From: Ulrich Herfort/esoc/ESA 19/04/2006 15:06
To: Mathias Lauer/esoc/ESA@ESA
Subject: VEX HGA1 calibration
The following plots show the path of the G/S relative to the nominal
and the calibrated boresight (HGA1XANG.JPG for X-band, HGA1SANG.JPG
for S-band):
The signal strength over time (as recorded in the IFMS AGC data files
and scaled by distance) is shown here (HGA1XAGC.JPG and HGA1SAGC.JPG):
The oscillations in the curves is a signature of the offset of the
actual boresight w.r.t. the nominal one.
Based on a least squares fit, the best estimate for the actual boresight
is off from the nominal one (as contained in the FDDB) by:
X-Band S-Band
total separation angle: 0.15 deg 0.32 deg
offset towards S/C y axis: -0.14 deg -0.32 deg
offset towards S/C z axis: 0.04 deg 0.02 deg
Relative signal strength over total depointing w.r.t. to both the
nominal and the estimated actual boresight is shown in the last two
plots (HGA1XCAL.JPG and HGA1SCAL.JPG). Also shown is
a) the theoretical prediction based on the 3dB half cone angle of 0.96
(X-band) or 3.5 (S-band) deg which is contained in the FDDB,
b) a biquadratic fit to the relative signal strength over total
depointing w.r.t. the estimated actual boresight (a quadratic fit was
not sufficient, see below).
In both cases the signal strength decreases considerably faster with
increasing depointing than predicted. This is due to the 4th order
term, which gives a non-negligible contribution compared to the
quadratic term even for an depointing equal to the 3 dB halfcone angle.
In case of X-band, even the quadratic term alone gives a lower 3 dB
halfcone angle than predicted, as can be seen from the following table:
3 dB halfcone angle X-Band S-Band
with 4th order term 0.86 deg 3.3 deg
quadratic term only 0.91 deg 3.5 deg
nominal 0.96 deg 3.5 deg
--
Ulrich Herfort
EDS Operations Services GmbH
ESOC Flight Dynamics Support, OPS-GFI
European Space Operations Centre
Robert Bosch Strasse 5, 64293 Darmstadt, Germany
Tel. +49-6151-90-2132 Fax: +49-6151-90-2271