[time-nuts] Long period variation of GPS PPS timing?
David C. Partridge
david.partridge at perdrix.co.uk
Fri Jul 2 05:19:37 EDT 2010
Is this a timing GPS receiver, or a generic fast start navigation receiver? 25 seconds to start from power on sounds rather short for a timing receiver.
The T-bolt start up from cold takes about 15 minutes for the crystal oven to warm, then the self survey is run which by default is 2000 samples (just over 33 minutes).
Once the survey is complete, the position is stored and the receiver runs in "over-determined" clock mode. I can't remember the "warm start" time but 25 seconds sounds on the short side.
Refer back to Mark Sims tests on the benefits of taking a 48 hour self survey to get a more accurate position (ISTR there was a cyclic error associated with an incorrect position).
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf Of Paul Nicholson
Sent: 02 July 2010 09:05
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Long period variation of GPS PPS timing?
This morning I waited for the VLF timing signals to settle into their steady daytime phase - still showing the slow cycle of phase variation.
Then I power-cycled the GPS, leaving everything else running.
During the 25 seconds it takes the GPS to begin sending PPS again, my software continues to time the VLF signal using the average PPS pulse interval measured over the previous 100 or so seconds.
See the phase plot of DCF 77.5kHz for the result,
GPS rebooted at 16686 seconds, just after the slow cycle started its upward ramp. It seems to have caused a phase jump in the slow cycle - it switched to starting a downward ramp.
To confirm, I rebooted the GPS again at 24915 seconds, as it was about half way down a downward ramp. Again a step change of phase, this time it restarted at the bottom of an upward ramp.
Just to eliminate any effect in my software, I unplugged the PPS signal from the soundcard for 30 seconds, leaving the GPS
running. My software is starved of PPS for the same length
of time as during the GPS reboots. This occurred at 27440
seconds, halfway along a downward ramp. The ramp continued down.
Just to 'shake the bag' of temperature effects, I opened the door of the cabinet containing the PCs and electronics, this quickly dropped the temperature by 10 C. Also took out the unit containing the RC pulse forming, took its lid off and left it in the open air. Result: no noticeable change to the slow cycle.
Well now, this sure looks to me like a GPS effect.
I think I'd better order another GPS, a different type, maybe a GlobalSat MR-350P, or something, for comparison.
Will also try to move the GPS16 so that it can see more of the sky - only about 50% visible at the moment, some trees block the other half.
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