[IGSMAIL-5304]: Warning: FORT radome change

[Jim Ray (NGS 301-713-2850 x112)]

******************************************************************************
IGS Electronic Mail      16 Feb 11:21:17 PST 2006      Message Number 5304
******************************************************************************

Author: Jim Ray

In order to perform an in situ test of the effect of the conical radome
at FORT, Arthur Niell has suggested that we remove it for some period
before we decommission the station and then compare any position change
to BRFT (see IGS Mail #5303, 15 Feb. 2006).  Antoni Macilio at Fortaleza
has kindly agreed to make this change on Monday, 27 Feb. 2006.

Users should be aware that the FORT position is subject to change, possibly
by several cm.  It is recommended that FORT no longer be used in
realizations of the IGb00 frame (aligned to ITRF2000) by applying apriori
position constraints, starting on 27 Feb. 2006.  If FORT data are used,
its position should be unconstrained.  Alternatively, users can substitute
BRFT data with the previously reported IGb00 inferred coordinates:

   BRFT (41602M002) coordinates in IGb00 frame
                 (epoch 1998.0)
   -------------------------------------------
          X =  4985393.5481  +/-  0.00413  m
          Y = -3954993.3972  +/-  0.00367  m
          Z =  -428426.8034  +/-  0.00221  m

      vel X = -0.001887  +/-  0.001378  m/y
      vel Y = -0.003463  +/-  0.001189  m/y
      vel Z =  0.010888  +/-  0.000530  m/y
   -------------------------------------------

The indicated uncertainties, propagated to the data epoch, should be used
for weighting.  This should achieve the same frame realization as FORT
(although the BRFT data yield and quality is considerably better).

It is not possible to separately test in situ the effect of the aluminum
backplace because it supports the antenna in place.

After some weeks of observations without the radome, FORT will probably
be decommissioned.

Our thanks to Arthur and Macilio for their assistance.