Re: [time-nuts] Using GPS for space-based instrument
Both Symmetricom and Frequency Electronics provide specialist space qualified products. It's not just a case of ruggedisation, but radiation hardening, g sensitivity which need to be designed in. Rob Kimberley -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of michael taylor Sent: 11 November 2008 00:01 To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Using GPS for space-based instrument On Mon, Nov 10, 2008 at 5:27 PM, Strauss, Karl F [EMAIL PROTECTED] wrote: I've been tasked (or was it I was volunteered?) to do some basic design definition work on an ultra-stable master frequency system for a proposed instrument that is currently planned to be in an Earth-trailing orbit. Given the first order accuracy requirement of 1 part in 1E-10, my first thought was to grab the GPS timing signal. Depending on your application requirements, I wonder if an OCXO (Oven Controlled Crystal Oscillators) in a ruggedised packaging might be suitable for your needs. For example one well-known frequency source vendor, Symmetricom offers a number of OCXO packagings that may be suitable for satellites. http://www.symmetricom.com/products/frequency-references/high-reliabilityru ggedized-frequency-sources/ Another option would be a Rubidium (Rb) frequency standard. A lot depends on your accuracy definition and requirements. Does it need to be accurate relative to UTC / UT / TAI? Depending on your application, dealing with details like leap seconds may not be worth staying in sync with UTC / GPS. Or do you need a stable local clock (within the SV)? Are you concerned with accuracy / stability over a short period of time (e.g. 1 second) or a longer period of time (e.g. 1 day / month / etc.)? ___ 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] Using GPS for space-based instrument
At 09.37 11/11/2008, you wrote: Both Symmetricom and Frequency Electronics provide specialist space qualified products. It's not just a case of ruggedisation, but radiation hardening, g sensitivity which need to be designed in. Rob Kimberley Also, space qualified GPS must handle the larger doppler shifts due to the relative spacecraft velocity. A standard GPS will not work, it is necessary to modify the algorhytms. Karl, if you know nothing about GPS, possibly it's best to start reading a tutorial, like http://www.trimble.com/gps/index.shtml - the amount of information to digest before putting a GPS receiver on a spacecraft is huge. Marco ___ 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] Link for T Mon
I have this and other Trimble software and documentation on my web site: http://www.ko4bb.com/cgi-bin/manuals.pl Search for Trimble Didier KO4BB -Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED] Sent: Sunday, November 09, 2008 8:50 PM To: time-nuts@febo.com Subject: [time-nuts] Link for T Mon I had an old laptop with T-Bolt mon and another program that synced the time with the T-Bolt but the hard drive died. I dug out a 486-300 to replace it. The link I had to down load the Thunderbolt software is no longer valid. Does any one have a valid link? TIA Chris ___ 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] Checking the Frequency of a Rubidium Oscillator
All the satellites are at the same frequency, and they are CDMA (each satellite has a different PN sequence on its signal) What's the bandwidth of an individual satellite? It may have been a different thread, but the Doppler shift is up to 2 KHz. Even if you could tune to an individual satellite signal, you still have to go through the whole GPS calculation in order to correct for Doppler. -- These are my opinions, not necessarily my employer's. 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] Checking the Frequency of a Rubidium Oscillator
-Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Hal Murray Sent: Tuesday, November 11, 2008 10:10 AM To: Discussion of precise time and frequency measurement Subject: Re: [time-nuts] Checking the Frequency of a Rubidium Oscillator All the satellites are at the same frequency, and they are CDMA (each satellite has a different PN sequence on its signal) What's the bandwidth of an individual satellite? Megahertz (the 1 MHz C/A code + the 10MHz P/Y code) It may have been a different thread, but the Doppler shift is up to 2 KHz. Even if you could tune to an individual satellite signal, you still have to go through the whole GPS calculation in order to correct for Doppler. A GPS receiver actually solves for the state vector of the receiver (including the local clock error) using the raw observables from the tracking loop (code phase). The nav equations calculate (apparent) range and range rate from the known state vector of each satellite and the (estimated) state vector of the receiver. Range rate is the doppler. The 1.xxx Megachip/second C/A code is 1023 bits long, so the classical approach is to step the receiver through all possible phases of the code, integrating at each one to see if it can detect the signal. If your integration time is, say, 10 milliseconds, it takes 10 seconds to step through them all. Once the signal is detected, the PN tracking loop tracks that signal. If you have some a-priori knowledge of the expected code phase, that reduces your search space quite a bit. You can also search for multiple codes at once with parallel receivers (really, parallel code tracking loops, because the RF receiver is usually just a single bit quantizer, and the same bits go to all loops), either acquiring different satellites in parallel, or speeding up the acquisition of a single satellite. This is where the proprietary nature of each manufacturer really comes in, because time spent acquiring is time not deriving a nav fix, and in a energy sensitive design (which many GPS receivers are.. E.g. in cell phones or battery powered), time is of the essence. For instance, if you know your approximate position and date/time, you can not bother trying to search for satellites that aren't above the horizon. If you've characterized your local oscillator properties, you might be able to do a more clever acquisition by modeling the drift. If the cellular system can tell the receiver in the phone an approximate position and estimated range/range rate, it can greatly reduce the acquisition time. (in fact, most phones don't actually implement a full GPS receiver.. They use assistance from the cell site to acquire, and just return the raw observables, and the centralized system turns that into a position) All very interesting stuff.. Jim ___ 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] Checking the Frequency of a Rubidium Oscillator
Hi Hal, What's the bandwidth of an individual satellite? the bandwidth is defined by the ~ 1 MHz chipping rate that phase-modulates the carrier, so it's roughly 1 MHz to both sides of the carrier (for the civilian signal). Search google images for gps spectrum to see plots... :-) Chris ___ 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] Checking the Frequency of a Rubidium Oscillator
On Tue, 2008-11-11 at 10:10 -0800, Hal Murray wrote: All the satellites are at the same frequency, and they are CDMA (each satellite has a different PN sequence on its signal) What's the bandwidth of an individual satellite? As said before. The carrier is chopped by a 1.023MHz PRN sequence on C/A and a 10.23MHz chipping rate on P(Y). -- Björn ___ 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] Checking the Frequency of a Rubidium Oscillator
Hi Bruce: It's my understanding that if you look at the signal from a common GPS antenna and feed it into a spectrum analyzer you will not see the signal. My guess is that when developed by the military it was designed to be a stealth system. GPS is what's called a Spread spectrum signal. Also the best possible s/n radio is determined by how orthogonal the different PN codes are to each other. These are described in ICD-GPS-200 which is on line at: http://www.navcen.uscg.gov/gps/geninfo/ along with other GPS info. Here's a National Instruments page about GPS signal generation: http://zone.ni.com/devzone/cda/tut/p/id/8015 The definitions on this page for the various Time To First Fix flavors may not be accurate. Have Fun, Brooke Clarke http://www.prc68.com [EMAIL PROTECTED] wrote: I have an EIP Model 548 counter with a YIG-tuned front end that can be programmed to scan over narrow frequency ranges. By feeding the rubidium oscillator under test into the 10 MHz clock input of the counter, is there any reasonably simple way to directly measure the frequency of a GPS satellite transmission so as to ascertain the accuracy of the rubidium source? The counter has an input sensitivity in the order of about –25 dBm -- not sufficient to measure directly from an amplified antenna, but perhaps through an amplifier. I am not sure whether the input YIG tuner selectivity is sufficient to separate transmissions from the various satellite’s (or are they TDMS?). What do you think? Bruce, KG6OJI **AOL Search: Your one stop for directions, recipes and all other Holiday needs. Search Now. (http://pr.atwola.com/promoclk/10075x1212792382x1200798498/aol?redir=http://searchblog.aol.com/2008/11/04/happy-holidays-from -aol-search/?ncid=emlcntussear0001) ___ 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] The other Trimble - NTPX26AB-06
I have just received a Trimble NTPX26AB-06, to add to the TBolt which performs perfectly. I am aware that some exchanges have taken place in the past with reference to this unit which it would seem was a Trimble design to offer a less expensive alternative to the HP- Z3801A. The obvious difference with the TBolt's facilities would seem to be the single BNC 10 MHz output, and the I/O (25 way D connector) which provides an RS-422 serial interface with TBoltMon and your PC. I do not have an RS-422 adaptor or PC card at present, and I am wondering if it is necessary to obtain one in light of the article investigating the HP-Z3801A. This makes it clear that the Z3801A had the facility to be simply modified by strapping, to change to RS-232 interface. I have examined my NTPX26AB-06, and find that on the narrow secondary PCB that carries the six status LED's on the front panel, there is a location (unpopulated), for a 9 pin PCB mount D connector, which if it was fitted, would protrude out of the front of the unit. More to the point, there are tracks to pins 2,3 and 5 which are carried back to the main PCB via the interconnection cable, which would appear to be carrying data to this location. I have not had time to investigate this further, but it would seem that Trimble had made this facility available as part of the specification, but it was unused ? Question, if it is a normal RS-232 (Trimble) interface, is it likely that it would be usable without modification ? - - is it possible that both interfaces are accessible at the same time - - or would one have to switch the other output off ? Further, I find that the I/O ( 25 way D ), offers the same additional 10 MHz and 1 pps signals as the Z3801A, although at non-standard levels. If anybody has investigated this unit and has been successful in modifying this facility, I would be pleased to have the information. I would rather have this unit interfaced with my existing RS-232 interface, than seek additional RS-422 equipment. I apologize if I am covering old ground, but I would appreciate any information on this item. Thank you Roy Phillips ___ 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] Checking the Frequency of a Rubidium Oscillator
-Original Message- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of [EMAIL PROTECTED] Sent: Tuesday, November 11, 2008 9:28 AM To: time-nuts@febo.com Subject: [time-nuts] Checking the Frequency of a Rubidium Oscillator I have an EIP Model 548 counter with a YIG-tuned front end that can be programmed to scan over narrow frequency ranges. By feeding the rubidium oscillator under test into the 10 MHz clock input of the counter, is there any reasonably simple way to directly measure the frequency of a GPS satellite transmission so as to ascertain the accuracy of the rubidium source? Not a chance.. The signal is a PN code at about 1 Megachip/second, and the power spectral density is probably comparable to the thermal noise floor of the receiver. The counter has an input sensitivity in the order of about -25 dBm -- not sufficient to measure directly from an amplified antenna, but perhaps through an amplifier. I am not sure whether the input YIG tuner selectivity is sufficient to separate transmissions from the various satellite's (or are they TDMS?). What do you think? All the satellites are at the same frequency, and they are CDMA (each satellite has a different PN sequence on its signal) ___ 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] Checking the Frequency of a Rubidium Oscillator
I have an EIP Model 548 counter with a YIG-tuned front end that can be programmed to scan over narrow frequency ranges. By feeding the rubidium oscillator under test into the 10 MHz clock input of the counter, is there any reasonably simple way to directly measure the frequency of a GPS satellite transmission so as to ascertain the accuracy of the rubidium source? The counter has an input sensitivity in the order of about –25 dBm -- not sufficient to measure directly from an amplified antenna, but perhaps through an amplifier. I am not sure whether the input YIG tuner selectivity is sufficient to separate transmissions from the various satellite’s (or are they TDMS?). What do you think? Bruce, KG6OJI **AOL Search: Your one stop for directions, recipes and all other Holiday needs. Search Now. (http://pr.atwola.com/promoclk/10075x1212792382x1200798498/aol?redir=http://searchblog.aol.com/2008/11/04/happy-holidays-from -aol-search/?ncid=emlcntussear0001) ___ 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] Checking the Frequency of a Rubidium Oscillator
On Tue, 2008-11-11 at 10:28 -0800, Lux, James P wrote: A GPS receiver actually solves for the state vector of the receiver (including the local clock error) using the raw observables from the tracking loop (code phase). The nav equations calculate (apparent) range and range rate from the known state vector of each satellite and the (estimated) state vector of the receiver. Range rate is the doppler. The 1.xxx Megachip/second C/A code is 1023 bits long, so the classical approach is to step the receiver through all possible phases of the code, integrating at each one to see if it can detect the signal. If your integration time is, say, 10 milliseconds, it takes 10 seconds to step through them all. Once the signal is detected, the PN tracking loop tracks that signal. You also need to check different doppler bins. 500Hz bins are a classic choice. -- Björn ___ 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] Using GPS for space-based instrument
On 11/11/08 2:55 AM, Brian Kirby [EMAIL PROTECTED] wrote: Somewhere out there is the specs that GPS was designed to. It list some of what they had to do, to make the rubidiums and cesiums work in the environment they put them in. Believe they are called ICD-GPS-200 or something like that ICD-GPS-200 is the spec that defines the interface among the various GPS segments, specifically, the RF waveform and nav message is defined there. I don't know that it gives the environmental requirements for the s/v hardware. In any event, space qualified GPS receivers are an off-the-shelf item (as much as anything space qualified is) and will set you back a good chunk of a million dollars, by the time you get it and the accompanying paperwork. For that matter, flight qualified Xos and OCXOs are readily available, and much cheaper, but 1E-10 is a pretty stringent tolerance. Check into the UltraStableOscillator (USO)s made by, e.g., Applied Physics Lab, which are used for deep space missions. Such things are used for doing precise measurements, not only of of s/c position, but also radio science (occultations, gravity measurements), although two way coherent ranging is also done. For the latter, you basically send a signal locked to a hydrogen maser signal to the spacecraft, where it is recovered and used to synthesize a return signal. A typical spec might be ADEV4E-16 in 1000 seconds. (and, why yes, verifying that the box sitting on the bench can do that performance is a non-trivial matter) Jim Lux ___ 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] Using GPS for space-based instrument
Somewhere out there is the specs that GPS was designed to. It list some of what they had to do, to make the rubidiums and cesiums work in the environment they put them in. Believe they are called ICD-GPS-200 or something like that Marco IK1ODO -2 wrote: At 09.37 11/11/2008, you wrote: Both Symmetricom and Frequency Electronics provide specialist space qualified products. It's not just a case of ruggedisation, but radiation hardening, g sensitivity which need to be designed in. Rob Kimberley Also, space qualified GPS must handle the larger doppler shifts due to the relative spacecraft velocity. A standard GPS will not work, it is necessary to modify the algorhytms. Karl, if you know nothing about GPS, possibly it's best to start reading a tutorial, like http://www.trimble.com/gps/index.shtml - the amount of information to digest before putting a GPS receiver on a spacecraft is huge. Marco ___ 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] Checking the Frequency of a Rubidium Oscillator
Björn Gabrielsson skrev: On Tue, 2008-11-11 at 10:28 -0800, Lux, James P wrote: A GPS receiver actually solves for the state vector of the receiver (including the local clock error) using the raw observables from the tracking loop (code phase). The nav equations calculate (apparent) range and range rate from the known state vector of each satellite and the (estimated) state vector of the receiver. Range rate is the doppler. The 1.xxx Megachip/second C/A code is 1023 bits long, so the classical approach is to step the receiver through all possible phases of the code, integrating at each one to see if it can detect the signal. If your integration time is, say, 10 milliseconds, it takes 10 seconds to step through them all. Once the signal is detected, the PN tracking loop tracks that signal. You also need to check different doppler bins. 500Hz bins are a classic choice. To elaborate on that. The C/A code is 1023 chips long, at 1,023 MChips/s which cause a cycle period of 1 ms. If you now consider sampling at 1 ms, the sampling rate is 1 kHz giving the Nyquist frequency of 500 Hz and thus 500 Hz doppler bins. For a earth bound GPS receiver, as extreme as 6 kHz doppler offsets can be seen on the carrier. The chipping rate shift is 1/1540 as low, so it can be almost neglected in comparision. The traditional search is a two-dimensional search in doppler bins +/- 6000 Hz in 500 Hz blocks and 0-1022 phase stages for each of 1-32 PRN codes. a search space totaling of 818400 combinations taking 818,4 s for a single integrator and 1/N for N integrators so roughly a minute or two for a now classic receiver of 8 to 12 channels. A more efficient algorithm is to sample the signal, FFT it and make the correlation in the frequency domain. It will crank out the phase and correlation amplitude for each PRN attempted with much less processing. This needs ot be performed for each doppler bin, but is certainly worthwhile the effort. Extending the search for all the WAAS/EGNOS sats is trivial and worthwhile. Once doppler bin and phase has been achieved for each PRN, picking the top N correlations and initiate channels is a quick process. The correlation phase can be initiated into the channel together with a rought initial frequency guess from the doppler bin and phase locking is quickly achieved in a traditional correlation channel. Data channel phase locking is the next thing, but that hunt is quickly achieved. This can be aided by having an existing total lock in which case even fundamental things such as bit phase on pseudo-code has a very limitied range between sats. A very rought idea of current position can give a correct model of full subcode phase. A sat based receiver must handle higher doppler offsets due to its higher speed, but as long as the per channel mix-down carrier NCO can be set wide enought, and that search patterns include the needed range, it will not be much of a problem. Naturally, tracking PLLs needs to handle the higher dynamics. As the orbit is fairly stable, orbit predictions can be fed into loop for better performance as it allows tighter bandwidth. The full benefit of code and carrier phase measurements should be used. Also, considering L2C is becomming more and more common, it should also be used. Preparation for L1C should also be done. As for signal bandwidth, while the C/A chiping rate is 1,023 MChips/s, we can expect a 2,046 MHz range between the first nulls offset from the carrier. However, the traditional sats uses a full 20,46 MHz bandwidth since it also transmitts the P(Y) code. Buliding a receiver that uses the full bandwidth provides certain benefits, but standard off the shelf chips usually stays within 2,046 MHz. The front end design is basically the same thought, just 10 MHz higher bandwidth. For civilian receivers, only code-less tracking receivers usually have that bandwidht. Modern GPS signals extend to a 24 MHz bandwidth. It is especially the M-code that mandates this shift. The M-code should be of no major interest for civilian receivers. Sorry for the short write-up, but there is certainly more to tell about this. 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] Checking the Frequency of a Rubidium Oscillator
In message [EMAIL PROTECTED], Magnus Danielson writes: Once doppler bin and phase has been achieved for each PRN, [...] Just a footnote to say that as soon as you start receiving ephemerides from the first sat, the search-space can be significantly reduced if you care to do the, rather longhaired, trignometric math. A sat based receiver must handle higher doppler offsets due to its higher speed, [...] While this is true for any non-geo-stationary satellite, it may not be true for the project the initial poster talked about. As I remember it, he said that the mission would be in an earth-following orbit, ie: in the same orbit as the earth around the sun, but trailing it by some distance. Given that the distance in GPS terms is vast and furthermore that the GPS orbits have a pretty steep angle relative to the earths orbital path, I would expect the doppler offsets to be much smaller than here on earth. Obviously, getting a position fix will suck with the worst DOP seen to date, but a frequency fix should not be out of the question. Obviously, the situation on the way to the final orbit is entirely different, and there I would expect doppler to be totally out of the lower end of the window. Remember to figure out the relevant relativistic corrections. Poul-Henning -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ 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] Checking the Frequency of a Rubidium Oscillator
GEEZ, After all this discussion, it sounds like he should consider 2 Cs space devices, one main and a secondary. BillWB6BNQ Poul-Henning Kamp wrote: In message [EMAIL PROTECTED], Magnus Danielson writes: Once doppler bin and phase has been achieved for each PRN, [...] Just a footnote to say that as soon as you start receiving ephemerides from the first sat, the search-space can be significantly reduced if you care to do the, rather longhaired, trignometric math. A sat based receiver must handle higher doppler offsets due to its higher speed, [...] While this is true for any non-geo-stationary satellite, it may not be true for the project the initial poster talked about. As I remember it, he said that the mission would be in an earth-following orbit, ie: in the same orbit as the earth around the sun, but trailing it by some distance. Given that the distance in GPS terms is vast and furthermore that the GPS orbits have a pretty steep angle relative to the earths orbital path, I would expect the doppler offsets to be much smaller than here on earth. Obviously, getting a position fix will suck with the worst DOP seen to date, but a frequency fix should not be out of the question. Obviously, the situation on the way to the final orbit is entirely different, and there I would expect doppler to be totally out of the lower end of the window. Remember to figure out the relevant relativistic corrections. Poul-Henning -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. ___ 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] Checking the Frequency of a Rubidium Oscillator
WB6BNQ skrev: GEEZ, After all this discussion, it sounds like he should consider 2 Cs space devices, one main and a secondary. Actually, I would pick rubidium sources unless extreme stability and offset is needed. The longer lifetime and less weight compared to Cs devices would be a better fit. Another option would be to use an OCXO and tune it over the telemetry channel which may prove sufficient if telemetry is timely spaced such that worst case drifts can be compensated out. The benefit in weight of such a solution is even greater. Weight and power budget is much more limiting factor than any of the labs we guys run. 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] Checking the Frequency of a Rubidium Oscillator
Poul-Henning Kamp skrev: In message [EMAIL PROTECTED], Magnus Danielson writes: Once doppler bin and phase has been achieved for each PRN, [...] Just a footnote to say that as soon as you start receiving ephemerides from the first sat, the search-space can be significantly reduced if you care to do the, rather longhaired, trignometric math. True, but breaking into the code phase for each sat is nowdays fairly quick, and after setting up the receive channels the rest is much more parallelized. It is simply just quicker to do the code phase break in and start tracking than receiving the ephemerides data from the first sat. The time it takes for a calender to be received is fairly long. A sat based receiver must handle higher doppler offsets due to its higher speed, [...] While this is true for any non-geo-stationary satellite, it may not be true for the project the initial poster talked about. As I remember it, he said that the mission would be in an earth-following orbit, ie: in the same orbit as the earth around the sun, but trailing it by some distance. That was never clear in my mind. Ah well, if so then that part would not need any specific modifications, not that they are particular hard. However, which ever orbit we are discussing, the doppler aspect needs to be studied. Given that the distance in GPS terms is vast and furthermore that the GPS orbits have a pretty steep angle relative to the earths orbital path, I would expect the doppler offsets to be much smaller than here on earth. Another aspect to remember is that there is usually a earth bound assumption used to bootstrap the position calculation. This would naturally need to be adapted. Fortunately it can be adapted and tested very easily if needed. Obviously, getting a position fix will suck with the worst DOP seen to date, but a frequency fix should not be out of the question. Obviously, the situation on the way to the final orbit is entirely different, and there I would expect doppler to be totally out of the lower end of the window. Actually, you can expect both ends of the doppler spectrum. As long as you are below the GPS sats, you will also see high positive dopplers as you goes towards them and negative from those you are leaving behind. As you go past them, all will show up on the negative side. However, those closest to you will not be looking at you any more due to directivity of the antenna array. Remember to figure out the relevant relativistic corrections. There are several relativistic corrections that needs to be considered. Also, while the sat is in transit to its final orbit one can expect these to be with a higher dynamic than a circular orbit. An elliptic orbit would always need orbit-based relativistic correction for that extra correctness. The intended orbit and transit-orbit is certainly of great importance for a number of key processing requirements. It is however not extremely hard to handle it. 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.
