Minimize Ground Segment

The GOCE mission has a main ground station in Kiruna, Sweden, supported by stations located in Svalbard, Norway, and in Antarctica. ESA-ESOC, in Germany does the satellite control and data acquisition. Processing to Level 1b, archiving, product distribution and mission performance monitoring will take place in ESA-ESRIN in Italy. This is an integral part of the Payload Data Ground Segment, with higher-level processing performed by the High level Processing Facility for GOCE (HPF), a distributed system made up of a consortium of ten European universities and research institutes.

The GOCE ground segment is composed of four main elements:
  • Command and Data Acquisition Element (CDAE)
  • Mission and Satellite Control Element (MSCE)
  • Payload Data Ground Segment (PDGS)
  • High Level Processing Facility Element (HPF)
Command and Data Acquisition element (CDAE)

The CDAE handles all direct interactions between the ground and the satellite, under the control of the MSCE. It is responsible for telemetry reception, checking and temporary storage, monitoring the health status of the satellite, and telecommand validation and uplinking. The CDAE comprises the ground antenna with acquisition and tracking modules, facilities for data storage and pre-processing, and operations and data distribution interfaces.

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 at S-band (1.2 Mb/s), and has reached operational maturity after being used successfully during the ERS and 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, are made at a maximum telemetry rate of 850 Kb/s. The fourth, afternoon pass will be used, when needed, for sending telecommands. This strategy ensures that the on-board 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 is also open.

The CDAE provides immediate storage of the received telemetry stream and near real-time 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 SSTI receiver data processing. Transmission to the MSCE for operational purposes occurs via the electronic link on a near real-time basis. Transmissions to the PDGS can occur on a more relaxed schedule, for example by using the file transfer protocol (FTP) service during night hours.

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 manages, monitors 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 if required. In fact, 24 hour/day service was given 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 engineering support on-call.

Normal satellite operations follow a regular routine of measurements by the on-board instruments, supported by operation of the attitude and drag-free control. No attitude or orbit manoeuvres are needed, and neither are 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 apply during instrument set-up and calibration phases, and in case of contingencies. The iterative calibration procedures are autonomous, based on dedicated operations schedules which will be validated prior to uplinking by means of the spacecraft software simulator. Severe contingencies will 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 '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 predicted one 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 on-board 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.

Payload Data Ground Segment (PDGS)

The PDGS performs data collection, processing to Level 1, archiving and delivery to the HPF 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 PDGS 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 HPF (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 the gravity anomalies and geoid, needing extensive data processing and analysis by specialists. This analysis is performed by the HPF 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 PDGS on a daily basis.

High Level Processing Facility for GOCE

The purpose of the High Level Processing Facility for GOCE is the systematic production of GOCE level 2 data products, which are orbits and gravity field solutions of various kinds. While so-called quick looks or rapid products are mainly of interest for the monitoring of satellite performance, the final and precise products represent the official GOCE level 2 products, used in turn for level 3 processing by the GOCE end-users (oceanographers, solid Earth scientists, geodesists and others).

The generation of level 2 products - orbits and gravity fields - is carried out based on level 1b data generated at ESRIN. The level 1b products consist mainly of gravity gradients in the gradiometer reference frame and pseudo-ranges and phases from the GPS receiver. These require comprehensive scientific data processing before they can be translated into geophysical values, the main products expected by the scientific users.

For level 2 processing a wide range of ancillary data are required on an ongoing basis, including Earth rotation parameters, GPS orbit, clock and ground station data, satellite laser ranging data and atmospheric parameters. Additional supporting data including planetary ephemeris, solar flux, geomagnetic indices, tide models, digital terrain models, external gravity field information and others also have to be acquired.

The HPF is a distributed system made up of a consortium of ten European universities and research institutes, developed and operated through an ESA contract and run by a principal investigator and management team.

The HPF is coordinated for ESA by the IAPG: Institute of Astronomical and Physical Geodesy at the Technical University Munich in Germany, and comprises the following partners:

  • SRON: National Institute for Space Research, Utrecht, the Netherlands.
  • FAE/A&S: Delft University of Technology, Faculty of Aerospace Engineering, Astrodynamics and Satellite Systems, the Netherlands.
  • CNES: Centre Nationale d'Etudes Spatiale, Toulouse, France.
  • TUG: Institute of Navigation and Satellite Geodesy, Technical University Graz, Milan, Italy.
  • POLIMI DIIAR: Sez. Rilevamento Politecnico di Milano, Italy.
  • AIUB: Astronomical Institute, University Bern, Switzerland.
  • GFZ: GeoForschungsZentrum Potsdam, Dep. 1 Geodesy and Remote Sensing, Germany.
  • ITG: Institute for Theoretical Geodesy, University Bonn, Germany.
  • UCPH: Department of Geophysics, University Copenhagen, Denmark.