GPS for satellite obs

Karl Cunningham (karlc@cnsii.com)
Thu, 23 Jan 1997 08:48:24 -0800

At 08:31 AM 1/23/97 +0200, you wrote:
>Has anyone had experience using A GPS receiver for satellite positional work?

Greetings --

I've used a handheld GPS receiver for timing of satellite obs for several
years.  In order to get a position solution within 30 meters or so, GPS
receivers need to have time to less than a microsecond, at least
internally.  Mine (and I suspect others) gives a time display as well as a
time output over a serial port.  HOWEVER, there is latency involved in
displaying and electronically outputting the time that is on the order of
0.5 sec, and it's not constant (it seems to depend on how hard the GPS's
processor is working on tracking satellites).

I'm aware of other GPS receivers that do give accurate time information via
a digital output (often to nanosecond accuracy), but I'm not aware of this
feature in any handheld ones.

Another thing to consider:  As I understand it, "GPS time" is not the same
as UT or GMT.  It is a special time that is not subject to leapseconds,
which would apparently glitch the system.  The "GPS clock" was started when
the system was first brought up, and is now off by some 10 seconds or so
from other times.  I think that most GPS receivers convert to GMT for
output.  Be sure that when you use a GPS receiver for time, it is giving
you the time you desire.

-- Karl Cunningham
karlc@cnsii.com
La Mesa, CA. 116.9775W 32.7705N +308M
interested in almost anything to do with science