[time-nuts] Some results of PRS10 and Trimble Resolution
Tom Van Baak
tvb at leapsecond.com
Tue Jun 27 20:41:49 EDT 2006
> In short, the sawtooth correction describes, to the best
> guess of the receiver, how early or late the hardware
> 1 PPS was.
Let me go on a bit more.
If the GPS OEM board knows what nanosecond the
1 PPS should be generated, why then doesn't it just
issue the 1 PPS pulse precisely at that instant?
Well, if there were a 1 GHz oscillator and ASIC on the
GPS board then this would be easy, yes? But it doesn't
surprise me that market economics argues against this
level of overkill.
Few people need ns granularity of their 1 PPS; those
that do often don't care if it comes in fine hardware or
in coarse hardware with fine software correction. And
besides, 1 ns granularity doesn't always make sense
when the actual UTC accuracy of the pulse is on the
order of 10 to 100 ns.
Also none of this makes sense if you're applying post
corrections to your GPS data; e.g., to back out UTC
errors in UTC(USNO), or GPS orbit errors, etc. So
most GPS boards just issue the 1 PPS synchronous
to the main clock; which is on the order of tens of
MHz; giving 1 PPS jitter on the order of tens of ns.
Now, another way to handle this is to run the hardware
1 PPS output, with all its jitter, though a programmable
nanosecond-resolution digital delay generator. Each
second you use the firmware reported future negative
sawtooth as the programmed delay value for the next
1 PPS pulse. Tom Clark and I talked about this years
ago and Rick Hambly implemented this as an option
in his CNS Clock II ( http://www.cnssys.com/ ).
Note this hardware trick only works for GPS OEM
boards that predict sawtooth error for the next 1PPS
(vs. reporting the sawtooth error of the previous 1PPS).
Motorola is OK here.
If you're tempted, see some programmable delay lines at:
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