Re: [time-nuts] MH370 Doppler
Joe Leikhim jleikhim@... writes: I raised this on the Duncan Steel website and was pretty much blown off. Oh there is a nice stable OCXO aboard etc. Well DUHH yes there is an OCXO aboard and if it is good to -20 to +75C, or just -20 to +60C and there is a huge fire raging around it for an hour, and then perhaps later the plane decompresses at 32,000 feet and ice forms inside the aircraft that all has to be a factor to consider. Funny, I also raised this issue on the Duncan Steel website and was also blown off. I think you are dead on about the response of an OCXO to temperature shocks. I designed a satcom frequency converter for 14 GHz that was phase locked to an internal OCXO. When we were trying to make phase noise measurements at 100 and 10 Hz offsets, we had to allow the units to warm up and sit in still air for 20 minutes so the frequency would stabilize enough so that the carrier would remain on the spectrum analyzer screen when set at 1 Hz per division. I also recall that when the equipment lid was opened, you could blow on the outside of the OCXO case and watch in real time the carrier shift side to side on the spectrum analyzer screen. I think people are seeing the Oven-Controlled phrase and assuming this means they are immune to temperature effects. In reality, the oven is what gets a crystal from the 1E-5 stability range down to 1E-7 or 1E- 8, but at 1600 MHz that is still 16 to 160 Hz. Unless the OCXO is double-ovened, my experience was that the OCXO specified stability is really achieved in steady-state conditions, when all the internal parts of the OCXO are at their nominal temperature gradients. During temperature transients, especially fast changes, the heater circuit may respond with overshoot, undershoot, etc. Plus, the case-to-crystal temperature gradients are all different than the conditions in which it is calibrated. Similarly, during a loss of power and reset, all bets are off, and our OCXOs would way overshoot as the oven heater circuits suddenly kicked in full blast. The number of minutes to gradually re-converge on stable operation may depend on many factors such as OCXO size and thermal inertia. I think given all the unknowns, all kinds of situations like power outages and sudden temperature changes could have occurred right before any of the hourly pings. Comparisons to BFOs from other flights that were completely normal level flight with nearly zero temperature changes may not apply. I can't find any schematics of what equipment was inside the MH370 aero classic terminal; hard to guess if the OCXO is a compact, low cost OCXO or some super-performing NIST marvel. In general, all the discussion of BFO talking so confidently about 7Hz frequency variations at 1600 MHz, and the inherent assumption of 4 parts per billion frequency stability under possible temperature changes just feels unrealistic to me. ___ 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] MH370 Doppler (Magnus Danielson)
In message 53f8060b.7020...@leikhim.com, Joe Leikhim writes: I applaud those applying their rather extensive math skills at this problem, but from the outside, it appears to me that the problem is so very complex (error prone) and so many assumptions are being applied, that folks time would be better spent scouring the beaches for flotsam from the aircraft to wash up. It's certainly a big problem that we're in tan(almost_ninety) territory. As for debris on beaches, not so much. There's a gyre in the Indian ocean which is likely to trap a lot of it. It still bothers me that with all the space, ocean and ground based radar, sonar and imaging sensors, there is so little trace of the aircraft's travels that night. The earth is pretty damn vast and not a lot of people loiter in the indian ocean on the off-chance that a plane is going to ditch. A very big uncertainty in relation to electronic tracking is that it is pretty trivial to spoof another plane, and it has previously been done by two planes carrying out a rendez-vouz and swapping squawk and other identifiers over international water. As far as I can tell, that would not work with the Inmarsat transponder without physically swapping the radio modules. Given the hour-long off period that could have happened, but it would have been so much easier to just enable the squawk. If they did a rendez-vous with a co-conspirator plane, took over their squawk code, turned off livery-lights and followed the prefiled flightplan, MH370 could fly unchallenged by all airforces all the way to the Black Sea along the norther route. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 p...@freebsd.org | 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] MH370 Doppler (Magnus Danielson)
Joe, I did find it here: http://www.dca.gov.my/mainpage/MH370%20Data%20Communication%20Logs.pdf There is also some Inmarsat presentation giving a little more detail. At the same time, the data gathered is from a system not designed for navigation and positioning purposes. It gives a rough hint. The reported resolution of the time and doppler and the checking time all gives that indication. The ring of last communication is from the time observation, and the reduction of that ring to likely sectors is due to doppler and time observations in previous observations trailing back to known position. Not very good hints. It's a system that only after the fact became a positioning system. Doppler positioning is indeed possible, but that requires more continuous observations than once every hour. I think that the Inmarsat folks most probably did about as much as you can with the data at hand. The BTO jumps in 20 micro steps. That is 6 km of satellite-plane distance, which through geometry becomes wider as the angle from the satellite-earth-line increases. Good enough hint for a search party, but the lack of frequent pings only shows in what neighborhood to search. Making some assumptions on continuous flight path helps. The BTO jumps between different types of channels, so only R-channel is being used for estimates. This positioning problem isn't very complex, but is a good exercise as it is similar to the pseudo-range measurements and positioning from GPS. The doppler and time observations with some basic geometry gives most of it out. The systematic doppler shift needs to be canceled but there is base measurements included that helps with that. Cheers, Magnus On 08/23/2014 05:10 AM, Joe Leikhim wrote: You can probably find links to all of the data from the Duncan Steel Blog. You might start by looking at the questions the group has posed in an open letter to the ATSB and Inmarsat. Frankly, the data Inmarsat released appear to be rather scant and some believe to be doctored not RAW. I applaud those applying their rather extensive math skills at this problem, but from the outside, it appears to me that the problem is so very complex (error prone) and so many assumptions are being applied, that folks time would be better spent scouring the beaches for flotsam from the aircraft to wash up. But I could be wrong, maybe some time-nuttery will set them straight to an answer! It still bothers me that with all the space, ocean and ground based radar, sonar and imaging sensors, there is so little trace of the aircraft's travels that night. Somebody must have seen something. In fact some observers have reported seeing stuff, fire in the sky, low flying aircraft and even a fire bottle washed up on the beach in the Maldives, but so much stock is being put in the southward arc, that nobody is listening. Does anyone has the set of timing and doppler measurements, and position of the observing satellite? 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] MH370 Doppler
Time-error would be rings for sure. Doppler errors could also behave with these rings, but there is a much more complex scenario of ground speed and angle of observation. As the transponder passes under the satellite-earth line there is no observeable doppler, just like a train passing by has no doppler just as it passes by. Similarly as you fly in a ring pattern centered around the sat-earth line there is no doppler, it takes that you travel orthogonally to these rings for maximum doppler, it depends on the ground speed and geometry. Thus doppler gives by itself fuggy info. It's only when you combine doppler info and timing info that you can start making some reasonable observations about ground speed and to some degree rule out parts of a later ring as it would have required too high air speed to be that airplane. If sufficient distance in time, you can draw a expanding path of likely true-track and get a rough idea of what area it could be. For this to give reasonable result, initial vector would need to use other observations for approximate startingpoint. Does anyone has the set of timing and doppler measurements, and position of the observing satellite? Cheers, Magnus On 08/21/2014 02:32 AM, Chris Albertson wrote: Yes, that is what they are doing. A given Doppler shift corresponds to a certain ring on the Earth's surface. Each Hertz of Soppler shift corresponds to a certain number of miles on the radius of the ring. At 1.6GHz one part per billion is 1.6Hz.175Hz of shift gives something like a 2,400 mile radius ring. On Wed, Aug 20, 2014 at 8:47 AM, Joe Leikhim jleik...@leikhim.com wrote: The L Band uplink was reported to be transmitting at around 1.6435/ghz. / Assuming, we actually knew what the tolerance of the OCXO (If it is an OCXO) was under the environment of the mishap, and assuming it /was/ 10ppm for example. The error would be (1,650 X1,000,000) * 10ppm or _16,500 hz_. I think we can discount the error being that large, but could still rationalize it being a significant portion of the reported BFO value./ / Also the ground track is unknown, they are attempting to reconstruct the ground track from the BFO (Burst Frequency Offset Doppler) and from the BTO (timing pings) the BTO supposedly offers range information, hence the concentric rings corresponding to pings._ _ _Chris Alb__ertson wrote:_ The total Doppler in this case is on the order of 100 Hz. The tiny frequency shifts of an out of spec OCXO is just to small to measure. The data says at UTC 18:30 the shift was in the mid range and was about 175Hz. Assume the OCXO drifts 10 parts per million. That is a lot for an OCXO. But maybe the effect is only about 50 feet on the ground. The OCXO error of even 1E-5 is just not very important as it does not move the aircrafts ground track enough to matter. -- Joe Leikhim Leikhim and Associates Communications Consultants Oviedo, Florida jleik...@leikhim.com 407-982-0446 WWW.LEIKHIM.COM ___ 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] MH370 Doppler (Magnus Danielson)
You can probably find links to all of the data from the Duncan Steel Blog. You might start by looking at the questions the group has posed in an open letter to the ATSB and Inmarsat. Frankly, the data Inmarsat released appear to be rather scant and some believe to be doctored not RAW. I applaud those applying their rather extensive math skills at this problem, but from the outside, it appears to me that the problem is so very complex (error prone) and so many assumptions are being applied, that folks time would be better spent scouring the beaches for flotsam from the aircraft to wash up. But I could be wrong, maybe some time-nuttery will set them straight to an answer! It still bothers me that with all the space, ocean and ground based radar, sonar and imaging sensors, there is so little trace of the aircraft's travels that night. Somebody must have seen something. In fact some observers have reported seeing stuff, fire in the sky, low flying aircraft and even a fire bottle washed up on the beach in the Maldives, but so much stock is being put in the southward arc, that nobody is listening. Does anyone has the set of timing and doppler measurements, and position of the observing satellite? Cheers, Magnus -- Joe Leikhim Leikhim and Associates Communications Consultants Oviedo, Florida jleik...@leikhim.com 407-982-0446 WWW.LEIKHIM.COM ___ 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] MH370 Doppler
The L Band uplink was reported to be transmitting at around 1.6435/ghz. / Assuming, we actually knew what the tolerance of the OCXO (If it is an OCXO) was under the environment of the mishap, and assuming it /was/ 10ppm for example. The error would be (1,650 X1,000,000) * 10ppm or _16,500 hz_. I think we can discount the error being that large, but could still rationalize it being a significant portion of the reported BFO value./ / Also the ground track is unknown, they are attempting to reconstruct the ground track from the BFO (Burst Frequency Offset Doppler) and from the BTO (timing pings) the BTO supposedly offers range information, hence the concentric rings corresponding to pings._ _ _Chris Alb__ertson wrote:_ The total Doppler in this case is on the order of 100 Hz. The tiny frequency shifts of an out of spec OCXO is just to small to measure. The data says at UTC 18:30 the shift was in the mid range and was about 175Hz. Assume the OCXO drifts 10 parts per million. That is a lot for an OCXO. But maybe the effect is only about 50 feet on the ground. The OCXO error of even 1E-5 is just not very important as it does not move the aircrafts ground track enough to matter. -- Joe Leikhim Leikhim and Associates Communications Consultants Oviedo, Florida jleik...@leikhim.com 407-982-0446 WWW.LEIKHIM.COM ___ 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] MH370 Doppler
Yes, that is what they are doing. A given Doppler shift corresponds to a certain ring on the Earth's surface. Each Hertz of Soppler shift corresponds to a certain number of miles on the radius of the ring. At 1.6GHz one part per billion is 1.6Hz.175Hz of shift gives something like a 2,400 mile radius ring. On Wed, Aug 20, 2014 at 8:47 AM, Joe Leikhim jleik...@leikhim.com wrote: The L Band uplink was reported to be transmitting at around 1.6435/ghz. / Assuming, we actually knew what the tolerance of the OCXO (If it is an OCXO) was under the environment of the mishap, and assuming it /was/ 10ppm for example. The error would be (1,650 X1,000,000) * 10ppm or _16,500 hz_. I think we can discount the error being that large, but could still rationalize it being a significant portion of the reported BFO value./ / Also the ground track is unknown, they are attempting to reconstruct the ground track from the BFO (Burst Frequency Offset Doppler) and from the BTO (timing pings) the BTO supposedly offers range information, hence the concentric rings corresponding to pings._ _ _Chris Alb__ertson wrote:_ The total Doppler in this case is on the order of 100 Hz. The tiny frequency shifts of an out of spec OCXO is just to small to measure. The data says at UTC 18:30 the shift was in the mid range and was about 175Hz. Assume the OCXO drifts 10 parts per million. That is a lot for an OCXO. But maybe the effect is only about 50 feet on the ground. The OCXO error of even 1E-5 is just not very important as it does not move the aircrafts ground track enough to matter. -- Joe Leikhim Leikhim and Associates Communications Consultants Oviedo, Florida jleik...@leikhim.com 407-982-0446 WWW.LEIKHIM.COM ___ 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] MH370 Doppler
My understanding is that the BFO and BTO values are self reported from the SDU (Terminal in the aircraft) and they represent adjustments made by the SDU. If so the BFO value would be the AFC adjustment relative to the OCXO onboard. My contention is that if the investigators are assuming the OCXO is 2Hz high and reporting an +88Hz offset as 86Hz Doppler, what if in fact the OCXO is 10Hz high? Then the doppler is 78Hz and that means the velocity and location at each of the pings is way off. It would also be good to know if the terminal has any sort of holdover battery to keep it running. So far I have heard only that the IRU has battery. Even so, the terminal has possibly three power sources, left and right and APU busses. Unlikely as it seems, what if they ditched successfully at sea and the APU ran for hours? From: David I. Emeryd...@dieconsulting.com IIRC the plane is expected to adjust its burst uplink frequency and timing to come out right at the satellite receive antenna... thus compensating for the uplink Doppler at L band and the time delay too. But I do remember that the ground supplies feedback on the control channel as to how much the plane is off so it can adjust... Guess it might be time to dig out the docs again. -- Joe Leikhim Leikhim and Associates Communications Consultants Oviedo, Florida jleik...@leikhim.com 407-982-0446 WWW.LEIKHIM.COM ___ 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] MH370 Doppler
As I understand it, Doppler will give you the magnitude of the velocity vector for an aircraft with respect to the satellite, but it won't give you the actual direction of the aircraft. Why does the stability of the oscillators matter if you can't determine the direction? Is there another satellite involved? Can you learn something if you assume a velocity for the aircraft? In which case the error in the assumed velocity would swamp the oscillator error, no? Bill Hawkins -Original Message- From: Joe Leikhim Sent: Monday, August 18, 2014 11:53 AM Is anyone paying attention to all the chatter about the lost aircraft MH370, Inmarsat's supposed flight tracks based on 6 or 7 pings (1 per hour), the Doppler shift (BFO) and transaction timing (BTO) etc?? Basically from my perspective they are putting too much stock into the Doppler which relies in part upon the stability of the satellite terminal in the 777 aircraft. My question is how stable an oscillator (reported OCXO - not confirmed) would be under the extremes of either or both a cabin fire or decompression event. There is a website (Duncan Steel Blog) where some math brains are trying to sort out the raw data provided by Inmarsat. They have made assumptions about the stability of the local oscillator in the satellite, but I think the aircraft satellite terminal's master oscillator is a variable they have pushed aside. ___ 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] MH370 Doppler
You CAN determine the ground track if you assume the altitude above sea level is constant and the aircraft's speed is also constant. But you are correct that Doppler alone would not be enough. The question I have to people here is: How does error in the dopler translate to error in the ground track. In other words what is the function that maps oscillator stability to distance on the ground. On Tue, Aug 19, 2014 at 3:25 PM, Bill Hawkins b...@iaxs.net wrote: As I understand it, Doppler will give you the magnitude of the velocity vector for an aircraft with respect to the satellite, but it won't give you the actual direction of the aircraft. Why does the stability of the oscillators matter if you can't determine the direction? Is there another satellite involved? Can you learn something if you assume a velocity for the aircraft? In which case the error in the assumed velocity would swamp the oscillator error, no? Bill Hawkins -Original Message- From: Joe Leikhim Sent: Monday, August 18, 2014 11:53 AM Is anyone paying attention to all the chatter about the lost aircraft MH370, Inmarsat's supposed flight tracks based on 6 or 7 pings (1 per hour), the Doppler shift (BFO) and transaction timing (BTO) etc?? Basically from my perspective they are putting too much stock into the Doppler which relies in part upon the stability of the satellite terminal in the 777 aircraft. My question is how stable an oscillator (reported OCXO - not confirmed) would be under the extremes of either or both a cabin fire or decompression event. There is a website (Duncan Steel Blog) where some math brains are trying to sort out the raw data provided by Inmarsat. They have made assumptions about the stability of the local oscillator in the satellite, but I think the aircraft satellite terminal's master oscillator is a variable they have pushed aside. ___ 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] MH370 Doppler
On Tue, Aug 19, 2014 at 12:08:24PM -0400, Joe Leikhim wrote: My understanding is that the BFO and BTO values are self reported from the SDU (Terminal in the aircraft) and they represent adjustments made by the SDU. If so the BFO value would be the AFC adjustment relative to the OCXO onboard. My contention is that if the investigators are assuming the OCXO is 2Hz high and reporting an +88Hz offset as 86Hz Doppler, what if in fact the OCXO is 10Hz high? Then the doppler is 78Hz and that means the velocity and location at each of the pings is way off. Clearly they have a history of the MH370 Aero Clasic terminal measured burst frequency at the ground earth station and the BFO value the SDU reports it used WHEN the aircraft was on the ground before it took off and WHEN it was being tracked by radar/mode-s/ads-b and was in a known position going at a known velocity on a known heading.This should presumably allow determination of the baseline OCXO long term error, and some indication of its short term drift as well. Whether that particular SDU attempts to use any form of EFC of its OCXO based on measured satellite L band downlink frequency error corrected for doppler or not I do not know. It is quite possible that any correction for OCXO error is just a value factored into computing the BFO to use and not used for actually correcting the standard with a EFC DAC. If that is true then the drift should be presumably be pretty typical of the class of OCXO used in the SDU which I suspect should be fairly small once it warms up - over a 6-8 hour period after warmup. And there may be some history of that particular terminal from previous flights to validate this. Of course if environment significantly changes the drift performance of that particular OCXO it is possible that temperature, or pressure or power conditions were so different on the fatal flight that the drift might be larger and unknown in character... not sure. It is an error to consider of course. Not clear to me how carefully it has been or what possible factors have been considered. But surely the folks doing the analysis know about these issues. -- Dave Emery N1PRE/AE, d...@dieconsulting.com DIE Consulting, Weston, Mass 02493 An empty zombie mind with a forlorn barely readable weatherbeaten 'For Rent' sign still vainly flapping outside on the weed encrusted pole - in celebration of what could have been, but wasn't and is not to be now either. ___ 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] MH370 Doppler
I raised this on the Duncan Steel website and was pretty much blown off. Oh there is a nice stable OCXO aboard etc. Well DUHH yes there is an OCXO aboard and if it is good to -20 to +75C, or just -20 to +60C and there is a huge fire raging around it for an hour, and then perhaps later the plane decompresses at 32,000 feet and ice forms inside the aircraft that all has to be a factor to consider. The ATSB (Australian NTSB) report is mute on this as well. Plus the Doppler reports are only every hour or so, so there isn't much of a trendline. But some interesting excursions. I was surprised no time-nuts have ventured over to that blog. David I. Emery wrote: Of course if environment significantly changes the drift performance of that particular OCXO it is possible that temperature, or pressure or power conditions were so different on the fatal flight that the drift might be larger and unknown in character... not sure. It is an error to consider of course. Not clear to me how carefully it has been or what possible factors have been considered. But surely the folks doing the analysis know about these issues. -- Joe Leikhim Leikhim and Associates Communications Consultants Oviedo, Florida jleik...@leikhim.com 407-982-0446 WWW.LEIKHIM.COM ___ 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] MH370 Doppler
The total Doppler in this case is on the order of 100 Hz. The tiny frequency shifts of an out of spec OCXO is just to small to measure. The data says at UTC 18:30 the shift was in the mid range and was about 175Hz. Assume the OCXO drifts 10 parts per million. That is a lot for an OCXO. But maybe the effect is only about 50 feet on the ground. The OCXO error of even 1E-5 is just not very important as it does not move the aircrafts ground track enough to matter. On Tue, Aug 19, 2014 at 6:26 PM, Joe Leikhim jleik...@leikhim.com wrote: I raised this on the Duncan Steel website and was pretty much blown off. Oh there is a nice stable OCXO aboard etc. Well DUHH yes there is an OCXO aboard and if it is good to -20 to +75C, or just -20 to +60C and there is a huge fire raging around it for an hour, and then perhaps later the plane decompresses at 32,000 feet and ice forms inside the aircraft that all has to be a factor to consider. The ATSB (Australian NTSB) report is mute on this as well. Plus the Doppler reports are only every hour or so, so there isn't much of a trendline. But some interesting excursions. I was surprised no time-nuts have ventured over to that blog. David I. Emery wrote: Of course if environment significantly changes the drift performance of that particular OCXO it is possible that temperature, or pressure or power conditions were so different on the fatal flight that the drift might be larger and unknown in character... not sure. It is an error to consider of course. Not clear to me how carefully it has been or what possible factors have been considered. But surely the folks doing the analysis know about these issues. -- Joe Leikhim Leikhim and Associates Communications Consultants Oviedo, Florida jleik...@leikhim.com 407-982-0446 WWW.LEIKHIM.COM ___ 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] MH370 Doppler
I thought that Inmarsat terminals had AFC to the sat's down-link. Not to the degree of true phase-lock like DSN has but enough so that the sat's abillity to do doppler correction on the uplink is valid to help with BER, etc... Otherwise the doppler correction would be of no help and not be needed. -Brian, WA1ZMS/4 iPhone On Aug 18, 2014, at 12:53 PM, Joe Leikhim jleik...@leikhim.com wrote: Is anyone paying attention to all the chatter about the lost aircraft MH370, Inmarsat's supposed flight tracks based on 6 or 7 pings (1 per hour), the Doppler shift (BFO) and transaction timing (BTO) etc?? Basically from my perspective they are putting too much stock into the Doppler which relies in part upon the stability of the satellite terminal in the 777 aircraft. My question is how stable an oscillator (reported OCXO - not confirmed) would be under the extremes of either or both a cabin fire or decompression event. There is a website (Duncan Steel Blog) where some math brains are trying to sort out the raw data provided by Inmarsat. They have made assumptions about the stability of the local oscillator in the satellite, but I think the aircraft satellite terminal's master oscillator is a variable they have pushed aside. -- Joe Leikhim Leikhim and Associates Communications Consultants Oviedo, Florida jleik...@leikhim.com 407-982-0446 WWW.LEIKHIM.COM ___ 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] MH370 Doppler
On Mon, Aug 18, 2014 at 04:18:30PM -0400, Brian, WA1ZMS wrote: I thought that Inmarsat terminals had AFC to the sat's down-link. Not to the degree of true phase-lock like DSN has but enough so that the sat's abillity to do doppler correction on the uplink is valid to help with BER, etc... Otherwise the doppler correction would be of no help and not be needed. I beleive most Aero Classic terminals use a fairly good OCXO. Somewhere I may have a limit spec on stability, but those docs are not immediately handy. Normally a demod in the terminal is kept tuned to one of the continuous L band control channels which I believe may be Doppler compensated in the ground uplink transmitter for the 6 Ghz C band uplink Doppler and LO drift on the satellite so it is correctly on frequency as radiated on the L band downlink. This could supply a frequency reference to the terminal that could be used to AFC the terminal frequency standard so it is close to right on. Doing this would require terminal firmware to determine estimated Doppler at the L band control channel downlink frequency from the satellite based on some estimate of the planes position, satellite position and relative velocities. The QPSK DSP modems used at both ends would be easily able to supply estimated frequency offset, both on the ground at ground earth station and in the plane. It is presumably true that this measurement is corrected on the ground end for the Doppler due to movement of the satellite relative to the ground station on the C band downlink relaying the L band uplinks from the plane so it reflects frequency error as seen at the satellite on the L band uplink with the downlink and satellite LO drift terms removed. I presume this is what INMARSAT is reporting, but am not sure. IIRC the plane is expected to adjust its burst uplink frequency and timing to come out right at the satellite receive antenna... thus compensating for the uplink Doppler at L band and the time delay too. But I do remember that the ground supplies feedback on the control channel as to how much the plane is off so it can adjust... Guess it might be time to dig out the docs again. -- Dave Emery N1PRE/AE, d...@dieconsulting.com DIE Consulting, Weston, Mass 02493 An empty zombie mind with a forlorn barely readable weatherbeaten 'For Rent' sign still vainly flapping outside on the weed encrusted pole - in celebration of what could have been, but wasn't and is not to be now either. ___ 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.
