The two Galileo navigation satellites left in incorrect orbits due to a rocket malfunction will be moved to improve their usability.
Controllers from the Galileo Control Centre in Oberpfaffenhofen, Germany, will perform a series of manoeuvres this month to change the parameters of the satellites’ faulty orbits to better match the original requirements.
“The new orbit will fly over the same location every 20 days,” explained Daniel Navarro-Reyes, Galileo mission analyst from the European Space Agency (Esa). “The standard Galileo repeat pattern is every 10 days, so achieving this will synchronise the ground track with the rest of the Galileo satellites.
The spacecraft, comprising the Galileo satellite navigation constellation, are designed to circle the Earth in perfectly circular orbits at an altitude of 23,222km. However, an anomaly of the fourth-stage of the Russian-made Soyuz rocket that carried the satellites to space in August this year left them stranded in elongated orbits with the highest point at nearly 26,000km and the lowest at barely 14,000km.
Using the satellites’ on-board fuel, the controllers will attempt to raise the lowest point of the elliptical orbit to 17,339 km. There is not enough fuel on board the satellites to bring them all the way up to the originally required 23,222km
The manoeuvre will not only allow the satellites to function, at least to a certain extent, as part of the constellation, but also to avoid the most extreme environment of the so-called Van Allen radiation belts – areas containing highly charged particles held around the Earth by the planet’s gravity.
“In addition, from a user receiver point of view, the revised orbit will reduce the variation in signal levels, reduce the Doppler shift of the signal, and increase the satellite’s visibility,” Navarro-Reyes said.
“The orbit will also allow Galileo’s Earth Sensor to hold a stable direction for the satellite’s main antenna to point at Earth. Right now, when the satellite dips to its lowest point, Earth appears so large that the sensor is unusable. The satellite relies on gyroscopes alone, degrading its altitude precision.”
Multiple European space agencies including Esa and the French space agency CNES are taking part in the recovery of the two satellites, the first of the 26 Full Operating Capability Galileo satellites to have been launched. Four In-Orbit Validation satellites had been successfully launched previously.
Over the next two weeks, the controllers will adjust the orbit of the first of the two satellites, followed by the second one if the operation proves successful.