Message-ID: <3A8A85DB.FF4B58AB@usa.net>
Date: Wed, 14 Feb 2001 14:19:23 +0100
From: "Alberto di Bene" <dibene@usa.net>
Organization: Undisclosed
MIME-Version: 1.0
To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: GPS-Disciplined BPSK
References: <008601c09682$e7f3bfc0$f06e97d4@oemcomputer>
Content-Type: text/plain; charset=us-ascii; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org
Sender: <majordom@post.thorcom.com>

Johan Bodin wrote:

> Oh! That's bad news. I checked the Laipac data sheet and the 1PPS accuracy
> is +/-1 microsecond - about 20 times worse than the UT+... It's good enough to
> be used as a bit clock but probably not good enough for disciplining a frequency
> standard.
>

Johan,
    I asked about this OEM unit on the TAPR bulletin board TACGPS.
Here are a couple of answers I received .

----------------------------------
According to the data on the web  site, the receivers are based on SiRF
technology, which is a pretty fair pedigree.  While the 1us jitter
appears excessive, perhaps it's not as bad as all that.  I seem to
recall hearing about a timing app using these (SiRF) receivers that was
rivalling the OnCores, but I'm not sure where...

----------------------------------
A few experimental SiRFstar-I receivers were equipped with experimental
timing firmware. The test results showed the 1PPS far better than any
other receiver we had seen before, 5-10 times better than an Oncore VP
or UT+.  For a copy of the paper or viewgraphs given at ION-2000 back in
September check out http://www.gpstime.com/. This is an ongoing project.
New SiRF firmware was released for testing this morning.  Unfortunately,
this is not for sale at this time.

The 1PPS performance of a GPS receiver is determined primarily by two
things - the hardware capability of the receiver and the firmware.  The
best hardware will still only provide roughly +-500nsec (half a
microsecond) performance with plenty of jitter if the firmware is
optimized only for navigation.  The best firmware will be limited by the
clock speed of the CPU in setting the edges of the 1PPS, although this
can be removed through post processing of the "sawtooth correction"
data.

Rick
W2GPS
-------------------------------------

Another (longish) answer on the topic, by Dr Thomas A Clark, is the
following :  http://www.tapr.org/tapr/list-archive/tacgps/0101/msg00024.html


73   Alberto   I2PHD