[time-nuts] GPS Timing prob

Tom Clark, K3IO K3IO at verizon.net
Wed Aug 23 00:06:33 EDT 2006


Faisal Khan wrote:
> Hi Tom,
> Thank you for your reply. This is Faisal, who is working on the GPS
> timing problem and you recommended me some papers from GPStime
> website. I looked those papers. I am getting similar sawtooths as
> well. But could you explain me what is causing the suspension bridge
> effects? Some ppl say that this is due to some sort of phase reversal,
> but i could not get that. Could you help me in this regard.
Faisal -- basically what you see is the beat between some harmonic of
the clock derived from the receiver's crystal oscillator and the
equivalent 1575.42 MHz GPS (actually, the aggregate of all the
satellites in view, corrected for Doppler and geometry) satellite
carrier. The 1PPS clock aliases this down to a very low frequency which
you see as the sawtooth "dither". Normally this error is ~0.1 Hz
(resulting in a 10 second typical sawtooth period) but the clock can
drift thru a zero-beat and even change sign (which you see because the
sawtooth changes phase from (the direction of the teeth changes).

When the Motorola receiver's navigation software makes its computation
of the "best" integer to load into the counter to produce the next
pulse, it also computes the residual error which it sends out (truncated
to 1 nsec) as an 8-bit number embedded in one of the Motorola binary
messages. In Rick's TAC32 software, it is possible to correct the
reading from one particular breed of time interval counter (The
HP/Agilent 53131 & 53132), and in the plots you see that correction
being applied in real-time. Rick and I also developed the simple
hardware "widget" I described in the last message which applies the
correction in real-time to the receiver's 1PPS pulse.

For the Time-Nuts folks: Would there be any interest in a PCB "widget"
that would correct the Motorola receiver if we were to make it available?

73, Tom




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