From: Yehuda Bock
Date: 16 Dec 1994 12:31:20 
Subject: [SCIGN-0035] Re

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SCIGN Electronic Mail    Fri Dec 16 12:31:20 PST 1994      Message Number 0035
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Author: Yehuda Bock
Subject: Re: Phase Center

Subject: Re: Phase Centers

Scripps Orbit and Permanent Array Center
Date: 16 December 1994

I would refer Ken Hurst to SCIGN mail 0014 in which we describe
the antenna offset problem at SIO3, and the fact that we have
found different offsets among several new Ashtech antennas (with
the larger ground planes). (Maybe he could enlighten us on the
other station problems that we raised in that message.)

It is a straightforward (albeit annoying) to determine any antenna offsets
from the PGGA time series and apply those corrections in velocity
determinations. I suggest that Ken and I swap our computation
of these offsets and make them available in the station log files.
Unfortunately, for the new Ashtech antennas this needs to
be done antenna by antenna. Currently, we have a large ground plane antenna
only at the TRAK station. Orange County bought their own GPS unit for
this site and were kind enough to share the data with us. We had loaned them
equipment for several months prior to this, with the old style antenna.
The older antennas do not appear to exhibit this variability. In any case,
it is important to keep track of antenna and receiver serial numbers.

I agree with Ken that we should be conservative in our exchange of
antennas and other equipment at the sites. The antenna issue as we
heard at the special session at AGU is one of the most serious technical
problems in GPS processing today. It would be a good idea to try
to standardize antennas in SCIGN. As a test, we are currently retrofitting
two Dorne Margolin antennas (with choke rings) for use with the Z-12's.

There are several other technical issues that will detract from mm-level accuracy
for SCIGN. These include, for example, monument stability, radio interference,
receiver inter-channel biases and multipath. The large amounts of data
that we are collecting now in SCIGN will allow us to isolate these errors
and choose the best possible equipment and procedures for the hoped-for large
densification.




[Mailed From: pgga@pgga.UCSD.EDU (Geodesy Lab account)]