Re: [time-nuts] 1PPS accuracy of commercial GPS receivers
Hi Ulrich, > mind. On the other hand: If a good timing receiver has ended his "site > survey" it's position messages stay constant, even if you move it around. > Obviously that is not what you expect from a navigation system. Keep > navigation and timing receivers clear apart from each other. They are as > different as horse and zebra. We had this discussion a few years ago here! You are still wrong. The best timing receivers available are the geodetic quality receivers that have an external frequency input. Sometimes they are slightly modified versions of the standard geodetic receivers, but they remain top class navigation receivers never-the-less. Tell me a modern receiver used by a national time-lab for time-transfer to other labs that is a bad navigation receiver! -- 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] 1PPS accuracy of commercial GPS receivers
Hal Murray skrev: 2D positioning requires at least 3 sats for resolving Lat, Long, T (really X, Y, Z and T which a fixed relationship between X, Y and Z so given two the third will be given, as the heigth is assumed). This has been discussed before, but I still don't really understand it. I assume they take the data and solve what they can. The answer will be a line in X,Y, Z and T. Do they just pick the point on Z=0? How much timing error does that turn into? OK, X, Y and Z is not arbitrary axes, they have origo at the earth mass center, X sticks out through 0 E 0 N (0 meridian at equator), Y sticks out through 90 E 0 N and Z sticks out through the north pole. T is in GPS time (GPS week, Z-count, data bit in frame, C/A cycle in bit, C/A phase and carrier phase). The sats position in X_i, Y_i, Z_i and T_i is also known in this coordinate system as transmitted and calculated. The actual pseudo-range to sat i is p_i = sqrt((X-X_i)^2 + (Y-Y_i)^2 + (Z-Z_i)^2)) but this pseudo-range is skewed by c(T-T_i) where c is the speed of light. Additional time-skew components is found from ionosphere and troposphere among others. The receivers time T needs to be in the neighborhood of correct, but as soon as the first sat is being tracked, just taking the time of that sat brings it within 100 ms at all times and the first 4-sat solution will remove the major part of that. That is, if I move up 100 meters along that line, how much does T change? Notice how the above equations is per sat, but speed of light is the conversion factor you need and it is "line of sigth" (not entierly true as the signal goes through dispersive athmosphere) to each sat which decides the time-skew. I assume the answer is "not much", but I don't have a good feel for the numbers. Is it lost in the other sources of noise? Or at least not big relative to them? 1 ns is 30 cm. 10 ns is 3 m. 100 ns is 30 m. 333 ns is 100 m. However, your errors goes in different directions depending on sat, so this is why a good constellation helps to accuratly correct errors in all directions. At 1ft per ns, 100 meters is 300 ns. But lots of places on Earth are much higher elevation than 100 meters. (I just used 100 meters as an example.) But that's the number for a satellite directly overhead. The geometry fudges things. If the satellites are low, the change in time will be close to zero. What's the average angle of a satellite? (or ones used in a GPS solution when you can only get 3 of them?) One rarely speaks of average angle, and it is highly dependent on position on earth, so it is kind of not interesting. There is another layer of fudging involved in the how-many-satellites discussion. Sometimes the geometry is degenerate. If the satellites are in (or close) to a line, you don't get a position offset to the side of the projection of that line on the surface of the Earth. You can rule out those which gives too high deviation in T for the approximated X, Y, Z and T and cancel those contributions out before making another calculation. 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] 1PPS accuracy of commercial GPS receivers
> 2D positioning requires at least 3 sats for resolving Lat, Long, T > (really X, Y, Z and T which a fixed relationship between X, Y and Z so > given two the third will be given, as the heigth is assumed). This has been discussed before, but I still don't really understand it. I assume they take the data and solve what they can. The answer will be a line in X,Y, Z and T. Do they just pick the point on Z=0? How much timing error does that turn into? That is, if I move up 100 meters along that line, how much does T change? I assume the answer is "not much", but I don't have a good feel for the numbers. Is it lost in the other sources of noise? Or at least not big relative to them? At 1ft per ns, 100 meters is 300 ns. But lots of places on Earth are much higher elevation than 100 meters. (I just used 100 meters as an example.) But that's the number for a satellite directly overhead. The geometry fudges things. If the satellites are low, the change in time will be close to zero. What's the average angle of a satellite? (or ones used in a GPS solution when you can only get 3 of them?) There is another layer of fudging involved in the how-many-satellites discussion. Sometimes the geometry is degenerate. If the satellites are in (or close) to a line, you don't get a position offset to the side of the projection of that line on the surface of the Earth. -- 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] 1PPS accuracy of commercial GPS receivers
Poul-Henning Kamp skrev: In message , "Ulrich Bangert" writes: The receiver can use this redundant information in two ways: a) to improve the solutions for the position or b) to improve the solutions for the time but not both at the same time. That's just bogus. First of all, you need four sats for a complete solution: X+Y+Z+T, second the more sats you add after that, provided they do contribute gainfully, will improve both the position and time solutions, for the very simple reasons that they are one and the same solution. Once you go to position-hold mode, all the sats contribute to is the time solution, and in principle one sat is enough to get a solution, because, as the name implies, you stop treating X+Y+Z as variables. I totally agree. This is well covered in the books that go into the deep details of GPS navigation. 3D positioning requires at least 4 sats for resolving X, Y, Z and T coordinates, which translates to Lat, Long, heigth and T. 2D positioning requires at least 3 sats for resolving Lat, Long, T (really X, Y, Z and T which a fixed relationship between X, Y and Z so given two the third will be given, as the heigth is assumed). T positioning requires at least 1 sat for resolving T. Also, you can use the redundant information to identify false-tickers and remove them before final position is calculated, this is done by making a preliminary calculation and then compare the calculated time with the pseudo-range value for each and let those being significantly off be removed. The pseudo-range system make the time of the receiver a critical variable to establish. The stability of the receives time will therefore also be a critical parameter in order to establish good quality positional values. High short-term stability oscillators is being deployed even in simple L1 receivers to reduce LO phase noise and its effect on code and carrier pseudo-range measures. All pseudo-ranges will depend on the actual distatance, but also on the time of the sat and the receiver. The sat time is being corrected into propper GPS time by additional correction values, such that remaining timing errors is to be found in the receiver. Phase offsets of the signal from the sats center of mass is also given, since it is the center of mass which the positional values of the sat indicate. The receiver uses the previous time estimates to correct its own clock and advanced receivers use Kalman filtering for optimum clock estimation. Each positional solution also feeds the clock algorithm so that the clock is steered towards a zero offest. The pseudo-ranges is samples with a sample clock, which has known deviation from the local clock. In the end, I can't see how this type of receiver would fit the claim that one has to optimize for position or time. It does not make sense to me, as I know the system. What is true is that not all receivers has the algorithms to provide optimum time solutions in the fixed geografical position (it's not fixed in time position). The same receivers where one has the time option performs the same on normal positioning. In fixed position the solution part of the receiver must know that the position is fixed in order to resolve all pseudo-ranges into time-offset only. So, 3D positioning does not give the same time-stability as a fixed position does, that is true, but it is not the same as being claimed. 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] 1PPS accuracy of commercial GPS receivers
In message , "Ulrich Bangert" writes: >The receiver can use this redundant information in two ways: >a) to improve the solutions for the position >or >b) to improve the solutions for the time > >but not both at the same time. That's just bogus. First of all, you need four sats for a complete solution: X+Y+Z+T, second the more sats you add after that, provided they do contribute gainfully, will improve both the position and time solutions, for the very simple reasons that they are one and the same solution. Once you go to position-hold mode, all the sats contribute to is the time solution, and in principle one sat is enough to get a solution, because, as the name implies, you stop treating X+Y+Z as variables. -- 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] 1PPS accuracy of commercial GPS receivers
Phil, as soon as a receiver sees more than 3 sats there is redundant information available. The receiver can use this redundant information in two ways: a) to improve the solutions for the position or b) to improve the solutions for the time but not both at the same time. That implies that there are good position receivers available and good timing receivers but not any that is suited well for both purposes (which matches the market siuation pretty well). On a not-timing receiver the PPS is more or less a useless gimick which may have accuracies as bad as 1 us or even more. Dont't worry: These receivers are not made with a precise PPS in mind. On the other hand: If a good timing receiver has ended his "site survey" it's position messages stay constant, even if you move it around. Obviously that is not what you expect from a navigation system. Keep navigation and timing receivers clear apart from each other. They are as different as horse and zebra. Best regards Ulrich Bangert > -Ursprungliche Nachricht- > Von: time-nuts-boun...@febo.com > [mailto:time-nuts-boun...@febo.com] Im Auftrag von Philip Pemberton > Gesendet: Mittwoch, 13. Mai 2009 23:06 > An: Discussion of precise time and frequency measurement > Betreff: [time-nuts] 1PPS accuracy of commercial GPS receivers > > > Just out of idle curiosity, is there any significant > difference in 1PPS > accuracy between different GPS modules? > > I've got a pair of Trimble SVeeSix CM3 boards (firmware 4.13 > if memory serves, > have to be reflashed to change the comm protocol, which can > be either TSIP or > NMEA) and an Axiom Sandpiper (SiRFStar II, RAM only, no > onboard NVM, SiRF > Binary or NMEA switchable on-the-fly) which are specified at > 1us and 40ns > accuracy respectively. As a comparison point, I've also been > looking at the > Fastrax iTrax321 (IT321) which is a solder-down "micro-GPS" > module based on > the "20-channel" SiRFStar III and is -- like the Trimble -- > specced at 1us > accuracy. This is one of the newer SiRFStar III based design. > > Am I missing something blindingly obvious here, or is there > really that much > spread in 1PPS accuracy on commercial receiver boards? > > Is 1us jitter really that good for a GPS module? > > Thanks, > -- > Phil. > li...@philpem.me.uk > http://www.philpem.me.uk/ > > ___ > 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] 1PPS accuracy of commercial GPS receivers
Philip Pemberton skrev: Just out of idle curiosity, is there any significant difference in 1PPS accuracy between different GPS modules? I've got a pair of Trimble SVeeSix CM3 boards (firmware 4.13 if memory serves, have to be reflashed to change the comm protocol, which can be either TSIP or NMEA) and an Axiom Sandpiper (SiRFStar II, RAM only, no onboard NVM, SiRF Binary or NMEA switchable on-the-fly) which are specified at 1us and 40ns accuracy respectively. As a comparison point, I've also been looking at the Fastrax iTrax321 (IT321) which is a solder-down "micro-GPS" module based on the "20-channel" SiRFStar III and is -- like the Trimble -- specced at 1us accuracy. This is one of the newer SiRFStar III based design. Am I missing something blindingly obvious here, or is there really that much spread in 1PPS accuracy on commercial receiver boards? Is 1us jitter really that good for a GPS module? The 1 us figure is a historic figure relating to a worst case degrades GPS constellation situation when the 24 sat constellation has degraded significantly etc. This number comes out of ICD-200. A more commonly referred figure is 340 ns which is what the GPS constellation with SA enabled. In the SA-disabled world seeing lower numbers as 60-40 ns is not unreasnoble. Old 6 or 8 channel receivers was adequate for the older constellation situation, but seing 10-12 sats in todays world is not unreasnoble and naturally will the bias effects and other noise processes be lower. In addition has receiver technology advanced to better suppress various imperfections such as multi-path, weak signals and quick locking. In the other end, awareness of how the PPS is being used have improved how the PPS signal is generated and producing "sawtooth corrections" enables lower time quantization noise. Old receivers can perform better in todays world, so we could modernize the specs by comparing them with newer boards in todays environment. I am not sure if you really got a real answer, but hopefully may some of the difference become explainable to some degree. 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.
[time-nuts] 1PPS accuracy of commercial GPS receivers
Just out of idle curiosity, is there any significant difference in 1PPS accuracy between different GPS modules? I've got a pair of Trimble SVeeSix CM3 boards (firmware 4.13 if memory serves, have to be reflashed to change the comm protocol, which can be either TSIP or NMEA) and an Axiom Sandpiper (SiRFStar II, RAM only, no onboard NVM, SiRF Binary or NMEA switchable on-the-fly) which are specified at 1us and 40ns accuracy respectively. As a comparison point, I've also been looking at the Fastrax iTrax321 (IT321) which is a solder-down "micro-GPS" module based on the "20-channel" SiRFStar III and is -- like the Trimble -- specced at 1us accuracy. This is one of the newer SiRFStar III based design. Am I missing something blindingly obvious here, or is there really that much spread in 1PPS accuracy on commercial receiver boards? Is 1us jitter really that good for a GPS module? Thanks, -- Phil. li...@philpem.me.uk http://www.philpem.me.uk/ ___ 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.
