Inside GNSS Media & Research

JUL-AUG 2018

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40 Inside GNSS J U L Y / A U G U S T 2 0 1 8 Limited fuel meant the satellites' originally assigned destination was out of reach, but their orbits could still be modified to make them suitable for nav- igation purposes. A recovery plan was devised between ESA's Galileo team, ESOC flight dynamics specialists, along with personnel from SpaceOpal, OHB with the support of the French (CNES), Italian (ASI), German (DLR) and Brit- ish (UKSA) space agencies experts. e scheme involved a multiple series of maneuvers, gradually raising the lowest point of the satellites' orbits more than 3,500 kilometers while making them more circular and positioned 180º of each other. e fih Galileo entered its correct- ed orbit at the end of November 2014, followed by the sixth Galileo in March 2015 with a total of 11 and 14 orbit rais- ing maneuvers, respectively. e com- mands were issued from the Galileo Control Centre at Oberpfaffenhofen, Germany by SpaceOpal, guided by cal- culations from a combined ESA-CNES flight dynamics team. Commands were uploaded to the satellite via an extended network of ground stations, made up of Galileo stations and additional sites coordinated by CNES. Satellite manu- facturer OHB also provided expertise throughout the recovery, helping to adapt the flight procedures. In the new orbit, the satellite's radiation exposure has also been greatly reduced, ensuring reliable performance for the long term. The corrected orbit of the satellites is shown in Figure 3 . The recovery strateg y achieved a slightly elliptic orbit due to insufficient fuel in the satellites' propulsion system to reach the nominal orbit (See Table 1 ). e major advantage in this slightly elliptic orbit with the two satellites sep- arated by 180º is that the ground track repetition pattern is 37 orbital revolu- tions in 20 siderial days, with a sub-cycle of 10 days where the two satellites swap locations in the geometry. is is com- patible to the nominal Galileo constel- lation having a repetition pattern of 17 revolutions in 10 siderial days ( Figure 4 ). is allows a proper predictability and repeatability of the constellation and an inclusion of the satellites into the plan- ning functions of the system. e revised, more circular and high- er altitude orbit means the satellites' Earth Sensors can be used continuously, ECCENTRIC SATELLITES FIGURE 3 Corrected orbit of Galileo 5 and 6 Satellite Semi- Major Axis (km) Eccentricity Inclination (deg) RAAN (deg) Argument of Perigee (deg) Mean Anomaly (deg) Galileo 5 27977.6 0.162 49.850 52.521 56.198 316.069 Galileo 6 27977.6 0.162 49.850 52.521 56.198 136.069 Nominal 29599.8 0.0 56.0 Sat. specific 0.0 Sat. Specific Table 1 Galileo 5 & 6 reference orbital parameters following orbital corrections FIGURE 4 Ground track comparison between recovery orbit (37 rev / 20 days in red) and nomi- nal Galileo orbit (17 rev / 10 days in blue)

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