Inside GNSS Media & Research

JUL-AUG 2018

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Page 38 of 59 J U L Y / A U G U S T 2 0 1 8 Inside GNSS 39 and to investigate possible operational scenarios. What followed was the his- tory of one of the most remarkable suc- cesses in the recovery of a failed launch. is success has already allowed to put these satellites in use for search and res- cue services and for an ambitious test of Einstein's General eory of Relativity. is article is the first in a series on the extraordinary journey of Galileo sat- ellites 5 and 6 from launch to mission recovery and exploitation. Ultimately, the final stage will bring these satellites into service for navigation usage. The Mishap At the beginning everything seemed well, but then the Galileo's Launch and Early Orbit Phase (LEOP) team at the European Space Operations Cen- tre (ESOC) in Darmstadt, Germany, responsible for controlling the satel- lites, raised the alarm. Unusually low power and instability in the radio signals received from the two satellites at the telemetry stations operated by the Euro- pean Space Agency (ESA), the French National Space Agency (CNES) and the Swedish Space Corporation (SSC) showed that the satellites were not in their expected orbit. e satellites were le in an elliptical orbit with the apogee at 25,900 kilometers above Earth and the perigee at 13,713 kilometers, with the orbit wrongly inclined by 49.77º with respect to the equator ( Figure 1 ). e first priority was to ensure the satellites were safe a nd s t able . Wit h o n e s o l a r w i n g d e p l o y e d , t h e r e was enough power to en s u re s p a c e - craft viability and safety. The second urgent priority was to determine a suf- f icient ly accurate orbit to ensu re a p r o p e r p o i n t i n g of t he a ntenna in order to mon itor and control the sat- ellites. Within four hours of the first sig- nal reception, the team determined the actual orbit, then generated new com- mands to repoint the ground anten- nas and established robust radio links. Over the following few days, with sup- port from ESA's Galileo project and satellite manufacturer OHB, the LEOP team developed, validated and rehearsed procedures to release the trapped solar wings, boosting the available power to its nominal value as they pointed toward the sun. Both satellites were left in a mode pointing to the sun, but they could not be used for navigation purposes nor be tested as the Earth Sensors used to point their navigation antennas stopped working around perigee because Earth's disc filled their field of view. e Independent Inquiry Commis- sion of Arianespace, European Com- mission (EC), ESA experts and Russia's Space Agency Roscosmos concluded that the problem was due to a misfir- ing Fregat upper stage. Corrective mea- sures were performed in the subsequent launches to prevent recurrence of the problem ( Figure 2 ). Mission Recovery In parallel to the Independent Inquiry Commission, a multidisciplinary team of engineers at ESA initiated a number of mission recovery analysis in order to investigate possible operational sce- narios which would allow to restore, as much as possible, the intended mission objectives and maximise the benefits for the program. Given the high eccentricity and low perigee of the injection orbit, the mission drivers for the recovery were to: • Reduce the L-band power dynamic range between apogee and perigee • Reduce Doppler as to faci l itate L-band ground receivers to lock the signal • Increase visibility time for receivers • Ensure perigee high enough to allow Earth Sensors to be operational over the complete orbit • Reduce exposure to the Van Allen radiation belts to minimize satellite equipment degradation • Improve contribution to the global constellation performance Different scenarios were prepared by ESA, supported by experts from indus- try and national space agencies, and proposed to the owner of the satellites, the EC, which gave the "green" light to initiate the recovery actions. In September 2014, the control of Gal- ileo satellites 5 and 6 was transferred from the LEOP team at ESOC in Darmstadt, Germany to the Galileo Control Centre (GCC) in Oberpfaffenhofen, Germany, operated by SpaceOpal. But the satellites' incorrect orbits meant their navigation payloads could not be switched on for testing due to the non-nominal opera- tion of the Earth Sensors. In addition, the lower orbits were exposing them to heightened levels of harmful radiation. FIGURE 1 Bottom view from orbital plane of nominal orbit (in blue) and injected orbit (in green) FIGURE 2 Soyuz ST-Fregat and Galileo satellites

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