Message-ID: <3A87DD0F.AAF4D15E@virgin.net>
Date: Mon, 12 Feb 2001 12:54:40 +0000
From: "Stewart Bryant" <stewart.bryant@virgin.net>
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To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: GPS-Disciplined BPSK
References: <5.0.2.1.2.20010212090705.00a69aa0@mail.pncl.co.uk>
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Why not use the signal from one of the LF time clocks, and
correct by offsetting the distance. Since this is being used
for LF transmission, the communications system must be
propagation phase change resistant, and timing from an
LF source will make the phase error order of 2 worse
that it might otherwise have been.

This would seem to need much less engineering than
processing the output of a GPS.

Stewart G3YSX


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.  My company
> used to make a lot of money out of selling specialist GPS timing receivers
> designed purely to recover absolute time - they cost about ten times as
> much as ordinary sets! Your basic idea is right  but do you really need
> microsecs?  Would millisecs do?  If so it becomes a lot easier although I
> still wouldn't advise using the 1 pps.  BTW you don't have to work out how
> many seconds from the start of week - the GPS message has what's called a
> Z-count which is exactly that - the number of seconds into the week
> starting at midnight Sunday UTC.
> If you can get the right data outputs from the receiver (some of the
> slightly more expensive receivers give them) what about this :
> Using only one satellite - pull out the Z-count (in the message); the delay
> between satellite and you (from measured pseudo-range); the UTC offset for
> that satellite (in the message).  Apply the transit delay to the UTC offset
> and label it with the Z-count. This will be a near approximation to
> absolute UTC that might do for your purpose. It only relies on the
> receiver's internal timing for a few millisecs while it gets the
> pseudorange although unavoidably it is still contaminated with software
> functions and timing. Can't be done with a cheapo because it won't let you
> into the fundamental GPS data message. NMEA data outputs are irrelevant and
> unusable for this purpose. Of course, there's a bit of external programming
> involved to manipulate the numbers.  I haven't looked at this method in any
> detail to see where the snags are but if it looks a runner I'll do a bit
> more work on it.
>
> Walter G3JKV.