The Gravity Field and Steady-State Ocean Circulation Explorer (GOCE; pronounced "go-chay") is measuring the Earth's gravity field and model the geoid with extremely high accuracy. It was launched on 17 March 2009. Owing to its sleek shape, GOCE is often cited as one of ESA's most elegant space probes.
The geoid, defined by the Earth's gravity field, is a surface of equal gravitational potential. It follows a hypothetical ocean surface at rest (in the absence of tides and currents). A precise model of the Earth's geoid is crucial for deriving accurate measurements of ocean circulation, sea-level change and terrestrial ice dynamics – all of which are affected by climate change.
The geoid is also used as a reference surface from which to map all topographical features on the planet. An improved knowledge of gravity anomalies will contribute to a better understanding of the Earth's interior, such as the physics and dynamics associated with volcanism and earthquakes and also further our knowledge of land uplift due to post-glacial rebound.
The GOCE Flight Operations Segment (FOS), is located at ESOC, Darmstadt, Germany.
|ROLE||Earth observation (EO)|
|LAUNCH DATE||17 March 2009|
|LAUNCH MASS||~1000 kg|
|ORBIT||Sun-synchronous, dawn-dusk, ~250 km|
|+ Will model Earth's geoid with extremely high accuracy & spatial resolution +|
The Flight Control Team
The GOCE Flight Control Team works from a combined Earth Observation Dedicated Control Room, located at ESOC.
Spacecraft Operations Manager Christoph Steiger, from Switzerland, manages a Flight Control team (FCT) of spacecraft operations engineers and spacecraft controllers. Most team members are shared with other Earth observation missions, thus increasing cost effectivesness. The FCT is responsible for all spacecraft operations, interfacing with other ground segment teams, including Flight Dynamics, Ground Facilities and Software Support.
Christoph Steiger joined the GOCE FCT in 2006 after having worked on ESA's Rosetta and Venus Express missions. He assumed SOM responsibilities in September 2009.
Mission operations overview
Launch and Early Orbit Phase (LEOP)
GOCE was launched on 17 March 2009 on a modified Russian intercontinental ballistic missile (ICBM) SS-19 launcher, which had been decommissioned as part of the Strategic Arms Reduction Treaty (START).
The adaptation of the SS-19, called 'Rockot', uses the original two lower liquid propellant stages of the ICBM, which has an excellent record of successful flights, in conjunction with a new third stage for commercial payloads. Rockot is marketed and operated by EUROCKOT, a German-Russian joint venture. Launch is from the Plesetsk Cosmodrome in northern Russia.
The launch went flawlessly. The launcher lifted off at 15:21 CET (14:21 GMT) and flew northward over the Arctic. About 90 minutes later, after one orbital revolution and two Breeze-KM upper-stage burns, the 1052 kg spacecraft was successfully released into a circular polar orbit at 283 km altitude with a 96.7º inclination to the Equator.
Contact with GOCE was established via ESA’s tracking station in Kiruna, Sweden, shortly after separation.
During the three days of LEOP, GOCE was operated around the clock from ESOC's Main Control Room with additional support by experts from the spacecraft manufacturer on site. The main operations in LEOP consisted of bringing the spacecraft to a stable configuration suitable for the full check-out of the spacecraft and ground segment in the commissioning phase.
Following completion of LEOP, operations were moved to the Earth observation Dedicated Control Room at ESOC, with team activities reduced to normal working hours.
Commissioning lasted until October 2009 and was dedicated to a full check-out of the spacecraft and ground segment. In particular, the ion propulsion system required to conteract the air drag at GOCE's low altitude had to be commissioned, as well as GOCE's highly-sensitive main payload, the Gradiometer.
As GOCE had been injected into its orbit with some additional altitude so as to perfom the complex commissioning activities, the orbit was left to decay naturally due to atmospheric drag from the initial altitude of 283 km to the desired altitude for science operations for 259 km.
Following completion of the orbit decay in September 2009, drag-free mode was resumed to stabilize the orbit. Routine science operations were started following a calibration of instruments.
Since September 2009 GOCE has been operating in drag-free mode at an altitude of 259 km, thereby acquiring data on Earth's gravity field. An occasional calibration of the Gradiometer is needed, however GOCE science operations do not require complex observation campaigns. Routine operations consist of a weekly planning and uplink of the operations time line covering all activities to be carried out, with the science data downlinked in each ground station contact.
Occasionally, drag-free mode is interrupted due to an anomaly on the spacecraft, which will then have to be investigated and resolved by the Flight Control Team. In 2010, some major problems on the main platform computer required extensive recovery operations, with the nominal mission eventually resumed.
The mission was originally planned to end in April 2011. Owing to the excellent scientific results, the good status of the spacecraft, and the large margin in the fuel budget for the ion propulsion system, the mission has been extended to the end of 2012.
The ground station - Kiruna
For routine operations, mission controllers use ESA's ESTRACK Kiruna antenna in Sweden as the prime station for command, control and tracking of GOCE.
Owing to the very low altitude of around 259 km at which GOCE is orbiting the Earth, the ground station contacts, or passes, are extremely short, with an average duration of five minutes (as compared to 15 min for a mission in a more common Earth orbit of around 800 km). GOCE's low orbit is also the reason why the spacecraft is only visable at Kiruna during six out GOCE's 16 daily orbits, despite the high latitude of the station.
To complement the six daily Kiruna passes, two passes per day are routinely scheduled on KSAT's Svalbard ground station. In case of contingencies, KSAT's Toll station located in Antarctica is used for providing additional visability.
A unique aspect of GOCE is that the orbit starts decaying rapidly due to the atmospheric drag whenever there is any interruption in drag-free operations. This may lead to problems when trying to acquire the spacecraft at the next ground station pass. Special measures are taken to allow for an efficient search for the spacecraft with the available antennas in such a case.
Ground segment & mission control system
The GOCE flight operations segment at ESOC uses the SCOS-2000 mission control system. With spacecraft command and control as its prime purpose the mission control system also comprises facilities for mission planning and for on-board software maintenance.
Other main components of the GOCE flight operations segment at ESOC are the flight dynamics system for orbit determination and prediction, a facility for remote control of the ground stations and the GOCE satellite simulator. This simulator is mainly used for validation of operational procedures and training of the Flight Control Team.
The preparation of the plan for GOCE's scientific operations is performed by the payload data ground segment located at ESA/ESRIN in Frascati, Italy. The scientific data downlinked from the spacecraft by ESOC are transferred to ESA/ESRIN for initial processing and distribution to scientists.
The platform and payload
The GOCE spacecraft has a unique design driven by the specifics of the mission. To minimize aerodynamic drag forces and torques at GOCE's low altitude, the spacecraft structure has a slim octagonal design (5m long, cross section of 1.1m2, launch mass 1050 kg), is fully symmetric and is provided with winglets for additional aerodynamic stability.
GOCE employs a drag-free attitude and orbit control system using an ion propulsion engine for continuous, closed-loop counteraction of the drag caused by the Earth's atmosphere. To minimize internal disturbances the satellite does not include any moving parts.
GOCE is the first spacecraft employing the concept of gradiometry, i.e. the measurement of acceleration differences over short baselines between proof masses of a set of accelerometers of an Electrostatic Gravity Gradiometer (EGG). The accelerations measurable are as small as one part in 10 million million of the gravity experienced on Earth, making the EGG about 100 times more sensitive than any accelerometers previously flown. The EGG is used to measure high-resolution features of the Earth's gravity field, while large-scale phenomena in the gravity field are obtained though analysis of GOCE's orbit as measured with a Global Navigation Satellite (GNSS) reciever on-board the spacecraft (the Satellite-to-Satellite Tracking Instrument - SSTI).
Unlike most other missions, GOCE has no clear separation between the 'platform' and the 'payload', with both the EGG and the SSTI measurements being used by the platfrom, i.e. the spacecraft, to keep the drag-free control working.
GOCE is also equipped with a GPS (global positioning satellite) receiver.