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