ESA - Galileo-GIOVE Mission patch.
12 April 2013
ESA’s retired GIOVE-A navigation mission has become the first civilian satellite to perform GPS position fixes from high orbit. Its results demonstrate that current satnav signals could guide missions much further away in space, up to geostationary orbit or even as far as the Moon.
GIOVE-A has been able to fix its position, velocity and time from GPS signals, despite orbiting more than 1000 km above the downward-pointing US satellites.
“Satellite navigation has become almost as indispensable for most low-orbiting satellites as it is for car drivers and other terrestrial users,” says ESA’s Steeve Kowaltschek.
GIOVE-A
“Satellites equipped with satnav receivers can continuously monitor their orbit in space, enabling largely autonomous operations with limited ground intervention.
“GIOVE-A’s three months of data show that future geostationary satellites could operate in the same way, bringing real competitive advantage to the multi-billion-euro telecommunications satellite market.”
Long-life satellite
Launched in 2005 to claim radio frequencies and test hardware for Europe’s Galileo satnav constellation, the Galileo In-Orbit Validation Element-A, or GIOVE-A, mission far outlasted its original two-year design life.
It was formally decommissioned by ESA in the middle of last year, once the first Galileo satellites completed their orbital commissioning. Having been moved into a graveyard orbit about 100 km above Galileo’s orbital altitude of 23 222 km, control was passed to its prime contractor Surrey Satellite Technology Ltd of Guildford, UK.
GIOVE-A on the launch pad
SSTL then collaborated with ESA experts to employ the aged satellite for experimental satnav reception.
The tests used a satnav receiver that had been activated for only 90 minutes during the very beginning of the satellite’s seven-year operational life.
“We have been really encouraged by the initial results from our receiver,” said Martin Unwin at SSTL. “Our patience has finally been rewarded, and we would like to make the best of this unique opportunity.”
SSTL is able to upload new software to the receiver in orbit, and has been able to apply sophisticated software algorithms to help detect faint satnav signals.
Further work is planned to refine operation through the use of an accurate onboard clock and orbit-estimating algorithms.
Taking a sideways look
GPS satellites – like those of Galileo, Russia’s Glonass or their Japanese, Chinese and Indian counterparts – aim their antennas directly at Earth.
Side lobe satnav signals available to satellites in higher orbits
Any satellite orbiting above the GPS constellation can only hope to detect signals from over Earth’s far side, but the majority are blocked by the planet. For a position fix, a satnav receiver requires a minimum of four satellites to be visible, but this is most of the time not possible if based solely on front-facing signals.
Instead, GIOVE-A makes use of signals emitted sideways from GPS antennas, within what is known as ‘side lobes’. Just like a flashlight, radio antennas shine energy to the side as well as directly forward.
Related links:
SSTL (UK): http://www.sstl.co.uk/
Mission accomplished for Galileo's pathfinder GIOVE-A: http://www.esa.int/Our_Activities/Navigation/Mission_accomplished_for_Galileo_s_pathfinder_GIOVE-A
Control Systems: http://www.esa.int/Our_Activities/Space_Engineering/Control_Systems
Images, Text, Credits: ESA / P. Carril / P. Müller.
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