[time-nuts] Some results of PRS10 and Trimble Resolution

Tom Van Baak tvb at leapsecond.com
Tue Jun 27 16:45:05 EDT 2006


> Where is the sawtooth coming from?

Hal,

The 1 PPS "sawtooth" effect is found on many OEM GPS
precise timing receiver boards.

The pulse output of a GPS board is usually derived from
the internal VCXO so the timing resolution of the 1 PPS
edge is thus tied to the frequency of the oscillator.

For example, a 25 MHz oscillator in a GPS engine allows
the processor to pick, each second, which one of 25 million
edges it wants to be the official 1 PPS edge for that second.
Now 25 MHz frequency is 40 ns period so that gives a 1 PPS
granularity of +/- 20 ns in this example. See:

http://www.leapsecond.com/pages/m12/sawtooth.htm

If that makes sense, then what's sawtooth correction?

Based on satellite ranging and oscillator time or frequency
drift calculations for the previous second, the GPS engine
can calculate where the 1 PPS pulse ideally should occur
(to within a few ns).

So even though the hardware 1 PPS is slightly limited by
the granularity of the clock, the firmware in some GPS
timing receivers, can report the difference (in ns) between
where it wanted the 1 PPS to be in software vs. where
the 1 PPS actually occurred in hardware. This +/- value
is called the "sawtooth correction", or sometimes the
"negative sawtooth correction".

In short, the sawtooth correction describes, to the best
guess of the receiver, how early or late the hardware
1 PPS was.

/tvb






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