Home Geospatial Applications Miscellaneous ESA fully salvages Artemis mission

ESA fully salvages Artemis mission

The unusual route taken by Artemis to get to geostationary orbit was long and hard, and beset with unfamiliar problems. But the mission was saved by the skills of a dedicated team of engineers and other specialists from the European Space Agency, Alenia Spazio, the prime contractor, Telespazio, responsible for satellite operations at the Fucino control centre, and Astrium, which designed the ion propulsion system, and by the use made of this experimental system, which had not been designed for such a task. The ion propulsion system originally on board Artemis to control small motion around its nominal position was the key to climbing the final 5000 km to reach geostationary height. Due to a malfunction in its upper stage, Ariane 5 left ESA’s telecommunications satellite Artemis in a lower than intended elliptical orbit. The apogee (maximum distance from Earth) was only 17 487 km, far short of the targeted geostationary transfer orbit with an apogee at 35 853 km.
From the start of orbit-raising operations, spacecraft controllers had to respond to many kinds of unforeseen situations, since the new strategy could only be tested realistically on the spacecraft itself. Unlike traditional pre-flight acceptance testing, no testbed is available to exactly replicate the current scenario. Thanks to the extreme flexibility and the redundancy inherent in the system design, steady progress in the orbit-raising process was maintained, albeit at a lower rate than would theoretically be possible.
In November/December 2001 payload tests were performed. These tests could only be done every fifth day, when the Artemis feeder link antenna beam “illuminated” ESA’s test station in Redu (Belgium). Further constraints arose from the fact that some payload frequencies can be used only when Artemis is at, or close to, its nominal orbit position. Nevertheless, enough opportunities were found to demonstrate that all payloads (S-band and Ka-band and optical data relay, navigation and L-band mobile payload) were available and that their performance was in line with pre-launch results. In other words, that they fully complied with specifications.
Correct operation of the closed-loop tracking system for the Ka-band inter-orbit antenna was also demonstrated. The antenna acquired a signal transmitted from Redu and maintained the link automatically while Artemis drifted slowly across the sky.
Following successful initial commissioning using ESA’s optical ground station on Tenerife, the optical link was established between Artemis and SPOT 4. On 30 November 2001, for the first time ever, image data collected by a low-flying spacecraft were transmitted by laser to a (quasi-) geostationary satellite and from there to the data processing centre in Toulouse.
In total, 26 attempts were made to establish the optical link and all 26 were successful. The link was never lost before the preprogrammed point in time. Link quality was almost perfect: a bit error rate better than 1 in 109 was measured. This means that 1 bit at most is received erroneously per 1 000 000 000 bits transmitted.
After the hectic and exciting orbital recovery operations in the days after launch, it was not easy to come to terms with the incremental progress provided by the ion propulsion, and for those not involved in the satellite operations it must have seemed a monotonous and uneventful activity. Nothing could be further from the truth for the operators and engineers responsible for maintaining a steady rate of climb.
Since the new attitude control mode was commissioned in February, and the ion engines started to expand the orbit with an almost imperceptible thrust, the workload has been gruelling and almost every week has brought new problems to be solved. Although generally minor, these anomalies needed investigation and sometimes resulted in an interruption in effective thrusting, slowing progress.
The planning and sequencing of satellite mode changes, including regular updating of critical parameters, and the management of ground station contacts involved steady but considerable background tasks.
In October the satellite left the third and final eclipse season since its launch. During eclipse the earth’s shadow hides the sun for some two hours each orbit and for reasons of power and attitude control the satellite has to be commanded from thrust mode to earth-pointing mode and the ion thrust turned off. These manoeuvres cost time and effort.
When the last manoeuvre was performed on 31 January it was an emotional moment. From the attitude control mode, which had sustained the ion thrusting for so long, the satellite was turned to point to earth for normal operations and the ion thrusters themselves were the toast of the day. Ground controllers were able to stand down the network of ground stations around the world that had helped in commanding the satellite. Now on station, Artemis will function as originally planned and there is sufficient chemical propellant for 10 years’ operation.
The satellite can now be made available to serve its first users: SPOT4, ENVISAT, EGNOS and EUTELSAT/Telespazio. A preparatory test will also be made with NASDA’s Earth observation mission ADEOS-II. Other users planning to use Artemis in future include ESA’s Automated Transfer Vehicle and Columbus elements of the International Space Station. Not only has Artemis clocked up a number of unique first-time applications during its recovery action – first optical inter-orbit satellite link; first major reprogramming of a telecommunications satellite; first orbital transfer to geostationary orbit using ion propulsion; longest ever operational drift orbit – but it will provide the promotional opportunity and stimulus for future European data relay services. A promising future for this incredible mission!