Re: [time-nuts] Quality of timing mode GPS vs survey accuracy
Just to add one datapoint to the discussion, I am presently looking at a professional grade receiver ($20k+++), tracking GPS/GLONASS/GALIELO/BEIDOU, 44 sats total as I type this. It reports standard deviation of the XYZ coordinates of 0.76m, 0.44m and 1.3m respectively. It is not crystal clear how this std is calculated, but I can only guess it is the std off x number of positions calculated on a per-epoch basis. Granted, the antenna I am using is *highly* suspicious, but it seems to track most signals with a decent SNR. The point of this is that even a *great* receiver can only get you so far. Some form of averaging must take place, either in the receiver itself (as is done with survey mode and "position hold") or externally (as is done with PPP). As I understand it. in the former case with a fixed position, the std quoted above will give an indication of the short term noise in the time pulse - which we already know to be horrible, and which the GPSDO is rigged to filter out. Multipath is a bigger issue, since the "moving average" of the receiver can/will skew the position appreciably in a 24 hour period. (And then ofcouse ionosphere and related cans of worms.) Ole On Sat, May 9, 2020 at 1:55 AM jimlux wrote: > On 5/8/20 3:38 PM, Charles Steinmetz wrote: > > Jim wrote: > > > >> I'm attaching a screen shot of 4 receivers, taken at the same time > >> (within seconds,not time-nuts same time) All sitting on the table > >> together. > > > > One might expect closer agreement if all of the receivers were fed from > > one antenna through a splitter, but even then each rx will choose its > > own series of constellations and will switch constellations at different > > times. There will also be phase dispersion among the rx input filters. > > And lots of other small effects. > > > > one might so expect. But I'm not too sure. > > There's >10 meters difference in the positions (in the X coordinate of > the ECEF position alone). > > They're all tracking the same satellites (except b203) > > but the S/N is noticeably different among them. > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. > ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
[time-nuts] Quality of timing mode GPS vs survey accuracy
Be aware the some receivers report altitude in MSL (mean sea level) and others use AGL (height above ground level). Depending upon where you are and the conversion model, there can be over 100 meters difference. --- I >if I take a survey > with several GPS receivers will they all get the same answer? ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Quality of timing mode GPS vs survey accuracy
On 5/8/20 3:38 PM, Charles Steinmetz wrote: Jim wrote: I'm attaching a screen shot of 4 receivers, taken at the same time (within seconds,not time-nuts same time) All sitting on the table together. One might expect closer agreement if all of the receivers were fed from one antenna through a splitter, but even then each rx will choose its own series of constellations and will switch constellations at different times. There will also be phase dispersion among the rx input filters. And lots of other small effects. one might so expect. But I'm not too sure. There's >10 meters difference in the positions (in the X coordinate of the ECEF position alone). They're all tracking the same satellites (except b203) but the S/N is noticeably different among them. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Quality of timing mode GPS vs survey accuracy
Jim wrote: I'm attaching a screen shot of 4 receivers, taken at the same time (within seconds,not time-nuts same time) All sitting on the table together. One might expect closer agreement if all of the receivers were fed from one antenna through a splitter, but even then each rx will choose its own series of constellations and will switch constellations at different times. There will also be phase dispersion among the rx input filters. And lots of other small effects. Best regards, Charles ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Quality of timing mode GPS vs survey accuracy
Hi > On May 8, 2020, at 5:12 PM, Hal Murray wrote: > > > Is there any published data on what happens to the quality of time if the > survey is off by xxx meters? Speed of light plus distance off “correct” plus constellation geometry are what are usually tossed into examples. Turning that into an exact error number for this receiver design at time = x …. never seen it taken that far. You could do it yourself for a hypothetical processing approach. > > Do all the GPS receivers use the same coordinate system? GPS uses a coordinate system, receivers translate this into another system to display the results. Many can be set to a variety of coordinate systems. Unless there are bug in the firmware, when set to the same system they should be talking the same way. Mr Google will give you a long list of hits going into this for each GNSS system. > If I take a survey > with several GPS receivers will they all get the same answer? Nope. If you take a survey with the *same* receiver, there will be some level of “this pass does not agree with that pass”. How big the errors are depends a lot on: 1) what sort of receiver you have (single band / multi band / design era / make / model) 2) what sort of antenna you have (fancy geodetic / simple pole mount / mag mount …) 3) how well you have things set up (sky view ….) 4) how bad your local situation is ( multi-path, jammers, RFI / EMI ….) 5) how long the survey is ( minutes / hours / days / weeks / month ….) 6) what the conditions are (space weather / local weather / GNSS system issues …) 7 )what sort of post processing you do ( antenna corrections …. IGS data sets used ….) (and on and on and on ….) For timing, you may not care about errors dimensioned in mm, but there are people who do. Yes, this is a basic issue with wide open questions :) Bob > > > -- > These are my opinions. I hate spam. > > > > > ___ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe, go to > http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com > and follow the instructions there. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
Re: [time-nuts] Quality of timing mode GPS vs survey accuracy
In message <20200508211209.315c1406...@ip-64-139-1-69.sjc.megapath.net>, Hal Mu rray writes: >Is there any published data on what happens to the quality of time if the >survey is off by xxx meters? I played with that many years ago, using the reported residuals from the OnCore to slowly sneak in on the position which averaged the residuals out. Made no measurable (for me) difference on the timing. http://phk.freebsd.dk/raga/sneak/ -- 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@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
[time-nuts] Quality of timing mode GPS vs survey accuracy
Is there any published data on what happens to the quality of time if the survey is off by xxx meters? Do all the GPS receivers use the same coordinate system? If I take a survey with several GPS receivers will they all get the same answer? -- These are my opinions. I hate spam. ___ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe, go to http://lists.febo.com/mailman/listinfo/time-nuts_lists.febo.com and follow the instructions there.
