Delivered-To: igsstation@igscb.jpl.nasa.gov
From: Jim Ray "(NGS" 301-713-2850 "x112)" <jimr@ngs.noaa.gov>
Message-Id: <200603071321.IAA02860@ness.ngs.noaa.gov>
Subject: [IGSSTATION-860]: FORT radome effect
To: igsstation@igscb.jpl.nasa.gov
Date: Tue, 7 Mar 2006 08:21:13 -0500 (EST)
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IGS Station Mail          07 Mar 05:21:52 PST 2006      Message Number 860
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Author: Jim Ray

I reduced the first 5 days of FORT-BRFT data after the FORT radome was
removed on 27 February 2006.  Assuming BRFT did not move, then I find:

  FORT(radome) -> FORT(no radome) = -0.1, 0.0, 17.6 mm in dN, dE, dU

In other words, the effect of the radome has been an apparent height
bias of the FORT position by about 18 mm downward.

In our paper "Evaluation of co-location ties relating the VLBI and GPS
reference frames" (J. Geodesy, 79, 189-195, 2005) Zuheir Altamimi and I
found the FORT GPS -> Fortaleza VLBI tie discrepancy at Fortaleza to be:

 (GPS -> VLBI)_space - (GPS -> VLBI)_tie = 3.0, 0.6, 22.7 mm for dN, dE, dU

based on a comparison of the global networks linked by nine weighted ties
that were consistent with each other and space geodesy to <4 mm (RMS) in
each component.

Since the (GPS -> VLBI)_space component was based on FORT(radome) but the
tie was presumably based on measurments to a physical point, the vertical
discrepancy would be reduced to 5.1 mm taking the radome effect into account.

The BRFT GPS -> Fortaleza VLBI tie can be derived based on the old
GPS -> VLBI survey and using the new no-radome GPS-GPS tie, which should be
much more accurate that the FORT tie with radome.  Of course, it would be
good to have a new site survey.

--Jim