use something like RTL-SDR for the first experiments and then gradually upgrade to better hardware to improve accuracy. You want to keep your chip-rate up to make the integer ambiguity of the carrier phase simple. The carrier frequency divided by chipping rate ratio indicate how difficult problem it is to solve (GPS L1 C/A code has 1540). The 70 cm band has rather narrow allocations. The 23 cm band allow for much wide allocations. The benefit of the 70 cm band is naturally the easy of getting hardware. Another benefit of a higher chipping rate is that it can allow for a higher bandwidth, allowing for tighter tracking of the rocket dynamics. The chipping rate at some code legnth creates the maximum tracking rate, and some fraction of that is the highest bandwidth tolerable. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Moin, On Fri, 03 Apr 2015 22:51:34 -0500 Robert Watzlavick roc...@watzlavick.com wrote: On 04/03/2015 10:12 PM, Robert Watzlavick wrote: I have an amateur radio license (mostly CW/HF and some VHF/UHF experience) and I've written some driver software for an IQ demodulation board but I have to admit, I would have no idea how to begin setting up that system as initially described by Attila and expanded by you and others. I have a rudimentary understanding of the modulation schemes involved but I don't fully understand how the various codes mentioned fit in. I've poked around a bit at some articles on PN codes and I can see how data would be transmitted but I think I'm missing something key that allows you to extract positions, velocities, etc. out of the various links. I think I have some more reading to do :) The basic system is that of an DSSS modulator/demodulator. The best text on spread spectrum systems I have found sofar is [1]. I explains modulation and demodulation in a hands on fashion. But, due to the age of the book, it does not contain any of the advanced stuff done today. But I think you don't need anything more fancy than an early-prompt-late correlator architecture for tracking. For the way how GPS works and how correlation and everything is done, I would suggest [2,3,4]. [2] is a good overview of how GPS is done and contains 99% of everything you need to know (special thanks to Magnus for mentioning it). It lacks some details on how to actually implement the system though. There [3] helps a lot, as it's a book specifically on building a GPS/Galileo receiver. I only skimmed trough a digital copy of [4] yet, so I cannot say too much about it, but that it's probably the most complete book on radio and inertial navigation I have seen sofar. The level of detail seems to vary from topic to topic quite a bit, but it is a treasure trove of references for everything the book covers (which is a damn lot!) If you are tight on time I would probably recommend to start with [3] and have a look at [1] and [2] when things don't make sense. To head off a bunch of replies - I think I stumbled upon what is being suggested. To extract the pseudorange, you have to figure out the offset of the locally generated PN code against the one that is received. In this reverse GPS case, I assume each ground station would have to start their local PN codes at the same time? Then you would be able to get the pseudoranges at each ground station and use those values for the multilateration equations. You still would have an uncertainty of one clock cycle since the phases of the local clocks at the stations wouldn't be aligned but several folks have suggested ways around that. There are multiple things here: * PRN generation: The locally generated PRN has to be time synchronous with the one received from the rocket transmitter. If you are more than one clock period off, you will only get noise out of the demodulator. What you measure is the time difference of the locally generated PRN to your ground station system time. * Uncertainty: The autocorrelation function of a PRN sequence has a quite steep peak at \tau=0 with width of the clock period. Yes, this does mean that you get a one clock period uncertainty, if you do a hit/miss correlation. But as the correlation function is actually triangle shaped, you can get quite a bit better than that. The limit is afaik around your sampling clock period for naive approaches, which you can further improve with some statistics (you have multiple edges to work with, ie can average over those). * Synchronisation of ground stations: There are easy and diffuclt ways to do that. Probably the easiest is to use to use an additional transmitter at the launch point on the same frequency, but with a different PRN than the rocket. This way you can do a difference of the two PRN codes in your receiver, which gets away with a lot of nasty effects that you would need to account for otherwise. Another approach would be to use a GPSDO on each ground station and run all the receivers already synchronized. This also enables you to get the position of all stations very accurately, especially if you let the GPSDO average its position for some time. But for ultimate accuracy, you'd need to calibrate the GPSDO's (including antennas) against each other, to know what the systematic offsets are (ie set them up all together at the same location and measure the time difference of the PPS). Of course, it's possible to use a combination of multiple approaches. Eg a nice one would be to GPSDO's to provide position and a precise frequency reference, but then use a central transmitter for the synchronization. HTH Attila Kinali [1] Spread Spectrum Systems with Commercial Applications, 3rd edition, by Robert C. Dixon, 1994 [2] Global positioning system signals,
Re: [time-nuts] Need advice for multilateration setup
On Sat, 04 Apr 2015 08:49:01 +0200 Magnus Danielson mag...@rubidium.se wrote: This is on either side of the amateur 23 cm band. That's also the first band where you have bandwidth enough to fool around with stuff like this without breaking the bandplan. This shouldn't be much of a problem. Using a chiping rate of a couple of kHz should be enough for this application. The signal strength can be rather large, directive antennas can be used and the expected noise level is rather low. So there no need to use a high chipping rate to compensate for noise effects. Of course, using a higher chipping rate makes it also easier to get an higher accuracy, but I would start with something easy to do first, like a 100mW transmitter in the 70cm band with 10kHz chipping rate (or go to a sub-band, where 200kHz signals are allowed). With that kind of setup it should be possible to use something like RTL-SDR for the first experiments and then gradually upgrade to better hardware to improve accuracy. Attila Kinali -- _av500_ phd is easy _av500_ getting dsl is hard ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
On 4/6/15 2:14 AM, Attila Kinali wrote: Moin, On Fri, 03 Apr 2015 22:51:34 -0500 Robert Watzlavick roc...@watzlavick.com wrote: On 04/03/2015 10:12 PM, Robert Watzlavick wrote: I have an amateur radio license (mostly CW/HF and some VHF/UHF experience) and I've written some driver software for an IQ demodulation board but I have to admit, I would have no idea how to begin setting up that system as initially described by Attila and expanded by you and others. I have a rudimentary understanding of the modulation schemes involved but I don't fully understand how the various codes mentioned fit in. I've poked around a bit at some articles on PN codes and I can see how data would be transmitted but I think I'm missing something key that allows you to extract positions, velocities, etc. out of the various links. I think I have some more reading to do :) The basic system is that of an DSSS modulator/demodulator. The best text on spread spectrum systems I have found sofar is [1]. I explains modulation and demodulation in a hands on fashion. But, due to the age of the book, it does not contain any of the advanced stuff done today. But I think you don't need anything more fancy than an early-prompt-late correlator architecture for tracking. Actually, if you're post processing, you can just record raw bits and do the correlation in software. You don't really need to track it in real time. Although, that might not be a bad way to do it. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
On 4/6/15 2:21 AM, Attila Kinali wrote: On Sat, 04 Apr 2015 08:49:01 +0200 Magnus Danielson mag...@rubidium.se wrote: This is on either side of the amateur 23 cm band. That's also the first band where you have bandwidth enough to fool around with stuff like this without breaking the bandplan. This shouldn't be much of a problem. Using a chiping rate of a couple of kHz should be enough for this application. The signal strength can be rather large, directive antennas can be used and the expected noise level is rather low. So there no need to use a high chipping rate to compensate for noise effects. Of course, using a higher chipping rate makes it also easier to get an higher accuracy, but I would start with something easy to do first, like a 100mW transmitter in the 70cm band with 10kHz chipping rate (or go to a sub-band, where 200kHz signals are allowed). With that kind of setup it should be possible to use something like RTL-SDR for the first experiments and then gradually upgrade to better hardware to improve accuracy. One strategy for this kind of application is to do the fine measurement using carrier phase, and use the PN code to do ambiguity reduction. Then, a low chip rate is fine: you're basically using it as a check that you haven't slipped a cycle. I would think that the RTL dongles would work just fine, especially if you radiate a pilot tone from a fixed location as well as the tone from the rocket. You basically set up two PLLs in software one to track each tone, and subtract the phase of one from the other for each ground station. Attila Kinali ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Attila, Thank you very much for the references. I had come across [4] when searching on Kalman filters for GPS aiding of INS measurements. I didn't pay much attention to the GPS chapter at the time but I'll look at it again. I just downloaded [3] and it appears to have a good mix of practical vs. theoretical aspects. I appreciate the help! -Bob On 04/06/2015 04:14 AM, Attila Kinali wrote: The basic system is that of an DSSS modulator/demodulator. The best text on spread spectrum systems I have found sofar is [1]. I explains modulation and demodulation in a hands on fashion. But, due to the age of the book, it does not contain any of the advanced stuff done today. But I think you don't need anything more fancy than an early-prompt-late correlator architecture for tracking. For the way how GPS works and how correlation and everything is done, I would suggest [2,3,4]. [2] is a good overview of how GPS is done and contains 99% of everything you need to know (special thanks to Magnus for mentioning it). It lacks some details on how to actually implement the system though. There [3] helps a lot, as it's a book specifically on building a GPS/Galileo receiver. I only skimmed trough a digital copy of [4] yet, so I cannot say too much about it, but that it's probably the most complete book on radio and inertial navigation I have seen sofar. The level of detail seems to vary from topic to topic quite a bit, but it is a treasure trove of references for everything the book covers (which is a damn lot!) If you are tight on time I would probably recommend to start with [3] and have a look at [1] and [2] when things don't make sense. Attila Kinali [1] Spread Spectrum Systems with Commercial Applications, 3rd edition, by Robert C. Dixon, 1994 [2] Global positioning system signals, measurements, and performance, 2nd edition, by Partap Misra and Per Enge, 2012. [3] A Software-Defined GPS and Galileo Receiver, by Bore, Akos, Bertelsen, Rinder, Jensen, 2007 [4] Principles of GNSS, Inertial, and Multisensor Integrated Navigation Systems, 2nd edition, by Paul D. Groves, 2013 ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Jim, On 04/06/2015 03:13 PM, Jim Lux wrote: On 4/6/15 2:21 AM, Attila Kinali wrote: On Sat, 04 Apr 2015 08:49:01 +0200 Magnus Danielson mag...@rubidium.se wrote: This is on either side of the amateur 23 cm band. That's also the first band where you have bandwidth enough to fool around with stuff like this without breaking the bandplan. This shouldn't be much of a problem. Using a chiping rate of a couple of kHz should be enough for this application. The signal strength can be rather large, directive antennas can be used and the expected noise level is rather low. So there no need to use a high chipping rate to compensate for noise effects. Of course, using a higher chipping rate makes it also easier to get an higher accuracy, but I would start with something easy to do first, like a 100mW transmitter in the 70cm band with 10kHz chipping rate (or go to a sub-band, where 200kHz signals are allowed). With that kind of setup it should be possible to use something like RTL-SDR for the first experiments and then gradually upgrade to better hardware to improve accuracy. One strategy for this kind of application is to do the fine measurement using carrier phase, and use the PN code to do ambiguity reduction. Then, a low chip rate is fine: you're basically using it as a check that you haven't slipped a cycle. I would think that the RTL dongles would work just fine, especially if you radiate a pilot tone from a fixed location as well as the tone from the rocket. You basically set up two PLLs in software one to track each tone, and subtract the phase of one from the other for each ground station. Indeed. Considering that in the start location you can solve integer ambiguity, especially with a pilot-tone or several radiated. Hacking in on the RTL-SDR to steer or replace the clock with a more stable clock might be considered. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
I have an amateur radio license (mostly CW/HF and some VHF/UHF experience) and I've written some driver software for an IQ demodulation board but I have to admit, I would have no idea how to begin setting up that system as initially described by Attila and expanded by you and others. I have a rudimentary understanding of the modulation schemes involved but I don't fully understand how the various codes mentioned fit in. I've poked around a bit at some articles on PN codes and I can see how data would be transmitted but I think I'm missing something key that allows you to extract positions, velocities, etc. out of the various links. I think I have some more reading to do :) Thanks, -Bob On 04/03/2015 06:08 AM, Magnus Danielson wrote: I think this is a good idea, and it is relatively straight-forward to do. You can observe both code and carrier phase this way, given that the transmitting radio is coherent with the code generation clock. Doppler also pops out of the tracking station. A good coding-gain reduces the need for a strong transmitter. The issue might be the allowed width of the signal being transmitted, forcing the chipping rate down. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Hi Bob, The actual receiver logic is that you have some sampling point in time, the tracking phase of a channel is being sampled. As you do for multiple channels, the relative phase of each channel is sampled. In order to extend this phase into a pseudo-range, one needs to guess how many integer multiples of the code there is from each GPS to the receiver. A bunch of multiples is assumed from the orbit, as there is at least the delay of the shortest distance, and then you can make a rough estimate by the sub-code phase of the birds, as they hook up like a set of clock-work gears. That gives you a first approximate guess, which might be wrong, but as we try to make it fit, we can solve this equation and out pops a first rough estimate, from that we can then maintain a correct guess from then on. For your rocket, you have a known stable situation at the launch-pad. That cuts out the guess-work, as at that point, you can assume that there is no multiple as your measurement nodes are within range. Cheers, Magnus On 04/04/2015 05:51 AM, Robert Watzlavick wrote: To head off a bunch of replies - I think I stumbled upon what is being suggested. To extract the pseudorange, you have to figure out the offset of the locally generated PN code against the one that is received. In this reverse GPS case, I assume each ground station would have to start their local PN codes at the same time? Then you would be able to get the pseudoranges at each ground station and use those values for the multilateration equations. You still would have an uncertainty of one clock cycle since the phases of the local clocks at the stations wouldn't be aligned but several folks have suggested ways around that. -Bob On 04/03/2015 10:12 PM, Robert Watzlavick wrote: I have an amateur radio license (mostly CW/HF and some VHF/UHF experience) and I've written some driver software for an IQ demodulation board but I have to admit, I would have no idea how to begin setting up that system as initially described by Attila and expanded by you and others. I have a rudimentary understanding of the modulation schemes involved but I don't fully understand how the various codes mentioned fit in. I've poked around a bit at some articles on PN codes and I can see how data would be transmitted but I think I'm missing something key that allows you to extract positions, velocities, etc. out of the various links. I think I have some more reading to do :) Thanks, -Bob On 04/03/2015 06:08 AM, Magnus Danielson wrote: I think this is a good idea, and it is relatively straight-forward to do. You can observe both code and carrier phase this way, given that the transmitting radio is coherent with the code generation clock. Doppler also pops out of the tracking station. A good coding-gain reduces the need for a strong transmitter. The issue might be the allowed width of the signal being transmitted, forcing the chipping rate down. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
To head off a bunch of replies - I think I stumbled upon what is being suggested. To extract the pseudorange, you have to figure out the offset of the locally generated PN code against the one that is received. In this reverse GPS case, I assume each ground station would have to start their local PN codes at the same time? Then you would be able to get the pseudoranges at each ground station and use those values for the multilateration equations. You still would have an uncertainty of one clock cycle since the phases of the local clocks at the stations wouldn't be aligned but several folks have suggested ways around that. -Bob On 04/03/2015 10:12 PM, Robert Watzlavick wrote: I have an amateur radio license (mostly CW/HF and some VHF/UHF experience) and I've written some driver software for an IQ demodulation board but I have to admit, I would have no idea how to begin setting up that system as initially described by Attila and expanded by you and others. I have a rudimentary understanding of the modulation schemes involved but I don't fully understand how the various codes mentioned fit in. I've poked around a bit at some articles on PN codes and I can see how data would be transmitted but I think I'm missing something key that allows you to extract positions, velocities, etc. out of the various links. I think I have some more reading to do :) Thanks, -Bob On 04/03/2015 06:08 AM, Magnus Danielson wrote: I think this is a good idea, and it is relatively straight-forward to do. You can observe both code and carrier phase this way, given that the transmitting radio is coherent with the code generation clock. Doppler also pops out of the tracking station. A good coding-gain reduces the need for a strong transmitter. The issue might be the allowed width of the signal being transmitted, forcing the chipping rate down. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
We essentially propose that you mimic the GPS system. The original GPS birds are relatively stupid. In GPS, the core clock produces 10,23 Mhz (modern GPS rubidiums output a different frequency, but that is not the point here), for C/A code it is divided down with 10 to produce the C/A chipping rate of 1,023 MHz and considering that the Gould-codes being used is 1023 chips long, they will wrap around every 1 ms. The same 10,23 MHz is then used to produce the carrier frequency which is 154 * 10,23 MHz. The produced PRN sequence alternate between +1 and -1 and when mixing this with the carrier frequency a BPSK signal is produced which is amplified and transmitted. A second carrier is also produced as 120 * 10,23 MHz. This is on either side of the amateur 23 cm band. That's also the first band where you have bandwidth enough to fool around with stuff like this without breaking the bandplan. Cheers, Magnus On 04/04/2015 05:12 AM, Robert Watzlavick wrote: I have an amateur radio license (mostly CW/HF and some VHF/UHF experience) and I've written some driver software for an IQ demodulation board but I have to admit, I would have no idea how to begin setting up that system as initially described by Attila and expanded by you and others. I have a rudimentary understanding of the modulation schemes involved but I don't fully understand how the various codes mentioned fit in. I've poked around a bit at some articles on PN codes and I can see how data would be transmitted but I think I'm missing something key that allows you to extract positions, velocities, etc. out of the various links. I think I have some more reading to do :) Thanks, -Bob On 04/03/2015 06:08 AM, Magnus Danielson wrote: I think this is a good idea, and it is relatively straight-forward to do. You can observe both code and carrier phase this way, given that the transmitting radio is coherent with the code generation clock. Doppler also pops out of the tracking station. A good coding-gain reduces the need for a strong transmitter. The issue might be the allowed width of the signal being transmitted, forcing the chipping rate down. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Hi, On 03/26/2015 01:25 PM, Attila Kinali wrote: On Wed, 25 Mar 2015 21:27:35 -0500 Robert Watzlavick roc...@watzlavick.com wrote: I'm working on a project that I could use some advice on and also might be of interest to the list. If it's not appropriate for the list, my apologies. The gods have apporved of your request. You may speak now. ;-) I want to develop a tracking system for an amateur rocket that can allow me to track the rocket even if onboard GPS is lost (as is typical during ascent and sometimes during descent) or if telemetry is lost. Given you can synchronize the clocks of the ground stations well enough, then the rest is easy. Then you can get away with having a simple signal generator that only needs an XO. Or you can go for a TCXO to make your signal processing life easier. What you need to do, is actually the same as GPS does: Create a direct spread spectrum signal and track it on all ground stations. The DSSS has the advantage over the single pulse, that it's more resilient against noise and interference. The disadvantage is, that you have to have more complicated hardware. One viable way would be, that you have precisly synchronized sampling systems (e.g. SDR's like the bladeRF which can take an external clock) and then feed the data to a PC where you do the heavy lifting. Then you don't need to build custom hardware at least. Also, if the precision by the DSSS signal is not good enough, you can apply various tricks from the GPS world, like carrier phase tracking, etc. I think this is a good idea, and it is relatively straight-forward to do. You can observe both code and carrier phase this way, given that the transmitting radio is coherent with the code generation clock. Doppler also pops out of the tracking station. A good coding-gain reduces the need for a strong transmitter. The issue might be the allowed width of the signal being transmitted, forcing the chipping rate down. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Remember that you can actually let each base-station transmit at a different code, and you can then monitor them that way. You could even keep them frequency and phase locked or just monitor it and adjust it in the post-processing. Such an approach would be a nice complementary solution to the GPS/GNSS receivers. Also, it's more of the same which helps in knowing your system. Cheers, Magnus On 03/26/2015 06:32 PM, Robert Watzlavick wrote: Thanks for the suggestion. Does the DSSS make it easier to correlate between ground stations? I'm not sure how to handle the phase offset on the 10 MHz ref clocks. -Bob On Mar 26, 2015, at 07:25, Attila Kinali att...@kinali.ch wrote: On Wed, 25 Mar 2015 21:27:35 -0500 Robert Watzlavick roc...@watzlavick.com wrote: I'm working on a project that I could use some advice on and also might be of interest to the list. If it's not appropriate for the list, my apologies. The gods have apporved of your request. You may speak now. ;-) I want to develop a tracking system for an amateur rocket that can allow me to track the rocket even if onboard GPS is lost (as is typical during ascent and sometimes during descent) or if telemetry is lost. Given you can synchronize the clocks of the ground stations well enough, then the rest is easy. Then you can get away with having a simple signal generator that only needs an XO. Or you can go for a TCXO to make your signal processing life easier. What you need to do, is actually the same as GPS does: Create a direct spread spectrum signal and track it on all ground stations. The DSSS has the advantage over the single pulse, that it's more resilient against noise and interference. The disadvantage is, that you have to have more complicated hardware. One viable way would be, that you have precisly synchronized sampling systems (e.g. SDR's like the bladeRF which can take an external clock) and then feed the data to a PC where you do the heavy lifting. Then you don't need to build custom hardware at least. Also, if the precision by the DSSS signal is not good enough, you can apply various tricks from the GPS world, like carrier phase tracking, etc. HTH Attila Kinali -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Remember, that if you have 4 receivers you get X, Y, Z and T of the source, and in this case T will be the phase-drift of the rocket. So, if logged with sufficient precision, the stability of the on-board clock may not become as important as the fact that it is there and has reasonably good phase-noise. That however, might be an issue for sounding-rockets, but can be addressed to some degree by mounting. Cheers, Magnus On 03/28/2015 01:25 PM, Peter Reilley wrote: Some crystal oscillators specify their sensitivity to G forces. Here is one: http://www.abracon.com/Precisiontiming/AOCJYR-24.576MHz-M6069LF.pdf Available here: http://www.digikey.com/product-detail/en/AOCJYR-24.576MHZ-M6069LF/535-12627- 1-ND/4989033 Others specify shock and vibration limits but say nothing about frequency stability. Pete. -Original Message- From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Chris Albertson Sent: Friday, March 27, 2015 9:55 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Need advice for multilateration setup On Fri, Mar 27, 2015 at 10:29 AM, Chuck Harris cfhar...@erols.com wrote: The biggest problem I see is the crystal oscillator in the rocket is going to notice the G forces during acceleration in a pretty big way. But all of the ground stations will see the same frequency shift on the rocket's transmitter. I think this can be backed out in processing. Someone needs to write the equations and post them here. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Jim, On 03/28/2015 10:01 PM, Jim Lux wrote: On 3/28/15 10:27 AM, Bob Camp wrote: Hi So If the rocket continuously accelerates at 10,000 G’s, you will get a 20 ppm shift with typical sensitivity. If you do this for very long, you will also get into time dilation issues. (you hit 0.1C in 2 minutes). 10,000G is more like an artillery shell. A large amateur rocket might be more like 20-30G maximum. Also, it's not 1 G for very long, it's the fireing moment, which is critical for any oscillator flying with it. The impact moment is somehow less important as it is intended to self-destruct most of the times. Cheers, Magnus ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Hi Calibrating the G sensitivity of the oscillator can be done much more easily by simply rotating it 360 degrees while carefully reading out the frequency. If you want the full vector, you will need to rotate it through two circles, with the plane of one 90 degrees out relative to the other. The net result is that you get a 2G change in acceleration in each axis. Measure the frequency to 1x10^-10 every 10 degrees and you have what you need. You will need to keep the temperature / voltage / whatever stable enough that you don’t have more than 1x10^-10 drift through the process. That’s the main reason for taking two readings at the the same angle, one at the start and one at the end of the process. Far easier to do in a static fixture on the ground than to extract it from telemetry after the fact. The temperature outside your rocket is dropping at around 3C for every 1,000 feet you go up. At 10G’s your are going through 1,000 feet pretty quick. Just the 3 C in the first 1,000 feet will move your frequency 3 ppm while you are trying to measure a 2x10^-8 shift. So, if you put a double oven in the rocket and put a thermal shield around it, (possibly using the lead acid batteries you are powering it with) - you could get around the thermal shift to some degree. Of course the extra 20 or 30 pounds of weight *might* impact your weight budget a bit :) Bottom line is still the same, you don’t need to worry about the acceleration impact on the static frequency. You do need to worry about it’s impact on phase noise and your carefully worked out modulation scheme. This does not just apply to amateur rockets and working out the RF systems on them. Some fairly *large* defense systems have run into this issue pretty hard. Bob On Mar 28, 2015, at 10:34 PM, Bill Hawkins b...@iaxs.net wrote: An idea occurred (always a surprise): The rocket's acceleration increases from 1 g as the mass of fuel is ejected energetically, according to f=ma, with pretty constant force from the motor. At some point, the fuel and oxidizer tanks are empty (MECO), causing the acceleration to revert to 1 g or less, depending on altitude. The change from max acceleration to free flight offers an opportunity to calibrate the effect of max g on the oscillator. The velocity is almost unchanged at that point, so the change in Doppler shift comes only from the effect of acceleration on the oscillator. It should be possible to use linear interpolation for the effect of acceleration during powered flight, since f=ma is a first order equation. Bill Hawkins -Original Message- From: Bob Camp Sent: Saturday, March 28, 2015 6:22 PM The point being that, to even get acceleration into the picture, you need have impossibly high accelerations . At 10 G, your oscillator needs to be temperature stable to 0.01C to even see the acceleration. If you are climbing 100K feet during the acceleration phase the oscillator will see a *lot* more than that. Bob On Mar 28, 2015, at 5:01 PM, Jim Lux jim...@earthlink.net wrote: On 3/28/15 10:27 AM, Bob Camp wrote: So If the rocket continuously accelerates at 10,000 G's, you will get a 20 ppm shift with typical sensitivity. If you do this for very long, you will also get into time dilation issues. (you hit 0.1C in 2 minutes). 10,000G is more like an artillery shell. A large amateur rocket might be more like 20-30G maximum. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
I want to thank everybody for their help on this. Thanks to the list, I have plenty of ideas that I can prototype so I'll keep you posted what I end up trying and how well it works eventually. -Bob On 03/25/2015 09:27 PM, Robert Watzlavick wrote: I want to develop a tracking system for an amateur rocket that can allow me to track the rocket even if onboard GPS is lost (as is typical during ascent and sometimes during descent) or if telemetry is lost. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Hi The point being that, to even get acceleration into the picture, you need have impossibly high accelerations … At 10 G, your oscillator needs to be temperature stable to 0.01C to even see the acceleration. If you are climbing 100K feet during the acceleration phase the oscillator will see a *lot* more than that. Bob On Mar 28, 2015, at 5:01 PM, Jim Lux jim...@earthlink.net wrote: On 3/28/15 10:27 AM, Bob Camp wrote: Hi So If the rocket continuously accelerates at 10,000 G’s, you will get a 20 ppm shift with typical sensitivity. If you do this for very long, you will also get into time dilation issues. (you hit 0.1C in 2 minutes). 10,000G is more like an artillery shell. A large amateur rocket might be more like 20-30G maximum. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Hi Depending on construction of the resonator, an oscillator can have maximum sensitivities anywhere from 5x10^-8 / g to 5x10^-11 per G. Typical numbers for “good but not great” parts are in the 5x10^-10 to 2x10^-9 per G. Since the sensitivity is *not* the same in every axis, a device with 2x10^-9 in (say) the X-Y axis might have a 1x10^-10 sensitivity in (say) the Z axis. In something like a rocket, your acceleration is likely to have a dominant axis. With characterization data on the individual oscillator, you might be able to reduce the impact by 10:1. So If the rocket continuously accelerates at 10,000 G’s, you will get a 20 ppm shift with typical sensitivity. If you do this for very long, you will also get into time dilation issues. (you hit 0.1C in 2 minutes). If the oscillator has a 1 ppm / C temperature coefficient, a 20C change will give you the same (static) frequency shift. If you change temperature quickly (as you would in this case, you hit outer space in a few seconds) figure a 5 to 10X increase in that shift. Simply put - temperature will get you before acceleration does in terms of static shift. There are other things that will be a problem before either of these get in your way. Most tracking *assumes* good phase noise on the signal. Oddly enough rockets are not very quiet devices while accelerating. The same sensitivities that give you the issues from static acceleration give you phase noise under vibration. It is not at all unusual to see phase noise degradation of 60 db on physical small platforms doing high levels of acceleration. Bob On Mar 28, 2015, at 8:25 AM, Peter Reilley pe...@reilley.com wrote: Some crystal oscillators specify their sensitivity to G forces. Here is one: http://www.abracon.com/Precisiontiming/AOCJYR-24.576MHz-M6069LF.pdf Available here: http://www.digikey.com/product-detail/en/AOCJYR-24.576MHZ-M6069LF/535-12627- 1-ND/4989033 Others specify shock and vibration limits but say nothing about frequency stability. Pete. -Original Message- From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Chris Albertson Sent: Friday, March 27, 2015 9:55 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Need advice for multilateration setup On Fri, Mar 27, 2015 at 10:29 AM, Chuck Harris cfhar...@erols.com wrote: The biggest problem I see is the crystal oscillator in the rocket is going to notice the G forces during acceleration in a pretty big way. But all of the ground stations will see the same frequency shift on the rocket's transmitter. I think this can be backed out in processing. Someone needs to write the equations and post them here. -- Chris Albertson Redondo Beach, California ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
An idea occurred (always a surprise): The rocket's acceleration increases from 1 g as the mass of fuel is ejected energetically, according to f=ma, with pretty constant force from the motor. At some point, the fuel and oxidizer tanks are empty (MECO), causing the acceleration to revert to 1 g or less, depending on altitude. The change from max acceleration to free flight offers an opportunity to calibrate the effect of max g on the oscillator. The velocity is almost unchanged at that point, so the change in Doppler shift comes only from the effect of acceleration on the oscillator. It should be possible to use linear interpolation for the effect of acceleration during powered flight, since f=ma is a first order equation. Bill Hawkins -Original Message- From: Bob Camp Sent: Saturday, March 28, 2015 6:22 PM The point being that, to even get acceleration into the picture, you need have impossibly high accelerations . At 10 G, your oscillator needs to be temperature stable to 0.01C to even see the acceleration. If you are climbing 100K feet during the acceleration phase the oscillator will see a *lot* more than that. Bob On Mar 28, 2015, at 5:01 PM, Jim Lux jim...@earthlink.net wrote: On 3/28/15 10:27 AM, Bob Camp wrote: So If the rocket continuously accelerates at 10,000 G's, you will get a 20 ppm shift with typical sensitivity. If you do this for very long, you will also get into time dilation issues. (you hit 0.1C in 2 minutes). 10,000G is more like an artillery shell. A large amateur rocket might be more like 20-30G maximum. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
On 3/28/15 10:27 AM, Bob Camp wrote: Hi So If the rocket continuously accelerates at 10,000 G’s, you will get a 20 ppm shift with typical sensitivity. If you do this for very long, you will also get into time dilation issues. (you hit 0.1C in 2 minutes). 10,000G is more like an artillery shell. A large amateur rocket might be more like 20-30G maximum. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Some crystal oscillators specify their sensitivity to G forces. Here is one: http://www.abracon.com/Precisiontiming/AOCJYR-24.576MHz-M6069LF.pdf Available here: http://www.digikey.com/product-detail/en/AOCJYR-24.576MHZ-M6069LF/535-12627- 1-ND/4989033 Others specify shock and vibration limits but say nothing about frequency stability. Pete. -Original Message- From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of Chris Albertson Sent: Friday, March 27, 2015 9:55 PM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Need advice for multilateration setup On Fri, Mar 27, 2015 at 10:29 AM, Chuck Harris cfhar...@erols.com wrote: The biggest problem I see is the crystal oscillator in the rocket is going to notice the G forces during acceleration in a pretty big way. But all of the ground stations will see the same frequency shift on the rocket's transmitter. I think this can be backed out in processing. Someone needs to write the equations and post them here. -- Chris Albertson Redondo Beach, California ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
On 03/26/2015 02:25 PM, Hal Murray wrote: I want to develop a tracking system for an amateur rocket ... Do you need the position in real time, or just after the rocket returns so you can find it? Near real-time would be nice but I guess not an absolute requirement. 40 ns is 25 MHz. It shouldn't be hard to find a uP with counter/timer that runs faster than that. I think you can get away without fancy oscillators. I'm assuming you can use GPS to get the the initial position of the rocket and the receiving stations. I'm also assuming that the rocket can start transmitting a few seconds/minutes before launch to calibrate things. Suppose the receiver puts out a pulse. Feed that to a uP with a counter/timer module that gives you a time stamp. Feed all the time-stamps to a central PC that will sort things out. If the pulses are far enough apart it will be easy to figure out which time-stamps go together. [1] The clocks used to make the time stamps don't need to agree on a base time. You can sort that out at the PC with data from before the rocket leaves the ground. Good idea - I hadn't thought about that. As long as they don't drift too far, I can calibrate out the initial drift. If a flight lasts 100 seconds (handy number for back of napkin calculations) and the calibration/drift is off by 1E9, that's 100 ns. So you will need an oscillator that is stable to better than 1E10 over 100 seconds. Ballpark/handwave. Powered flight will be less than 30 seconds. Depending on when the chute deploys, it may take a few minutes or tens minutes to make it all the way down. If the chute doesn't open (a common occurrence), then it will come down much faster :) You can also calibrate the receiver oscillators again after the rocket lands. Does the transmitter survive the landing? Does the antenna survive well enough? If I get the rocket back in a small number of pieces, it will be an achievement. The recovery success rate with large amateur liquids isn't that grea Is Z interesting? I'm assuming you are firing rockets in flat desert terrain. All the receivers will be in the same plane. I'll bet the math has troubles if you try to calculate the Z when the rocket is near the plane of the receivers. Have you looked into a different set of algorithms that assume the rocket is on the ground? Altitude (z) is not too important for finding it but will be useful in confirming the performance. From the multilateration simulations I've done so far, there are some bad areas and yes, near the ground isn't too good if all them are in the same plane. Maybe I can put one or more of the ground stations on a big hill or something. Good point though - if they're nearly in the same plane, the equations may be a bit simpler. -Bob ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
I've already integrated an onboard IMU (Analog Devices ADIS16xxx) but they have a lot of drift, especially in a high-g environment. I plan to record the raw IMU data to a flash card and assuming I can recover the card intact, I'll use it to tune a Kalman filter algorithm for the future version that will have active control. I understand your point - it is a complicated solution but that's some of the fun of the project, trying out new ideas and learning new concepts. -Bob On 03/26/2015 01:10 PM, Mike Cook wrote: Sounds over complicated. Why not use an onboard triple-axis accelerometer? A few mm of real-estate, milliamp consumption, up to 16g, 600+ samples a sec. The code is probably already available. Le 26 mars 2015 à 03:27, Robert Watzlavick roc...@watzlavick.com a écrit : I'm working on a project that I could use some advice on and also might be of interest to the list. If it's not appropriate for the list, my apologies. I want to develop a tracking system for an amateur rocket that can allow me to track the rocket even if onboard GPS is lost (as is typical during ascent and sometimes during descent) or if telemetry is lost. The idea is to use a transmitter in the rocket and have 4 or more ground stations about a mile apart each receive the signal. Multilateration based on TDOA (time difference of arrival) measurements would then be used to determine x, y, z, and t. With at least 4 ground stations, you don't need to know the time the pulse was transmitted. The main problem I'm running into is that most of the algorithms I've come across are very sensitive to the expected uncertainty in the time measurements. I had thought 100 ns of timing accuracy in the received signals would be good enough but I think I need to get down less than 40 ns to keep the algorithms from blowing up. My desired position accuracy is around 100 ft up to a range of 100k ft. There were two different methods I thought of. The first method would transmit a pulse from the rocket and then use a counter or TDC on the ground to measure the time difference between a GPS PPS and the pulse arrival. This is the most straightforward method but I'm worried about the timing accuracy of the pulse measurement. I should be able to find a timing GPS that has a PPS output with about +/- 30-40 ns of jitter (2 sigma) so that portion is in the ballpark. There also seem to be TDCs that have accuracy and resolution in the tens of picosecond range but they also have a maximum interval in the millisecond range. I'm not sure I can ensure the pulse sent from the rocket will be within a few miilliseconds of the 1 PPS value on the ground. I will have onboard GPS before launch so in theory I could initialize a counter to align the transmit pulse within a millisecond or so to the onboard PPS. But, once GPS is lost on ascent, unless I put an OCXO onboard that pulse may drift t oo far away (due to temperature, acceleration, etc.) for the TDC on the ground to pick it up. Plus an OCXO will add weight and require extra power for the heater. Another idea would be to send pulses at a very fast rate, say 1 kHz to stay within the TDC window. But then I need to worry about what happens if the pulses get too close to the edge of the TDC window. One other variable is the delay through the RF chain on the receive end but I figure I could calibrate that out. The other idea, and I'm not sure exactly how to implement it, would be to transmit a continuous tone (1 kHz for example) and perform some kind of phase measurement at each ground station against a reference. I could use a GPSDO to divide down the 10 MHz to 1 kHz to compare with the received signal but how can I assure the divided down 1 kHz clocks are synchronized between ground stations? Are the 10 MHz outputs from GPSDOs necessarily aligned to each other? I let two Thunderbolts sit for a couple of hours and the 10 MHz outputs seemed to stabilize with an offset of about 1/4 of a cycle, too much for this application. Another related idea would be to use the 10 MHz output to clock an ADC and then sample several thousand points using curve fitting, interpolation, and averaging to get a more accurate zero crossing than you could get based on the sample rate alone. Adding a TDC would allow the use of RIS (random interleaved sampling) for repetitive signals which could generate an effective sample rate of 1 GS/s. Does anybody have advice or practical experience on which method would work better? Thanks, -Bob ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. Ceux qui sont prêts à abandonner une liberté essentielle pour obtenir une petite et provisoire sécurité, ne méritent ni liberté ni sécurité. Benjimin Franklin ___ time-nuts mailing list
Re: [time-nuts] Need advice for multilateration setup
On 03/26/2015 01:56 PM, Jim Lux wrote: The key is that you don't need *real time* position.. a few seconds or minutes delay is probably ok, right? Seconds are probably ok, minutes might be a little long. PCs are pretty fast though these days for signal processing I would think. To compensate for the receiver variability, simultaneously transmit a signal with a different PN code, at the same frequency (roughly) as the rocket's transmitter.. The receiver will receive both, but the signal from your ground reference transmitter isn't moving, so you can use the non-rocket signal as a calibration reference. Now I didn't think of that - so you're saying to send another signal from a central ground station to all the receivers and then have them use that as a relative reference? Since I'll know where each ground station is, I should be able to subtract off the TOF so each station has a common reference point. That's a pretty cool idea. What's your budget? I was thinking in the $1k range so that would be about $200 per ground station. A couple of controllers I was considering for the ground stations include the Netburner MOD54415 (same one I'm using for the flight computer) or the BeagleBone Black. Both of those are under $100 and have counter/timers onboard although I have to see what the max clock rate is. As long as the channel-to-channel delay wan't too bad, I think using a 12-bit ADC to digitize the two signals would work because you can interpolate to get a higher-resolution zero crossing. -Bob ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Your second method is by far the best. But it can be simplified. All you need is two very stable oscillators, one in the rocket and one some known fixed location. Then you ground stations can be just dumb recorders that record both signals. In post processing you compare the relative phases. Likely the rock has a transmitter already so all you need is a very good oscillator on the ground. This one transmits to all you ground stations This technique has. Even been used to analyze serious failures of large rockets. Transmitters are packed with batteries and continue after the explosion. They have recovered spin rates and so on of falling derbies. On Wednesday, March 25, 2015, Robert Watzlavick roc...@watzlavick.com wrote: I'm working on a project that I could use some advice on and also might be of interest to the list. If it's not appropriate for the list, my apologies. I want to develop a tracking system for an amateur rocket that can allow me to track the rocket even if onboard GPS is lost (as is typical during ascent and sometimes during descent) or if telemetry is lost. The idea is to use a transmitter in the rocket and have 4 or more ground stations about a mile apart each receive the signal. Multilateration based on TDOA (time difference of arrival) measurements would then be used to determine x, y, z, and t. With at least 4 ground stations, you don't need to know the time the pulse was transmitted. The main problem I'm running into is that most of the algorithms I've come across are very sensitive to the expected uncertainty in the time measurements. I had thought 100 ns of timing accuracy in the received signals would be good enough but I think I need to get down less than 40 ns to keep the algorithms from blowing up. My desired position accuracy is around 100 ft up to a range of 100k ft. There were two different methods I thought of. The first method would transmit a pulse from the rocket and then use a counter or TDC on the ground to measure the time difference between a GPS PPS and the pulse arrival. This is the most straightforward method but I'm worried about the timing accuracy of the pulse measurement. I should be able to find a timing GPS that has a PPS output with about +/- 30-40 ns of jitter (2 sigma) so that portion is in the ballpark. There also seem to be TDCs that have accuracy and resolution in the tens of picosecond range but they also have a maximum interval in the millisecond range. I'm not sure I can ensure the pulse sent from the rocket will be within a few miilliseconds of the 1 PPS value on the ground. I will have onboard GPS before launch so in theory I could initialize a counter to align the transmit pulse within a millisecond or so to the onboard PPS. But, once GPS is lost on ascent, unless I put an OCXO onboard that pulse may drift too far away (due to temperature, acceleration, etc.) for the TDC on the ground to pick it up. Plus an OCXO will add weight and require extra power for the heater. Another idea would be to send pulses at a very fast rate, say 1 kHz to stay within the TDC window. But then I need to worry about what happens if the pulses get too close to the edge of the TDC window. One other variable is the delay through the RF chain on the receive end but I figure I could calibrate that out. The other idea, and I'm not sure exactly how to implement it, would be to transmit a continuous tone (1 kHz for example) and perform some kind of phase measurement at each ground station against a reference. I could use a GPSDO to divide down the 10 MHz to 1 kHz to compare with the received signal but how can I assure the divided down 1 kHz clocks are synchronized between ground stations? Are the 10 MHz outputs from GPSDOs necessarily aligned to each other? I let two Thunderbolts sit for a couple of hours and the 10 MHz outputs seemed to stabilize with an offset of about 1/4 of a cycle, too much for this application. Another related idea would be to use the 10 MHz output to clock an ADC and then sample several thousand points using curve fitting, interpolation, and averaging to get a more accurate zero crossing than you could get based on the sample rate alone. Adding a TDC would allow the use of RIS (random interleaved sampling) for repetitive signals which could generate an effective sample rate of 1 GS/s. Does anybody have advice or practical experience on which method would work better? Thanks, -Bob ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/ mailman/listinfo/time-nuts and follow the instructions there. -- Chris Albertson Redondo Beach, California ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Robert; It seems that a Doppler system should work for you. But first, you have a problem. If you want to track your rocket to 100K feet (20 miles) using some form of triangulation then you need your receiving stations further apart than 1 mile. Your triangle is too extreme and any measurement error will be greatly amplified. Here is what I suggest. Place a simple transmitter in the rocket of say 100 MHz. It really should be a legal frequency, 2 meter ham band? The transmitted frequency is not modulated and should be stable for the duration of the flight. The receiving stations should have a very narrow receive filter on the front end and mix the signal with a local oscillator that is 5 KHz off from the rocket frequency. For example: 100.005 MHz. A narrow audio filter will help as well. This is results in a very narrow bandwidth receiver which is very good in rejecting received noise. Take the audio signal and feed it into a computer's audio input. Sample the audio A/D converter as fast as you can and timestamp each sample. The computer's clock should be synchronized with your GPS receiver's time. This system measures velocity relative to your vantage point. Because distance is the integral of velocity you can calculate the distance during your flight. Since the initial positions are known you can calculate absolute position. If we assume a 100 MHz transmitter and with the speed of light at 300,000 KM/S you will see about 1/3 of a HZ shift for each 1 M/S of velocity. You do not need super stable oscillators. They only need to be stable for the duration of the flight. Here is how the flight will be tracked: Before the flight, the ground stations will receive the 100 MHz from the rocket and record the offset between the rocket's oscillator and the local oscillator. Any error will show up as the 5 KHz being somewhat off. This is not a problem if it remains constant during the flight. Before the flight the computer logs the audio input data with the timestamp. This is the reference data. When the rocket is launched the computer continues logging but should notice the shift in frequency. The entire set of logged data should show the velocity profile for the entire flight. This can be converted to distance since all of the initial positions of the ground stations and the rocket are known. Using the data from all the ground stations you can calculate the absolute position of the rocket for the entire flight. This setup should easily fit within your budget. The crystal oscillators do not need to be super precise or stable. They only need to be stable for the duration of the flight since the system calibrates itself immediately before launch. Pete. Robert Watzlavick wrote: I'm working on a project that I could use some advice on and also might be of interest to the list. If it's not appropriate for the list, my apologies. I want to develop a tracking system for an amateur rocket that can allow me to track the rocket even if onboard GPS is lost (as is typical during ascent and sometimes during descent) or if telemetry is lost. The idea is to use a transmitter in the rocket and have 4 or more ground stations about a mile apart each receive the signal. Multilateration based on TDOA (time difference of arrival) measurements would then be used to determine x, y, z, and t. With at least 4 ground stations, you don't need to know the time the pulse was transmitted. The main problem I'm running into is that most of the algorithms I've come across are very sensitive to the expected uncertainty in the time measurements. I had thought 100 ns of timing accuracy in the received signals would be good enough but I think I need to get down less than 40 ns to keep the algorithms from blowing up. My desired position accuracy is around 100 ft up to a range of 100k ft. There were two different methods I thought of. The first method would transmit a pulse from the rocket and then use a counter or TDC on the ground to measure the time difference between a GPS PPS and the pulse arrival. This is the most straightforward method but I'm worried about the timing accuracy of the pulse measurement. I should be able to find a timing GPS that has a PPS output with about +/- 30-40 ns of jitter (2 sigma) so that portion is in the ballpark. There also seem to be TDCs that have accuracy and resolution in the tens of picosecond range but they also have a maximum interval in the millisecond range. I'm not sure I can ensure the pulse sent from the rocket will be within a few miilliseconds of the 1 PPS value on the ground. I will have onboard GPS before launch so in theory I could initialize a counter to align the transmit pulse within a millisecond or so to the onboard PPS. But, once GPS is lost on ascent, unless I put an OCXO onboard that pulse may drift too far away (due to temperature, acceleration, etc.) for the TDC on the ground to pick
Re: [time-nuts] Need advice for multilateration setup
NASA uses the Doppler effect for deep space navigation, by integrating the velocity. You'd need a very stable oscillator, but you don't need a powered oven, due to the short duration of the flight. You only need one receiver. In fact, if it's possible for the rocket to hear a ground signal and return it at some offset or fractional frequency, you don't need an oscillator on the rocket. But if you do need a stable oscillator, consider enclosing it in aerogel, as we were discussing a few months ago. Bring it up to temp with ground power and let it go. There is still the matter of acceleration. If the oscillator can be calibrated, then the frequency versus acceleration is known and can be used to get the rocket's acceleration during powered flight. Double integration yields position. Taking the Doppler shift out of the integral could be tricky. Disclaimer: The last time I had anything to do with a rocket was 1959, with an Aerobee-Hi launched from White Sands, NM. We used Doppler to get altitude for upper air density measurement. The rocket went off course horizontally (determined by radar) and was destroyed before it crossed the border. Bill Hawkins ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
On Thu, 26 Mar 2015 12:32:33 -0500 Robert Watzlavick roc...@watzlavick.com wrote: Thanks for the suggestion. Does the DSSS make it easier to correlate between ground stations? I'm not sure how to handle the phase offset on the 10 MHz ref clocks. The DSSS allows you to make the integer ambiguity, you have with all periodic signals low enough that you dont care anymore. Ie. if you have a PRN that repeates every millisecond, then your you will have an ambiguity of n*300km, which you can easily resolve. The other advantage is that you have multiple edges (not just one, when you have a single pulse) over which you can average, thus getting a better precision. The downside of this is, that you have not only to solve for position and time, but for position, velocity and time (or rather frequency of the oscillator). The idea with the reference station on ground, to sync up all other stations is quite good. Then you can use simple DVB-T dongles (google RTL-SDR) as receivers, which you get almost for free on ebay. But you pay for that in higher calculation complexity. On the other hand, adding another measurment station is just another PC + USB dongle. I think that most of the receiver work can be done with gnu radio as basis. But i have never done any DSSS system in GR, so i cannot say for sure. HTH Attila Kinali -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
The biggest problem I see is the crystal oscillator in the rocket is going to notice the G forces during acceleration in a pretty big way. Time nuts easily notice the reversal in a 1G force on a laboratory oscillator caused by flipping it on its back for service. But all is not even close to lost. If your transmitter is amplitude modulated with a rate that is a digital division of your crystal's frequency, then you can remove any G-variation in the crystal's frequency by observing frequency variations in your modulation. Doppler will change the carrier frequency with speed, but it won't change the amplitude modulation frequency. Otherwise it should work beautifully. -Chuck Harris Peter Reilley wrote: Robert; It seems that a Doppler system should work for you. But first, you have a problem. If you want to track your rocket to 100K feet (20 miles) using some form of triangulation then you need your receiving stations further apart than 1 mile. Your triangle is too extreme and any measurement error will be greatly amplified. Here is what I suggest. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
On Fri, Mar 27, 2015 at 10:29 AM, Chuck Harris cfhar...@erols.com wrote: The biggest problem I see is the crystal oscillator in the rocket is going to notice the G forces during acceleration in a pretty big way. But all of the ground stations will see the same frequency shift on the rocket's transmitter. I think this can be backed out in processing. Someone needs to write the equations and post them here. -- Chris Albertson Redondo Beach, California ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Thanks for the suggestion. Does the DSSS make it easier to correlate between ground stations? I'm not sure how to handle the phase offset on the 10 MHz ref clocks. -Bob On Mar 26, 2015, at 07:25, Attila Kinali att...@kinali.ch wrote: On Wed, 25 Mar 2015 21:27:35 -0500 Robert Watzlavick roc...@watzlavick.com wrote: I'm working on a project that I could use some advice on and also might be of interest to the list. If it's not appropriate for the list, my apologies. The gods have apporved of your request. You may speak now. ;-) I want to develop a tracking system for an amateur rocket that can allow me to track the rocket even if onboard GPS is lost (as is typical during ascent and sometimes during descent) or if telemetry is lost. Given you can synchronize the clocks of the ground stations well enough, then the rest is easy. Then you can get away with having a simple signal generator that only needs an XO. Or you can go for a TCXO to make your signal processing life easier. What you need to do, is actually the same as GPS does: Create a direct spread spectrum signal and track it on all ground stations. The DSSS has the advantage over the single pulse, that it's more resilient against noise and interference. The disadvantage is, that you have to have more complicated hardware. One viable way would be, that you have precisly synchronized sampling systems (e.g. SDR's like the bladeRF which can take an external clock) and then feed the data to a PC where you do the heavy lifting. Then you don't need to build custom hardware at least. Also, if the precision by the DSSS signal is not good enough, you can apply various tricks from the GPS world, like carrier phase tracking, etc. HTH Attila Kinali -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
On Wed, 25 Mar 2015 21:27:35 -0500 Robert Watzlavick roc...@watzlavick.com wrote: I'm working on a project that I could use some advice on and also might be of interest to the list. If it's not appropriate for the list, my apologies. The gods have apporved of your request. You may speak now. ;-) I want to develop a tracking system for an amateur rocket that can allow me to track the rocket even if onboard GPS is lost (as is typical during ascent and sometimes during descent) or if telemetry is lost. Given you can synchronize the clocks of the ground stations well enough, then the rest is easy. Then you can get away with having a simple signal generator that only needs an XO. Or you can go for a TCXO to make your signal processing life easier. What you need to do, is actually the same as GPS does: Create a direct spread spectrum signal and track it on all ground stations. The DSSS has the advantage over the single pulse, that it's more resilient against noise and interference. The disadvantage is, that you have to have more complicated hardware. One viable way would be, that you have precisly synchronized sampling systems (e.g. SDR's like the bladeRF which can take an external clock) and then feed the data to a PC where you do the heavy lifting. Then you don't need to build custom hardware at least. Also, if the precision by the DSSS signal is not good enough, you can apply various tricks from the GPS world, like carrier phase tracking, etc. HTH Attila Kinali -- It is upon moral qualities that a society is ultimately founded. All the prosperity and technological sophistication in the world is of no use without that foundation. -- Miss Matheson, The Diamond Age, Neil Stephenson ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
What's your budget? Put a white-rabbit switch (3.5keur) in the middle, and install a mile of single-mode fiber to each rx-station. Then use TDC or FDEL SPEC-cards (1.5keur each) at the RX-stations to time-stamp the incoming pulse. 1 ns systematic and 50 ps RMS random error should be doable. The systematic constant error in time-stamp for each rx-station can maybe be calibrated out in the TDOA-algorithm? The FDEL-card can time-stamp up to 100 kEdges/s (that results in a ca 4 Mb/s datastream). Anders On Thu, Mar 26, 2015 at 4:27 AM, Robert Watzlavick roc...@watzlavick.com wrote: I'm working on a project that I could use some advice on and also might be of interest to the list. If it's not appropriate for the list, my apologies. I want to develop a tracking system for an amateur rocket that can allow me to track the rocket even if onboard GPS is lost (as is typical during ascent and sometimes during descent) or if telemetry is lost. The idea is to use a transmitter in the rocket and have 4 or more ground stations about a mile apart each receive the signal. Multilateration based on TDOA (time difference of arrival) measurements would then be used to determine x, y, z, and t. With at least 4 ground stations, you don't need to know the time the pulse was transmitted. The main problem I'm running into is that most of the algorithms I've come across are very sensitive to the expected uncertainty in the time measurements. I had thought 100 ns of timing accuracy in the received signals would be good enough but I think I need to get down less than 40 ns to keep the algorithms from blowing up. My desired position accuracy is around 100 ft up to a range of 100k ft. There were two different methods I thought of. The first method would transmit a pulse from the rocket and then use a counter or TDC on the ground to measure the time difference between a GPS PPS and the pulse arrival. This is the most straightforward method but I'm worried about the timing accuracy of the pulse measurement. I should be able to find a timing GPS that has a PPS output with about +/- 30-40 ns of jitter (2 sigma) so that portion is in the ballpark. There also seem to be TDCs that have accuracy and resolution in the tens of picosecond range but they also have a maximum interval in the millisecond range. I'm not sure I can ensure the pulse sent from the rocket will be within a few miilliseconds of the 1 PPS value on the ground. I will have onboard GPS before launch so in theory I could initialize a counter to align the transmit pulse within a millisecond or so to the onboard PPS. But, once GPS is lost on ascent, unless I put an OCXO onboard that pulse may drift too far away (due to temperature, acceleration, etc.) for the TDC on the ground to pick it up. Plus an OCXO will add weight and require extra power for the heater. Another idea would be to send pulses at a very fast rate, say 1 kHz to stay within the TDC window. But then I need to worry about what happens if the pulses get too close to the edge of the TDC window. One other variable is the delay through the RF chain on the receive end but I figure I could calibrate that out. The other idea, and I'm not sure exactly how to implement it, would be to transmit a continuous tone (1 kHz for example) and perform some kind of phase measurement at each ground station against a reference. I could use a GPSDO to divide down the 10 MHz to 1 kHz to compare with the received signal but how can I assure the divided down 1 kHz clocks are synchronized between ground stations? Are the 10 MHz outputs from GPSDOs necessarily aligned to each other? I let two Thunderbolts sit for a couple of hours and the 10 MHz outputs seemed to stabilize with an offset of about 1/4 of a cycle, too much for this application. Another related idea would be to use the 10 MHz output to clock an ADC and then sample several thousand points using curve fitting, interpolation, and averaging to get a more accurate zero crossing than you could get based on the sample rate alone. Adding a TDC would allow the use of RIS (random interleaved sampling) for repetitive signals which could generate an effective sample rate of 1 GS/s. Does anybody have advice or practical experience on which method would work better? Thanks, -Bob ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/ mailman/listinfo/time-nuts and follow the instructions there. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Budget is a concern but not an overriding concern. I'd like to keep the whole system around $1k. I was planning on making it as portable as possible with each ground station being self contained and sending their data to the launch site over a serial RF modem at 9600 baud. I agree though - fiber connections would make it a lot easier. -Bob On Mar 26, 2015, at 08:41, Anders Wallin anders.e.e.wal...@gmail.com wrote: What's your budget? Put a white-rabbit switch (3.5keur) in the middle, and install a mile of single-mode fiber to each rx-station. Then use TDC or FDEL SPEC-cards (1.5keur each) at the RX-stations to time-stamp the incoming pulse. 1 ns systematic and 50 ps RMS random error should be doable. The systematic constant error in time-stamp for each rx-station can maybe be calibrated out in the TDOA-algorithm? The FDEL-card can time-stamp up to 100 kEdges/s (that results in a ca 4 Mb/s datastream). Anders On Thu, Mar 26, 2015 at 4:27 AM, Robert Watzlavick roc...@watzlavick.com wrote: I'm working on a project that I could use some advice on and also might be of interest to the list. If it's not appropriate for the list, my apologies. I want to develop a tracking system for an amateur rocket that can allow me to track the rocket even if onboard GPS is lost (as is typical during ascent and sometimes during descent) or if telemetry is lost. The idea is to use a transmitter in the rocket and have 4 or more ground stations about a mile apart each receive the signal. Multilateration based on TDOA (time difference of arrival) measurements would then be used to determine x, y, z, and t. With at least 4 ground stations, you don't need to know the time the pulse was transmitted. The main problem I'm running into is that most of the algorithms I've come across are very sensitive to the expected uncertainty in the time measurements. I had thought 100 ns of timing accuracy in the received signals would be good enough but I think I need to get down less than 40 ns to keep the algorithms from blowing up. My desired position accuracy is around 100 ft up to a range of 100k ft. There were two different methods I thought of. The first method would transmit a pulse from the rocket and then use a counter or TDC on the ground to measure the time difference between a GPS PPS and the pulse arrival. This is the most straightforward method but I'm worried about the timing accuracy of the pulse measurement. I should be able to find a timing GPS that has a PPS output with about +/- 30-40 ns of jitter (2 sigma) so that portion is in the ballpark. There also seem to be TDCs that have accuracy and resolution in the tens of picosecond range but they also have a maximum interval in the millisecond range. I'm not sure I can ensure the pulse sent from the rocket will be within a few miilliseconds of the 1 PPS value on the ground. I will have onboard GPS before launch so in theory I could initialize a counter to align the transmit pulse within a millisecond or so to the onboard PPS. But, once GPS is lost on ascent, unless I put an OCXO onboard that pulse may drift too far away (due to temperature, acceleration, etc.) for the TDC on the ground to pick it up. Plus an OCXO will add weight and require extra power for the heater. Another idea would be to send pulses at a very fast rate, say 1 kHz to stay within the TDC window. But then I need to worry about what happens if the pulses get too close to the edge of the TDC window. One other variable is the delay through the RF chain on the receive end but I figure I could calibrate that out. The other idea, and I'm not sure exactly how to implement it, would be to transmit a continuous tone (1 kHz for example) and perform some kind of phase measurement at each ground station against a reference. I could use a GPSDO to divide down the 10 MHz to 1 kHz to compare with the received signal but how can I assure the divided down 1 kHz clocks are synchronized between ground stations? Are the 10 MHz outputs from GPSDOs necessarily aligned to each other? I let two Thunderbolts sit for a couple of hours and the 10 MHz outputs seemed to stabilize with an offset of about 1/4 of a cycle, too much for this application. Another related idea would be to use the 10 MHz output to clock an ADC and then sample several thousand points using curve fitting, interpolation, and averaging to get a more accurate zero crossing than you could get based on the sample rate alone. Adding a TDC would allow the use of RIS (random interleaved sampling) for repetitive signals which could generate an effective sample rate of 1 GS/s. Does anybody have advice or practical experience on which method would work better? Thanks, -Bob ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/ mailman/listinfo/time-nuts and follow the
Re: [time-nuts] Need advice for multilateration setup
Hi Bob: There are many ways of doing this. To test artillery shells they have a GPS front end in the shell and transmit the IF. A receiver at the gun is locked to the satellites prior to firing. You would want one of the 10 Hz update rage GPS receivers for this. Another method is to transmit a pulse of RF from the ground. When the rocket receives the pulse it sends out a pulse. When the receiver sees that pulse it makes another pulse. The repetition rate depends on the range (and fixed delays in the circuits). Doppler was used to determine the orbit of Sputnik. (Note: the transmitter was near a WWV frequency so the beat note was the Doppler.) If the rocket has a stable CW transmitter and you have a few receivers in known locations on the ground and record the Doppler for each receiver you can work out the path. A blinking light on the rocket and video cameras on the ground. Hollywood uses reflective dots on an actor's face and body which are watched with video cameras in a motion capture setup. A 3-axis accelerometer in the rocket and 3 channels of telemetry. Etc. Mail_Attachment -- Have Fun, Brooke Clarke http://www.PRC68.com http://www.end2partygovernment.com/2012Issues.html http://www.prc68.com/I/DietNutrition.html Robert Watzlavick wrote: I'm working on a project that I could use some advice on and also might be of interest to the list. If it's not appropriate for the list, my apologies. I want to develop a tracking system for an amateur rocket that can allow me to track the rocket even if onboard GPS is lost (as is typical during ascent and sometimes during descent) or if telemetry is lost. The idea is to use a transmitter in the rocket and have 4 or more ground stations about a mile apart each receive the signal. Multilateration based on TDOA (time difference of arrival) measurements would then be used to determine x, y, z, and t. With at least 4 ground stations, you don't need to know the time the pulse was transmitted. The main problem I'm running into is that most of the algorithms I've come across are very sensitive to the expected uncertainty in the time measurements. I had thought 100 ns of timing accuracy in the received signals would be good enough but I think I need to get down less than 40 ns to keep the algorithms from blowing up. My desired position accuracy is around 100 ft up to a range of 100k ft. There were two different methods I thought of. The first method would transmit a pulse from the rocket and then use a counter or TDC on the ground to measure the time difference between a GPS PPS and the pulse arrival. This is the most straightforward method but I'm worried about the timing accuracy of the pulse measurement. I should be able to find a timing GPS that has a PPS output with about +/- 30-40 ns of jitter (2 sigma) so that portion is in the ballpark. There also seem to be TDCs that have accuracy and resolution in the tens of picosecond range but they also have a maximum interval in the millisecond range. I'm not sure I can ensure the pulse sent from the rocket will be within a few miilliseconds of the 1 PPS value on the ground. I will have onboard GPS before launch so in theory I could initialize a counter to align the transmit pulse within a millisecond or so to the onboard PPS. But, once GPS is lost on ascent, unless I put an OCXO onboard that pulse may drift too far away (due to temperature, acceleration, etc.) for the TDC on the ground to pick it up. Plus an OCXO will add weight and require extra power for the heater. Another idea would be to send pulses at a very fast rate, say 1 kHz to stay within the TDC window. But then I need to worry about what happens if the pulses get too close to the edge of the TDC window. One other variable is the delay through the RF chain on the receive end but I figure I could calibrate that out. The other idea, and I'm not sure exactly how to implement it, would be to transmit a continuous tone (1 kHz for example) and perform some kind of phase measurement at each ground station against a reference. I could use a GPSDO to divide down the 10 MHz to 1 kHz to compare with the received signal but how can I assure the divided down 1 kHz clocks are synchronized between ground stations? Are the 10 MHz outputs from GPSDOs necessarily aligned to each other? I let two Thunderbolts sit for a couple of hours and the 10 MHz outputs seemed to stabilize with an offset of about 1/4 of a cycle, too much for this application. Another related idea would be to use the 10 MHz output to clock an ADC and then sample several thousand points using curve fitting, interpolation, and averaging to get a more accurate zero crossing than you could get based on the sample rate alone. Adding a TDC would allow the use of RIS (random interleaved sampling) for repetitive signals which could generate an effective sample rate of 1 GS/s. Does anybody have advice or practical experience on which
Re: [time-nuts] Need advice for multilateration setup
I want to develop a tracking system for an amateur rocket ... Do you need the position in real time, or just after the rocket returns so you can find it? I had thought 100 ns of timing accuracy in the received signals would be good enough but I think I need to get down less than 40 ns to keep the algorithms from blowing up 40 ns is 25 MHz. It shouldn't be hard to find a uP with counter/timer that runs faster than that. I think you can get away without fancy oscillators. I'm assuming you can use GPS to get the the initial position of the rocket and the receiving stations. I'm also assuming that the rocket can start transmitting a few seconds/minutes before launch to calibrate things. Suppose the receiver puts out a pulse. Feed that to a uP with a counter/timer module that gives you a time stamp. Feed all the time-stamps to a central PC that will sort things out. If the pulses are far enough apart it will be easy to figure out which time-stamps go together. [1] The clocks used to make the time stamps don't need to agree on a base time. You can sort that out at the PC with data from before the rocket leaves the ground. How accurate do the oscillators need to be? If you can listen for a while before launch you can calibrate the individual oscillators. So the question becomes how long does it take to do the calibration? How stable do the oscillators need to be? How long does the flight last? The calibration error and noise/wander from calibration is part of your error budget. If a flight lasts 100 seconds (handy number for back of napkin calculations) and the calibration/drift is off by 1E9, that's 100 ns. So you will need an oscillator that is stable to better than 1E10 over 100 seconds. Ballpark/handwave. You can also calibrate the receiver oscillators again after the rocket lands. Does the transmitter survive the landing? Does the antenna survive well enough? measurements would then be used to determine x, y, z, and t Is Z interesting? I'm assuming you are firing rockets in flat desert terrain. All the receivers will be in the same plane. I'll bet the math has troubles if you try to calculate the Z when the rocket is near the plane of the receivers. Have you looked into a different set of algorithms that assume the rocket is on the ground? - 1) If you need more data, you can still sort things out if the transmitter sends pulses with non-uniform spacing. I think there is a whole branch of math for that problem but I don't know the name/term. -- These are my opinions. I hate spam. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
Sounds over complicated. Why not use an onboard triple-axis accelerometer? A few mm of real-estate, milliamp consumption, up to 16g, 600+ samples a sec. The code is probably already available. Le 26 mars 2015 à 03:27, Robert Watzlavick roc...@watzlavick.com a écrit : I'm working on a project that I could use some advice on and also might be of interest to the list. If it's not appropriate for the list, my apologies. I want to develop a tracking system for an amateur rocket that can allow me to track the rocket even if onboard GPS is lost (as is typical during ascent and sometimes during descent) or if telemetry is lost. The idea is to use a transmitter in the rocket and have 4 or more ground stations about a mile apart each receive the signal. Multilateration based on TDOA (time difference of arrival) measurements would then be used to determine x, y, z, and t. With at least 4 ground stations, you don't need to know the time the pulse was transmitted. The main problem I'm running into is that most of the algorithms I've come across are very sensitive to the expected uncertainty in the time measurements. I had thought 100 ns of timing accuracy in the received signals would be good enough but I think I need to get down less than 40 ns to keep the algorithms from blowing up. My desired position accuracy is around 100 ft up to a range of 100k ft. There were two different methods I thought of. The first method would transmit a pulse from the rocket and then use a counter or TDC on the ground to measure the time difference between a GPS PPS and the pulse arrival. This is the most straightforward method but I'm worried about the timing accuracy of the pulse measurement. I should be able to find a timing GPS that has a PPS output with about +/- 30-40 ns of jitter (2 sigma) so that portion is in the ballpark. There also seem to be TDCs that have accuracy and resolution in the tens of picosecond range but they also have a maximum interval in the millisecond range. I'm not sure I can ensure the pulse sent from the rocket will be within a few miilliseconds of the 1 PPS value on the ground. I will have onboard GPS before launch so in theory I could initialize a counter to align the transmit pulse within a millisecond or so to the onboard PPS. But, once GPS is lost on ascent, unless I put an OCXO onboard that pulse may drift t oo far away (due to temperature, acceleration, etc.) for the TDC on the ground to pick it up. Plus an OCXO will add weight and require extra power for the heater. Another idea would be to send pulses at a very fast rate, say 1 kHz to stay within the TDC window. But then I need to worry about what happens if the pulses get too close to the edge of the TDC window. One other variable is the delay through the RF chain on the receive end but I figure I could calibrate that out. The other idea, and I'm not sure exactly how to implement it, would be to transmit a continuous tone (1 kHz for example) and perform some kind of phase measurement at each ground station against a reference. I could use a GPSDO to divide down the 10 MHz to 1 kHz to compare with the received signal but how can I assure the divided down 1 kHz clocks are synchronized between ground stations? Are the 10 MHz outputs from GPSDOs necessarily aligned to each other? I let two Thunderbolts sit for a couple of hours and the 10 MHz outputs seemed to stabilize with an offset of about 1/4 of a cycle, too much for this application. Another related idea would be to use the 10 MHz output to clock an ADC and then sample several thousand points using curve fitting, interpolation, and averaging to get a more accurate zero crossing than you could get based on the sample rate alone. Adding a TDC would allow the use of RIS (random interleaved sampling) for repetitive signals which could generate an effective sample rate of 1 GS/s. Does anybody have advice or practical experience on which method would work better? Thanks, -Bob ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. Ceux qui sont prêts à abandonner une liberté essentielle pour obtenir une petite et provisoire sécurité, ne méritent ni liberté ni sécurité. Benjimin Franklin ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
Re: [time-nuts] Need advice for multilateration setup
On 3/25/15 7:27 PM, Robert Watzlavick wrote: I'm working on a project that I could use some advice on and also might be of interest to the list. If it's not appropriate for the list, my apologies. I want to develop a tracking system for an amateur rocket that can allow me to track the rocket even if onboard GPS is lost (as is typical during ascent and sometimes during descent) or if telemetry is lost. The idea is to use a transmitter in the rocket and have 4 or more ground stations about a mile apart each receive the signal. Multilateration based on TDOA (time difference of arrival) measurements would then be used to determine x, y, z, and t. With at least 4 ground stations, you don't need to know the time the pulse was transmitted. The main problem I'm running into is that most of the algorithms I've come across are very sensitive to the expected uncertainty in the time measurements. I had thought 100 ns of timing accuracy in the received signals would be good enough but I think I need to get down less than 40 ns to keep the algorithms from blowing up. My desired position accuracy is around 100 ft up to a range of 100k ft. The key is that you don't need *real time* position.. a few seconds or minutes delay is probably ok, right? So transmit a PN code modulated onto a carrier from your rocket at some convenient frequency that's legal. Drive the PN shift register from your carrier, divided down, so there's an integer number of carrier cycles per chip. Receive that signal and digitize it on the ground at a suitably high rate. Post process the sampled data to recover the timing of the PN (and carrier). To compensate for the receiver variability, simultaneously transmit a signal with a different PN code, at the same frequency (roughly) as the rocket's transmitter.. The receiver will receive both, but the signal from your ground reference transmitter isn't moving, so you can use the non-rocket signal as a calibration reference. What's your budget? The transmitter can be very cheap. The receiver is going to be the pricey part, depending on how it's implemented. A sort of brute force approach would be to use a USRP and a portable PC at each receiver site. ___ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
