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: VEX-T.ASTR-TCN.00349
Issue : 2
Date
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Rev. : 0
: 06/02/2004
: 5.24
5.6 HIGH GAIN ANTENNA MANAGEMENT
Specific need for Venus Express
On Venus Express, in order to fulfill thermal constraints at spacecraft level, 2 HGAs are used for the
communications with the Earth. The switch from one HGA to another has to be performed around
quadratures, in order to ensure that HGA1 is selected on superior conjunction side, and HGA2 on
inferior conjunction side of the mission.
At AOCS level, both HGA1 and HGA2 direction unit vectors are stored onboard, and the AOCS will
use the appropriate vector to compute the final attitude of the SHM, or the attitude of the Normal
Mode / GSEP.
In SHM, the adequate HGA parameters will be used when entering in this mode, the final attitude and
the attitude manoeuvres being computed autonomously by the software.
In Normal Mode, the adequate HGA parameters will be used when entering in GSEP after a ground
slew ensuring the correct pointing of the spacecraft (no attitude manoeuvre is computed autonomously
in Normal Mode).
Management of the HGA switching at DMS and AOCS level
The HGA switching is performed only 4 times during the mission and is not considered as “time
critical”. It is therefore performed from ground through a dedicated TC. The onboard software ensures
however several tasks autonomously :
q
The appropriate storage of parameters in the Safeguard Memory (SGM),
q
The consistency between DMS data and AOCS data for the HGA selection is performed
autonomously by the DMS, in order to avoid a critical situation where the RF configuration
selected by the DMS is different from the HGA pointed towards the Earth by the AOCS,
q
The adequate selection of parameters during Safe Mode is also ensured by the DMS.
For this purpose, the DMS uses a “Selected HGA” flag in DMS PM RAM, which is also stored in
SGM, and an internal command sent to AOCS for the HGA switching.
On receipt of the DMS command, the AOCS updates its own “HGA selected” flag and selects the
associated directors cosines of the HGA. This direction to be pointed will be effective at AOCS level
only at the next entry in SHM or NM / GSEP, as recalled before.
During a Software or Hardware Safe Mode, the DMS also sends this command to the AOCS, ensuring
that the safe mode is started with the same data at DMS and AOCS level.
This procedure is defined for the “SSCE option” of the Guidance, which is the baseline, and
especially mandatory at quadrature time. The AOCS software ensures autonomously the sign change
necessary in this guidance law when changing from one HGA to the other.