Message 5/468 Dr Thomas A Clark (W3IWI)          Oct 12, 2000 06:01:12 pm +0000

Date: Thu, 12 Oct 2000 18:01:12 +0000
Reply-To: tac@clark.net
X-Accept-Language: en
To: Exploder for EVN Techies <evntech@jb.man.ac.uk>
CC: Rick Hambly <rick@cnssys.com>, Reza Abtahi <reza@cnst.com>
Subject: EVNtech: GPS Timing Report for the TOG
Sender: owner-evntech@jb.man.ac.uk
Precedence: bulk

I thought you might find it interesting to get an update on my activities
to develop low-cost GPS Timing suitable for VLBI stations.

In the past year, I saw several significant happenings:

1. We deployed my stand-alone timing system based on my TAC (and its
commercial clone, the CNS Clock) plus an HP53131/2 time-interval
counter(TIC) -- the combinations sometimes being called "TIC-TAC". The two
pieces of hardware are glued together by the Windoze (Win 95/98/NT/2000,
with Win2K being my current recommendations) TAC32Plus software package.
The Win PC should be connected to the station's LAN for full access to the
timing data. This includes the use of this PC as as STRATUM-1 network time
server and a telnet connection to the VLBI Field System software. The
software is available from www.cnssys.com/tac32plus/

2. On May 2nd, 2000 the DoD turned off the clock dither called Selective
Availability, and GPS timing improved markedly overnight.

3. The TAC was always based on the Motorola ONCORE VP receiver. Motorola
discontinued the VP series receiver in late 1999 and now only supports the
poorer performance of the UT+ timing receiver.

4. When SA was turned off in May, a number of small timing "glitches"
showed up in the VP and UT+ receivers, and we started seeing more
significant differences between different Motorola hardware and firmware
releases. In the collection of receivers ranging from the early 6-channel
Motorola PVT-6 ONCOREs to the latest versions, I suspect upwards of ~100
nsec (peak-to-peak) of "DC" timing biases associated with the different
releases.

5. Starting in early 2000 we began an effort to develop an alternative to
the ONCORE VP, and just last month I made a report on the effort at the
Institute of Navigation meeting in Salt Lake City. With the new receiver,
we are now achieving sub-nsec short-term performance with daily noise
totally set by the ionosphere. The ION paper is available at by clicking on
the two items at the bottom of

   http://www.gpstime.com/

he first is the PDF format text that will appear in the ION Proceedings and
the second is the set of PowerPoint slides I used (including photos of
Urumqi and GGAO). You might find it interesting how I described VLBI's
timing needs to the GPS timing world.

To anticipate your question about the new timing receiver described in the
paper -- it isn't available yet, and only a few copies exist. I hope to
have news soon about it. Needless to say, I would be quite interesting to
hear of your interest.

FYI -- this note is posted from my home. In Salt Lake, the day following
the presentation of this paper, I fell down some stairs and broke two bones
in my left leg. The surgeon implanted enough metal in my leg to set off
airport metal detectors and I'm a bit slow hobbling around on crutches!

Hope the Torun meeting is fun and interesting. I wish I could be there.

Regards, Tom