Message-ID: <000a01c09517$64913c00$938901d4@g4jnt>
From: "Andrew Talbot" <G4JNT@thersgb.net>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: GPS-Disciplined BPSK
Date: Mon, 12 Feb 2001 17:15:25 -0000
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The GPS25 module - one of those recommended by TAPR for the "GPS Disciplined
Frequency Standard" - is specified as having a 1 PPS output accurate to
within 1 microsecond of UTC.  Additionally, it is supposed to be possible to
lock in the exact location giving a further improvement in timing, but this
does not appear to be documented in the notes that came with mine.  Does
anyone have this info ?

Andy G4JNT

-----Original Message-----
From: Walter Blanchard <blanch@pncl.co.uk>
To: rsgb_lf_group@blacksheep.org <rsgb_lf_group@blacksheep.org>
Date: 12 February 2001 10:55
Subject: Re: LF: GPS-Disciplined BPSK


>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.
>
>
>