The Bus

Payload Data Handling and Transmission

The video data are output from the instrument at 4.5 Gbits/s total output rate, are compressed in the Payload Data Compression Unit. A wavelets transform algorithm is used, that enables the compression ratio to go up to 7, while in standard operation the ratio is 5.

The compressed data are then memorized in the Solid State Mass Memory (SSMM). This memory has a storage capacity of 600 Gbits. The maximum image data input rate is 1.5 Gbits/s. The output rate is nominally of 465 Mbits/s, on three individual channels of 155 Mbits/s each.

The data are then coded following a trellis-coded scheme in 8-PSK type modulators coupled to Travelling Waves Tube power Amplifiers (TWTA). They are multiplexed and down-linked with an omni-directional 64° aperture horn antenna.

These high storage capacity and high transmission rate allow high reactivity of the Pléiades system with few ground image receiving stations. Coupled with high agility, a great amount of user's requests can be satisfied within one day due to 12 hours reprogramming capabilities offered by centralized data collection.

Power

The power system uses a Li-ion battery, and triple junction solar cells. The 150 Amps-hour battery is directly connected to the power lines, and imposes its voltage. It is charged out of eclipse by the Gallium Arsenide (GaAs) cells of the 5 m² solar arrays. To ensure a balanced power budget over one day, the satellite points its arrays towards the Sun before and after every orbit imaging sequence.

Li-ion batteries are well suited because of their high energy to mass ratio and their very efficient charge sequence, which is highly indicated in the case of the fixed solar arrays solution. The solar array size is therefore minimized. The use of triple junction GaAs cells also pushes towards smaller arrays.

Attitude and Orbit Determination

In order to reach very high level of ground location accuracy, i.e. 10 m for 90 % probability ground circular error without ground control points (GCP), new very high precision technological developments have been taken into account for Pleiades satellite attitude restitution.

The autonomous orbit determination is performed by a Doris receiver, which allows reaching an accuracy of about 1 m (on the three axes).

The attitude determination is performed by a gyro-stellar system. Very accurate solid state gyroscopes are used to ensure high accuracy attitude determination while maneuvering. Fiber Optic Gyros (FOG) allow high performances, such as a scale factor stability of a few ppm, a random drift of 0.002 deg/h, and an angular random walk of 0.0002 deg/root-hour. Both star trackers and inertial measurement unit have separated optical heads and electronic units. The optical heads are placed onto the instrument structure to minimize the thermal distortion with respect to the instrument line of sight.

FOG Inertial Measurement Unit Optical Head Configuration

Attitude Control and agility

In order to fulfill the high imaging capacity coming from mission requirements, very demanding maneuvering capabilities are necessary for the Pleiades satellites, and Control Moment Gyros (CMGs) become mandatory. A cluster of four 15 Nms actuators is used.

Actuator

Innovative guiding techniques are used to avoid the usual drawbacks of CMGs: instead of following a pre-defined attitude profile that always lock the cluster in singularities in some cases, a cluster re-orientation is realized taking into account the satellite trajectory and the cluster history to globally optimize the system. It has been shown that this reorientation strategy always avoids singularities, while ensuring a correct convergence towards the imaging dynamics. This new approach allows the use of the complete angular momentum capacity envelope, that is about 3.2 times elementary CMG momentum (15 Nms) in roll and pitch direction. Moreover, this open-loop guidance law can be realized autonomously on analysis of the programming message. It results dramatic simplification of GMG cluster management in the flight software.

The propulsion is grouped in a module that gathers all the related equipments and is used only in the orbit control phases.

The Acquisition and Safe Hold mode (ASH) is based on the use of the magnetic based attitude control called 'B dot' law.