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Date: Mon, 12 Feb 2001 09:43:22 -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.

Well, isn't that a bummer!

I had assumed that Garmin's specification of 1 microsecond meant that
the 1 PPS output was within 1 microsecond of UTC.

We don't really need microsecond accuracy.  Within one millisecond would
be good enough.  But now I'm very curious about this.

What is the difference between relative time and absolute time?  If all
GPS receivers output their 1 PPS at "relative" time then the system will
work just fine.  It doesn't matter what time it is absolutely, only that
everyone agrees on what time it is.

Bill VE2IQ