How was geostationary orbit created
ESA satellite ARTEMIS on geostationary orbit - How a quasi-loss became a complete success
ESA INFO 04-2003. Friday, January 31, 2003, late afternoon: After a final corrective maneuver, ESA's ARTEMIS communications satellite has reached its intended position in geostationary orbit. This brought a spectacular rescue operation to a successful conclusion after 18 months.
The unusual route that ultimately led ARTEMIS to its deployment position was long, difficult and lined with unexpected problems. However, thanks to the expertise of a team of engineers and other specialists from ESA, the main contractor Alenia Spazio, the Astrium company responsible for the development of the ion propulsion system and the operator of the control station in Fulcino, Telespazio, as well as the use of an experimental system not intended for this purpose, the mission was able to be carried out to be saved. The ion propulsion system that ARTEMIS is actually equipped with in order to be able to correct any drift from the target orbit became the key to negotiating the last 5,000 km of the satellite up to its geostationary orbit.
Because of a malfunction in its upper stage, the Ariane-5 had deployed the satellite in an elliptical orbit that was too low: the apogee (greatest distance from Earth) was only 17,487 km and thus far from that of the targeted geostationary transition orbit, i.e. 35,853 km. Specialists from ESA and industry responded immediately with a series of innovative steering maneuvers: Using almost all of its chemical fuel, ARTEMIS was able to escape just a few days after starting the initial orbit that led it through the deadly Van Allen radiation belt, and a circular parking orbit unscathed Reach at an altitude of 31,000 km.
A long way to go to geostationary orbit
Since then, rescue efforts have continued unabated using the four ion thrusters attached to the satellite in redundant pairs. These new types of engines do not work with conventional chemical fuel, but with ionized xenon gas. Originally, they were only intended to regulate the inclination of the satellite with pulses perpendicular to the plane of the orbit. However, the rescue procedure required impulses in the orbit level in order to lift the satellite onto its final orbit. This was made possible by the fact that the satellite was rotated in the orbit plane by 90 ° compared to its normal position.
With optimal use of the flight configuration of the satellite, new strategies were developed not only to raise the orbit height, but also to counteract the natural increase in the orbit inclination. In order to implement these new strategies, new path and attitude control procedures, a new station network and new flight control procedures were necessary.
The new method for controlling the ion thrusters included completely new control methods that had never been used on a communications satellite, as well as new interface functions for processing remote control, telemetry and other data. In total, around 20% of the original satellite control software had to be rewritten. Thanks to the reprogrammable on-board control concept, these changes could be transmitted to the satellite in the form of software packages via an uplink data link - a total of 15,000 words and thus the most extensive reprogramming of flight software that has ever been carried out on a communications satellite.
At the end of December 2001, work on the new software was completed; the satellite simulator was used as a test bench for their validation. With the characterization of the four ion thrusters, all preparatory work was completed, whereupon the orbit raising maneuvers with the ion propulsion system were initiated on February 19, 2002.
Since the start of the maneuvers, the satellite controllers had to react to numerous unforeseen situations, since a realistic test of the new strategy was only possible on the satellite itself. In contrast to conventional flight acceptance tests, no test bench was available for the detailed testing of this scenario.
Thanks to the high flexibility and redundancy of the system design, the orbit elevation could be continued steadily, albeit more slowly than theoretically possible. With the modest thrust of its ion thrusters of only 15 millinewtons - something like trying to propel a large cargo ship with an outboard motor - ARTEMIS covered an average of 15 km per day.
Payload testing and performance testing
Several months passed between the arrival at the park runway and the start of the runway lifting maneuvers, which were used for tests with the payloads to check their performance.
The payload tests extended over the period November / December 2001. They could only be carried out every five days, namely when the beam of the antenna on ARTEMIS intended for the uplink “illuminated” the ESA test station in Redu (Belgium). Further limitations resulted from the fact that some payload frequencies can only be used when ARTEMIS is on or near its nominal orbit position.
Nevertheless, enough opportunities were found to provide evidence that all payloads (S-band, Ka-band and optical data relay payload, navigation and L-band cellular payload) are working properly and that their performance is in line with the test results prior to launch ie fully meet the specifications.
It was also proven that the closed-loop system for tracking the Ka-band antenna for satellite-to-satellite connections works as planned. The antenna picked up a signal sent by the station in Redu and automatically maintained the connection while ARTEMIS slowly drifted across the sky.
World premiere before the finish line
Most spectacular, however, was the proof of the SILEX relay function. After an initial successful functional test using the optical ESA ground station on Tenerife, an optical connection was established between ARTEMIS and the French remote sensing satellite SPOT-4. On November 30, 2001, for the first time worldwide, image data from a low-flying spacecraft was transmitted by laser to an (almost) geostationary satellite, from where it was forwarded to the data processing center in Toulouse.
A total of 26 attempts to establish the optical connection were made, all of which were successful. In no case did the connection break before the scheduled time. The transmission quality was almost perfect: the measured bit error rate was 1: 109, which means that out of 1 billion bits sent, only one was received incorrectly.
Ion propulsion as a savior in an emergency
Compared to the hectic operation in the days immediately after take-off, when ARTEMIS had to be transported to a safe parking lane as quickly as possible, it was not easy to get used to the slow increase with the ion drive, and outsiders like this phase as monotonous and appear boring. The technicians and engineers who were responsible for maintaining a steady rate of climb felt quite differently.
From February 2002, when the new attitude control method came into use and the ion thrusters began to lift the satellite up with almost imperceptible thrust, they were constantly under high working pressure. Almost every week brought new problems with them, most of which only concerned minor anomalies, but had to be clarified and sometimes also resulted in an interruption in thrust.
In addition to carefully monitoring and optimizing the performance of the ion thrusters, the satellite controllers also tried out various attitude control techniques in order to align ARTEMIS in such a way that the momentum of the ion thrusters could be used as efficiently as possible. The planning and control of satellite operations, including regular fine-tuning of critical parameters and the management of ground station contacts, presented a constant challenge.
In October, ARTEMIS passed the third and final eclipse period since its start. During the eclipse, the earth's shadow hides the sun for two hours per revolution, during which the satellite has to be aligned with the earth for reasons of energy supply and position control and the ion drive has to be switched off. These maneuvers cost time and effort.
After overcoming all of these difficulties, the satellite controllers had to concentrate on planning the positioning of the satellite on the geostationary orbit and its initial use.
At heights only a few hundred kilometers below the geostationary ring, it takes several weeks for the satellite to drift once around the earth. In order not to miss the target, the drift speed must therefore be set so that the satellite arrives at the targeted longitude (21.5 ° East) exactly when it reaches the geostationary altitude.
The required orbit adjustments were made using small chemical thrusters that were ignited for the first time since launch. The first maneuver took place in December 2002, followed by two more in January 2003. The drift speed was reduced to a few degrees per day when the satellite made its last pass over Europe and finally took up its operational position in the geostationary orbit.
After the last maneuver on January 31, the joy was great. From its optimal flight position for ion propulsion, the satellite could now be aligned with earth for normal operation, and ion propulsion was hailed as a savior in an emergency. Finally, the satellite controllers were also able to release the global ground station network used to control the satellite for other tasks.
Now that ARTEMIS has achieved its goal, it will start its planned ten-year operation, for which it still has a sufficient supply of chemical fuel.
A large number of users are already waiting for ARTEMIS to go into operation. During the first few weeks on the mission track, his payloads were subjected to thorough functional tests by the facility responsible for orbital testing in Redu (Belgium), which showed that all payloads were working properly. An optical connection with SPOT-4 was established again.
The satellite can now be used for its first users: SPOT-4, ENVISAT, EGNOS and EUTELSAT / Telespazio. A preparatory experiment is also to be undertaken with NASDA's Japanese Earth observation satellite ADEOS-II. In the future, ARTEMIS will also serve as a relay station for the ESA's automatic transfer vehicle and the Columbus elements of the International Space Station.
After his rescue, in the course of which ARTEMIS performed a series of pioneering technical achievements - first optical relay connection between satellites, first comprehensive reprogramming of a communications satellite, first transfer to geostationary orbit by means of ion propulsion, the longest planned drift phase of a satellite to date - he is now ready to test basic technologies and to promote future European data relay services. A bright future for this incredible mission!
For more information:
Artemis project manager
ESA / ESTEC
Tel .: 00 31 71 565 3168
Fax: 00 31 71 565 4093
Email: [email protected]
ESA Public Relations Department
Media Relations Unit
Tel .: +33 (0) 184.108.40.20655
Fax: +33 (0) 220.127.116.1190
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