GIOVE-B Spacecraft In
Good Health
(29 April 2008) After its successful
launch by a Soyuz Rocket from Baiknour on 27 April and accurate insertion into
its target orbit by the Fregat autonomous upper stage, GIOVE-B is now
completing its Launch and Early Operations Phase (LEOP), which will shortly
give way to the platform commissioning phase.
Platform
commissioning includes the on-board verification of all primary and redundant
platform subsystems, namely telemetry and telecommand, propulsion, power,
thermal control, and attitude and orbit control. GIOVE-B operations are being
carried out from Telespazio's Fucino Space Centre in Italy.
Once the
spacecraft platform has been commissioned, switch-on of the various payload
elements can begin. First signal transmission is anticipated in the coming
days.
Artist's impression of GIOVE-B in orbit (courtesy: ESA)
Charging the batteries
Since
the spacecraft had been running on battery power from just before lift-off -
throughout the lengthy series of manoeuvres performed by Fregat to reach the
intended orbit - the first tasks after separation from Fregat were to deploy
the solar arrays and achieve Sun-pointing, so that battery charging could
begin.
The first-choice method to turn the spacecraft for Sun-pointing
is to use the reaction wheels. GIOVE-B is equipped with four gyroscope-like
wheels driven by brushless electric motors. Altering the speed of rotation of
these wheels allows the satellite to be rotated in space.
Use of the
reaction wheels for initial pointing is the first choice because, once they are
operational, the supply of electricity from the solar arrays is essentially
unlimited. However, due to the battery discharge that occurred during
pre-orbital flight, the spacecraft operations manual only allows a restricted
time for these manoeuvres to be accomplished.
The Telespazio satellite ground station at Fucino, Italy, from where GIOVE-B operations are being controlled. (courtesy: ESA)
Attitude and orbit
control
GIOVE-B's reaction wheels were slow to respond and stable
Sun-pointing was not achieved within the specified time period. As a
consequence, around five hours after launch it was decided to put the satellite
into safe mode, and Sun-pointing was achieved using small engines known as
thrusters.
The initial underperformance of the reaction wheels was found
to have occurred due to a mismatch between the on-board avionics software and
the reaction wheel calibration data. This has now been corrected by the
uploading of a software patch to the spacecraft, leading to nominal
performance.
Use of the thrusters is the second-choice method because
one of the factors determining the operational life of GIOVE-B is the amount of
propellant that the spacecraft carries. The thrusters are used for orbital
station-keeping and conservation of propellant is important to maximise the
lifetime of the spacecraft. Once the propellant is nearly exhausted, the
thrusters' final task is to lift the spacecraft into a 'graveyard' orbit at the
end of its mission - to free the orbit for the operational Galileo
constellation.
Payload switch-on
Once the GIOVE-B
spacecraft platform is fully commissioned, the navigation payload can be
switched on. The rubidium and passive hydrogen maser clocks will be started
first, followed by the navigation signal generator.
The final units to
be turned on will be the transmit chain consisting of L-band solid-state power
amplifiers that broadcast the Galileo signals to the Earth. First signal
transmission is anticipated in the coming days and will be received and
calibrated by In-Orbit Test (IOT) facilities deployed at ESA's ground station
in Redu (Belgium) and at Chilbolton Observatory in the UK.
(source:
ESA)
