Hi There is a lot of math in a GPS receiver. In a “typical” nav receiver, time is not a priority. The code may well be optimized to “shove” all the error into time rather than position. They also may not have spend much effort debugging the time related code ….
Bob On Mar 23, 2014, at 11:08 PM, Bob Stewart <[email protected]> wrote: > Random noise or not, wouldn't a position error in a nav receiver cause a > corresponding displacement of the 1PPS pulse? > > Also there's a bit more to it than just minor noise. It's probably > multipath, or perhaps even jammers passing on the freeway about a mile away. > Whatever the cause, take a look at this screen capture of "foxtrotgps" over > about 40 minutes of elapsed time. The red is the GPS movement around my > house that I periodically mention. I judge it to be about 15 ft on the > diagonal. Sometimes it is much much worse. Likewise I would expect the 1PPS > to move by that amount. > > http://www.evoria.net/AE6RV/Nav/NavWander.png > > Bob > > > > >> ________________________________ >> From: Chris Albertson <[email protected]> >> To: Bob Stewart <[email protected]>; Discussion of precise time and frequency >> measurement <[email protected]> >> Sent: Sunday, March 23, 2014 9:24 PM >> Subject: Re: [time-nuts] Nav Receiver Sawtooth Correction? >> >> >> What you are seeing in position error is a random noise. There is no >> pattern to it and it is not predictable. A sawtooth error is very >> nice and regular looking. It's not "noisy" and can be predicted in >> advance. Possition error is not at all like sawtooth. >> >> I think what you CAN do is look at the size of the error. Then you >> adjust the gain on the loop control based of the measured error sigma. >> >> >> >> >> On Sun, Mar 23, 2014 at 6:30 PM, Bob Stewart <[email protected]> wrote: >>> Hi Ignacio, >>> >>> Thanks for the response. I've got a UT+ in the parts box. But that's not >>> the problem I'm trying to solve. I'm trying to make the best GPSDO that I >>> can make using a nav receiver at the moment. Call it an obsession if you >>> like. It's OK if I don't have corrections to the nanosecond for each PPS. >>> But I can see the nav receiver wandering around; especially on cloudy days, >>> (the antenna is in the attic, so that's about the best I can do for that) >>> and it just seems to me that I should be able to do a general correction >>> for nav position errors. Sorry if my naive posts are starting to get on >>> people's nerves. >>> >>> Bob >>> >>> >>> >>> >>>> ________________________________ >>>> From: EB4APL <[email protected]> >>>> To: [email protected] >>>> Sent: Sunday, March 23, 2014 7:35 PM >>>> Subject: Re: [time-nuts] Nav Receiver Sawtooth Correction? >>>> >>>> >>>> Bob, >>>> >>>> The sawtooth is generated by the granularity of the GPS receiver clock >>>> not being synchronous with the recovered PPS. The receiver program can >>>> calculate the correction to be applied to the next PPS and outputs it in >>>> a message, bu only in timing receivers, this is not a useful thing in >>>> navigation receivers and I think that it cannot be calculated using the >>>> satellites' position, it is a receiver "defect". >>>> Why don't you buy a timing receiver? An used Motorola Encore M12+ >>>> timing receiver can be bought by $35 or less (ebay items 290656401551 or >>>> 301131583613. The seller is a known Time Nuts supplier). An UT+ or GT+ >>>> even for quite less. >>>> >>>> Regards, >>>> Ignacio EB4APL >>>> >>>> >>> _______________________________________________ >>> time-nuts mailing list -- [email protected] >>> 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 -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
