N° 40–1993: OLYMPUS: End of mission

Olympus entered its fifth year of operations in July of this year. Throughout its mission, the satellite payloads were used by a wide range of European and Canadian Organisations to develop and test advanced satellite communications technologies and provide new services. The results of these were the subject of an international Conference held in April 1993, in Seville, Spain, which brought together 300 scientists, engineers, educators and broadcasters, who reported on their achievements using the satellite. The proceedings of this conference, which contain the approximately 130 papers which were presented, are now being distributed.

Olympus carried four separate payloads, a two channel high power direct broadcasting payload operating at 18/12 GHz, a four channel 12/14 GHz Specialized Services Payload, a 20/30 GHz payload for advanced communications experiments, and a 12/20/30 GHz beacon package for propagation experiments.

The Ka band (20/30 GHz) transponders have been exploited extensively by numerous organisations to demonstrate the use of small terminal networks and for video conferencing and satellite news gathering applications including daily transatlantic transmissions. These have shown a high degree of service availability at these new frequencies. In addition, this payload was used to constitute the geostationary end of a data relay link between the Inter- Orbit Communications (IOC) terminal mounted on the Agency's EURECA spacecraft and ground controllers and experimenters during that highly successful recently completed mission.

Olympus has also been instrumental in the development of new applications, such as distance learning, data distribution and new commercial services. In the distance learning field alone, over 100 organisations, in 12 countries have used Olympus to develop training courses which are now part of the established satellite based educational infrastructure. Several of these operations have now been transferred to the EUTELSAT space segment.

In the broadcast field, Olympus was the initial test bed for a number of satellite broadcast programmes, which are now running on a commercial basis, including RAISAT and the BBC World Service. It was also used for experimental high definition TV broadcasts, to assist in the development of that new technology.

Olympus provided valuable opportunities to collect propagation data for hundreds of scientists across Europe and North America. The 20/30 GHz data, in particular, forms a major part of the statistical information available globally.

It will be remembered that control of OLYMPUS was accidentally lost in the summer of 1991 and, following an uncontrolled orbital drift around the world, in a frozen state at temperatures of some minus 60 degree Celsius, the satellite was the subject of a spectacular recovery action which allowed it to be put back into service again. The recovery at that time, however, used a large amount of fuel, and little was left on-board to complete the intended five year mission. By instituting new procedures to conserve fuel, it was nevertheless hoped that OLYMPUS could complete its nominal five year mission in early 1994 and then still have a reserve amount of fuel that could be used to re-orbit the satellite, i.e. remove it several hundred kilometres from the geostationary orbital ring. This re- orbiting at the end of a geostationary satellite mission is ESA policy in order to eliminate the probability that a satellite left to drift in that orbit could later strike or interfere with another satellite.
The event which occurred on the night of August 11 made it impossible to achieve the planned five years of operations. When the satellite lost earth pointing, an automatic mode change was initiated, but the on-board control loops were not immediately successful in despinning and reorienting the satellite. The firing of thrusters caused an orbital drift and used much of the remaining fuel. Actions then had to be taken by the spacecraft controllers on the ground to over-ride the automatic functions and manually control the satellite. These actions were successful, but the amount of fuel assessed to still be available would not have been sufficient to re-establish three-axis controlled attitude of the satellite, stop its orbital drift, return it to its proper position in the geostationary ring and still retain a margin for eventual withdrawal from the geostationary orbit. It was therefore decided to initiate re-orbiting at once. In order to achieve this, it was first necessary to despin the satellite to less than about 2 deg/second.

The re-orbiting process has been initiated and is being carried out several hours per day. The satellite is now in an orbit with respectively an apogee some 195 km, and a perigee some 390 km below geostationary. It was decided to re-orbit to a lower rather than higher altitude due to the fact that the incident itself caused an orbital perturbation in this direction, and, consequently, there would not have been sufficient fuel to retransfer the spacecraft to a higher orbit. This activity will continue for several more days, in order to reach the lowest possible orbit. Several end-of life tests will then be conducted with the satellite, after which steps will be taken to deplete the pressurant remaining in its tanks. Once the satellite is in this "safe" configuration, it will be turned off and its mission will have come to an end.