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Date: Tue, 13 Feb 2001 09:47:56 -0500
To: rsgb_lf_group@blacksheep.org
From: "Bill de Carle" <bill1@cgocable.ca>
Subject: Re: LF: GPS-Disciplined BPSK
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At 10:28 AM 2/12/01 +0000, Walter Blanchard wrote:
>At 21:17 11/02/01 Sunday, VE2IQ wrote:
>
>>...............I propose to use GPS ..........:
>>The only real requirement is a GPS receiver with a 1-PPS output good to
>>within a few microseconds of UTC.
>
>Be careful - don't confuse relative with absolute time. The 1 pps output 
>from cheapo GPS sets is only relative time, not absolute.  If you like, it 
>gives frequency not phase and phase is essential to your 
>application.  Recovering absolute time from GPS to microsec accuracy is not 
>easy and you won't do it with an ordinary positioning receiver.

I checked with Garmin on this and they're standing by their 1 microsecond
spec.  They suggest using the GPS25-LVS (which apparently has a mode where
you can specify that the location is fixed and let it concentrate on
calculating the time accurately) - and say that under good signal conditions
the 1PPS output will track UTC "well within" the 1 microsecond limit.
They say typical accuracy is 100 nsec RMS with 50 nsec jitter.

That is more than good enough for our purposes.
I still haven't sorted out what the difference is between absolute and relative time, but as long as everyone agrees on the same system it should
work fine.

Bill VE2IQ