Top Side View of CHAMP
The satellite is designed and manufactured under contract of GFZ by the Daimler Chrysler Aerospace Jena Optronik GmbH (DJO) in an integrated industry effort with Dornier Satellitensysteme GmbH (former DSS, now Astrium) and Raumfahrt und Umwelttechnik GmbH (RST).
The CHAMP satellite has a robust structure design with fixed solar panels. The primary structure is mainly based on aluminium sandwich panels with an additional kapton foam layer on the outer panels. The shape of the satellite is a compromise with respect to its aerodynamic behavior, accommodation of instruments and subsystems and fitting into the fairing of the launcher.
The two solar arrays are symmetrically inclined to the equipment panel with two additional solar panels on the roof side of the spacecraft. The satellite is closed at its aft and front side by sandwich panels of which the aft panel carries the GPS occultation antenna. The front panel carries the digital ion driftmeter and is covered by a conductive aluminium plate in order to avoid electrical charging in the space plasma. The nadir S-band antenna is mounted on short boom beneath the forward compartment, while the zenith S-band antenna is placed on the rear antenna support structure.
The power requirements of the on-board equipment as well as the chosen orbit necessitate about 7 m2 of body-mounted solar generator surface. A battery of NiH2-cells ensures the appropriate power supply during eclipses.
For magnetic cleanliness reasons, the magnetometry assembly must be kept at some distance from the satellite This is achieved by using a deployable boom mounted at the front end of the satellite. The length of the boom is approximately 4 m, thus guaranteeing that the magnetic strayfield at the location of the Overhauser magnetometer will be less than 0.5 nT.
The boom consists of three segments: the outer part, carrying the Overhauser magnetometer, the middle segment with the optical bench on which two star sensor heads and two Fluxgate magnetometers are mounted, and the inner segment with the deployable part of the hinge.
Bottom view of CHAMP
The primary structure houses the accelerometer at the satellite's Center of Gravity (CoG). The surrounding subsystems and especially the two tanks carrying gaseous nitrogen are accommodated in such a way that the proof-mass of the accelerometer will coincide with the satellite CoG within 2 mm accuracy throughout the whole mission. The laser retro-reflector is within a circle of 5 mm located directly below the CoG and the cold gas thrusters are accommodated in such a way that torques are generated with minimum resulting force.
For the Attitude and Orbit Control Subsystem (AOCS), a cold gas propulsion system for attitude and orbit change manoeuvres has been employed. A set of three magnet torquers for pre-compensation of environmental disturbances will support the cold gas propulsion system comprising 14 thrusters. The task is to control the orientation and angular rates within a deadband of ±2° and ±0.1°/s. Star Sensors, GPS, the Fluxgate magnetometer and coarse earth- and sunsensors will be used for exact attitude measurement and the calculation of the angular rates.
The Thermal Control Subsystem (TCS) guarantees a secure temperature for all instruments and subsystems during all possible space environment conditions. To achieve this, primarily paint with small a/e-values (absorptivity/emissivity), solar cells, Second Surface Mirrors (SSMs) and Multilayer Insulation (MLI) will serve for passive thermal control. For critical spots with high demands on temperature stability, active thermal control will be pursued by means of heaters.
Power generation, conditioning, distribution and storage will be the tasks of the power subsystem. Therefore approximately 7 m² of solar cell surface, a combined Power Control and Distribution Unit (PCDU) as well as a NiH2-battery consisting of 10 cells are implemented.
The On-Board Data Handling (OBDH) Subsystem manages all scientific and housekeeping (H/K) data. In addition, it has limited autonomous capability of Failure Detection, Isolation and Recovery (FDIR) occurring during data handling. Since most instruments provide scientific as well as H/K-data, the AOCS data handling and software will be integrated into the OBDH. The OBDH will accept a 1 Hz time reference pulse derived from the GPS signal and distribute it to all instruments and attitude sensors. Additionally a clock, synchronised to the GPS 0.1 Hz refresh pulse, will maintain the pulse in case of a missing GPS synchronisation. A Mass Memory Unit (MMU) of 1 Gigabit capacity will store all data acquired during observation periods and dump them when CHAMP is in visibility of the receiving ground station.
Telecommunication will be accomplished by means of the Telemetry, Tracking and Command (TT&C) Subsystem in S-band. This subsystem consists of a receiver, a transmitter, an encoding/decoding device and two antennas with complementary semi-spherical radiation patterns (one primarily for the Launch and Early Orbit Phase (LEOP) and another for nominal operation).
Specific characteristics of the CHAMP Satellite Subsystems are summarized in the following.
Attitude & Orbit Control System (AOCS)