GOCE ground segment
The GOCE mission has a single ground station in Kiruna, Sweden, and uses ESA-ESOC, in Germany for mission and satellite control. Processing, archiving, product distribution and mission performance monitoring take place at ESA-ESRIN in Italy.
The GOCE ground segment is composed of three main elements:
- Command and Data Acquisition Element (CDAE)
- Mission and Satellite Control Element (MSCE)
- Processing and Archiving Element (PAE)
Command and Data Acquisition Element (CDAE)
The CDAE selected for GOCE is the Kiruna S-band ground station. It is optimally located for a near-polar orbit, has a sufficient telemetry-rate capability and has reached operational maturity after being used successfully during the ERS and the Envisat missions. The station also has sophisticated antenna-pointing devices with scanning capabilities, high-performance-link quality control and range/range-rate tracking systems, as well as an on-line communications link with ESOC.
The launch window selected ensures that all ground contacts occur in the daytime, and, since the nominal operations schedule uses only four ground passes per day out of the six available, there are two passes available for any contingencies that may occur.
Of those four passes the first three, in the morning, will be made at a maximum telemetry rate of 850 Kb/s. The fourth, afternoon pass will be used, when needed, for telecommanding. This strategy ensures that the onboard memory is completely empty after the morning passes. The whole operations sequence can be run on a schedule provided by the MSCE one day in advance, and implementing all information of the orbital dynamics elaborated at the MSCE. The possibility of on-line direct remote control from the MSCE also exists.
The CDAE provides immediate storage of the received telemetry stream and near-realtime pre-processing to prepare the telemetry data for further use. Given the very limited data rate produced by GOCE (6 Kbit/s), a storage capacity of 4 Gb is sufficient for one week of data, providing wide margins for any contingency.
These Level 0 data products are sent to the MSCE for satellite-monitoring and flight-dynamics support, and to the PAE for gradiometer and SST-hl receiver data processing. Transmission to the MSCE for operational purposes occurs via the electronic link on a near-realtime basis. Transmissions to the PAE can occur on a more relaxed schedule, for example by using the file transfer protocol (FTP) service during the night.
Mission and Satellite Control Element (MSCE)
The mission and satellite control element (MSCE) is responsible for the management, monitoring and control of all operations, on the ground as well as on the satellite. It creates the operations plan, manages and co-ordinates the operations, and controls the satellite and the payload instruments. Moreover, the MSCE provides data quality assessment, control of the instrument behaviour, and generates auxiliary data for use by the PAE together with the scientific data. It has operational and data interfaces with the CDAE and PAE, and links with the ESA ground station network for early-orbit and contingency-phases support.
During the Launch and Early-Operations Phase (LEOP), round-the-clock service was provided, including the use of supplementary ESA ground stations when required. In fact, 24 hour/day service was provided for the first three months of the mission, while commissioning took place and the payload was calibrated. For the rest of the mission, only a single shift of 8 hours a day is necessary, with on-call engineering support.
Normal satellite operations follow a regular routine of measurements by the onboard instruments, supported by operation of the attitude and drag-free control. No attitude or orbit manoeuvres are needed, nor changes to the operational status of the instruments. Therefore, mission planning is basically be limited to the selection and scheduling of ground passes for telemetry and telecommand, satellite and instrument health checks, and data transmission, processing and delivery.
Different procedures applied during the instrument set-up and calibration phases, and in case of contingencies. The iterative calibration procedures are autonomous, based on dedicated operations schedules which are validated prior to uplinking, by means of the spacecraft software simulator.
Severe contingencies require quick reaction times, because of the very low operational altitude of the satellite. The most critical problem would be an orbit-control failure during the blind orbits. Knowledge of such a failure might only become possible up to 12 hours after the event, during which time the orbit will have evolved differently from the predictions and the visibility periods will have shifted away from the nominal pattern. Countermeasures for such cases are to be taken both at satellite level and at mission-control level. The first include robust control strategies and different levels of autonomous spacecraft safe modes, including autonomous spacecraft navigation based on GPS and onboard orbit propagation algorithms. On the ground-control side, contingency actions include increasing the frequency of satellite interrogation (by making use of all contact opportunities), search scans performed by the ground antenna, satellite initiated ground contacts, and calling on support from the other ESA network stations.
Processing and Archiving Elements (PAE)
The PAE performs data collection, processing to Level 1, archiving and delivery to the scientific consortia responsible for the production of the Level 2 geophysical products. As part of the generation of Level 1b data products, orbit determination based on the payload data is performed.
The PAE has data interfaces with the CDAE (to receive Level 0 data), and operational and data interfaces with the MSCE (for receiving operational status information and transmitting samples of Level 1 data for operational use).
In addition, the PAE has interfaces with the scientific consortia (for delivery of Level 1 data and reception of Level 2 data for archiving) and with the IGS for reception of GPS station data. None of these links require real-time action, and therefore transmission via normal commercial links is adequate.
The ultimate scientific output of GOCE is a map of gravity anomalies and the geoid, which need extensive data processing and analysis by specialists. This analysis is performed by the scientific consortia and requires large data sets (encompassing at least two months of observations).
Smaller data sets will be used on shorter time scales to check the consistency of the data (e.g. by comparing gravity gradients over areas where good-quality information already exists). Procedures for data validation for operational purposes will be prepared by the scientific consortia and implemented as standard routines to be run at the PAE on a daily basis.
Last update: 16 September 2010