In the Earth observation mission GOCE (Gravity field and steady state Ocean Circulation Explorer); the aim is to establish models of the earth's gravity field in form of a geoid (equipotential surface at normal sea level of a hypothetical ocean at rest).
The Gradiometer is the main payload and is to measure the gravity field anomalies with an accuracy of 1-2mGal (1mGal=10-5 m/s2). The measurement is based on 3 pairs of three-axes servo-controlled, capacitive accelerometers manufactured by ONERA.
The Gradiometer instrument (under the responsibility of Thales Alenia Space in Cannes) includes Syderal electronic boxes including:
- 3 x Front End Electronics Unit (FEEU)
- Gradiometer Accelerometers and Interface Electronics Unit (GAIEU)
- Thermal Control Electronics Unit (TCEU)
The FEEU performs the acquisition in 48 channels and provides the reference voltage of the capacitive sensors. The A/D and D/A conversion are also included in this unit i.e. the accelerometer control loop.
The TCEU manages the thermal control of the entire payload structure and boasts an accuracy of 0.07°C over 40 measurement points and 20 heaters delivering 200W.
The GAIEU interfaces with the 3 FEEU's, with the TCEU and with the platform CDMU. All telemetry and telecommanding as well as the main application software are included in this equipment.
The GAIEU includes:
- Power supply for FEEU and GAIEU
- Power switches for FEEU
- FDIR supervisor
- DSP21020 (including memory mapping)
- MIL-STD-1553 Bus with CDMU
- Communication with FEEU via RS-644
- Communication with TCEU via RS-422
The software runs on a DSP21020 processor with a virtuoso real-time operating system. A reliable scheduling algorithm with sub-milliseconds response time is implemented. It has been developed following the ECCS-E-40B and ECSS-Q-80B standards. The two main parts are:
- Boot software: Start-up at power-on and program or data patch in safe context
- Application software: It is based on a multi-tasking, real-time operating system. The operational program is in charge of the regulation of the probe-mass position every milisecond, and of the command and control interface with the CDMU. It is also in charge of a given level of autonomous fault detection, isolation, and recovery (FDIR) mechanism.
The payload electronics system included 5 separate units fully developed and realised by Syderal that worked successfully throughout the mission. The GOCE mission ended in November 2013 (three times the initial lifetime) after successfully meeting all its objectives.
The high accuracy required for the 48 acquisition chains with 24 bits AD conversion as well as the 0.1°C thermal control accuracy have shown outstanding results. The payload was controlled by a DSP 21020 based processor board including highly complex software with over 20,000 lines of code.
The project was completed successfully and required no update of the application software during the entire mission.