November 5, 2011
Galileo Satellites Handed Over To Control Center In Germany
Europe´s first two Galileo satellites have reached their final operating orbits, opening the way for activating and testing their navigation payloads.
Marking the formal end of their LEOP Launch and Early Operations Phase, control of the satellites was passed Nov. 3 from the CNES French space agency center in Toulouse to the Galileo Control Center in Oberpfaffenhofen in Germany.Oberfaffenhofen, operated by the German Aerospace Center DLR, will be in charge of the satellites' command and control for the whole of their 12-year operating lives.
The two Galileo satellites were launched by Soyuz from French Guiana on 21 October. Three hours and 49 minutes after launch, their Fregat-MT upper stage carried them into their planned 23 222 km orbit, where they were released simultaneously.
At this point, a joint team from CNES and ESA´s ESOC European Space Operations Center moved into action, beginning the crucial task of bringing the two satellites to life.
The first signals were heard almost simultaneously, confirming they were in good health — but there was still plenty of work to be done to keep them that way.
Like parachutists jumping from an aircraft, they were left tumbling through space. This spinning had to be brought under control before it was safe to deploy the power-giving solar panels, ending the satellites´ reliance on their rapidly dwindling battery power.
Once the reaction wheels steadied them, the satellites sought the Sun and began recharging their batteries, around 70 minutes after separation.
The thrusters were then tested — an important milestone because the Fregat had carried them most of the way into space but they would have to maneuver the last 100 km or so into their planned final orbits by themselves.
The pair then switched from Sun-pointing to Earth-pointing, using infrared sensors that detect our planet as a warm object in cold space.
After that, they settled into their normal mode, with the solar arrays tracking the Sun and the navigation antenna pointed towards Earth.
The section of satellite housing the sensitive atomic clocks — the most accurate ever flown in space for navigation purposes — is kept permanently cool in shadow to help stabilize their performance.
In this configuration, the Toulouse center commanded a set of thruster firings to relocate the satellites to their intended orbits inclined at 56º to the equator.
The handover to Oberpfaffenhofen occurred directly after LEOP was completed, taking place at 22:00 CET on 3 November.
Ground controllers began by encrypting the telemetry and telecommand links, ensuring secure satellite control. Then they started commissioning the platform, verifying that all prime and redundant subsystems perform as expected.
The next few days will see the navigation payload being switched on, marking the start of Galileo´s In-Orbit Test campaign. This rigorous check of the navigation signals is being conducted from ESA´s ground station in Redu, Belgium.
In particular, a 20 m-diameter antenna will measure the precise shape of the navigation signals to a very high degree of accuracy.
Once the navigation payload is fully checked-out and activated, a second Galileo Control Center in Fucino, Italy — operated by the Telespazio company - will oversee all navigation services.
All the entities participating in these activities - ESOC, CNES, DLR and Telespazio - do so under contract to SpaceOpal, a joint subsidiary company of DLR and Telespazio for Galileo operations.
Image 1: The first two of four Galileo In-Orbit Validation satellites were launched on 21 October 2011. Credits: ESA - P. Carril
Image 2: The Galileo launch dispenser, designed for Arianespace by RUAG Space Sweden, secured the twin Galileo IOV satellites in place during the take-off of their Soyuz ST-B. The satellites remained attached to the dispenser for about three hours 40 minutes after launch. Then, when the three-stage Soyuz ST-B´s additional Fregat MT upper stage reached the planned 23,222 km orbit, the dispenser fired a pyrotechnic separation system to release them in opposing directions. Credits: ESA - P. Carril
Image 3: Galileo's Ground Control Segment (GCS) in the Oberpfaffenhofen Control Center in Germany is in charge of overseeing the performance of the Galileo satellites Credits: DLR
On the Net: