Satellite navigation systems were originally designed for terrestrial use. Some fifteen years ago, operators began equipping satellites in low earth orbit (LEO) with GNSS receivers for positioning purposes. Since satellites in LEO are below GNSS orbits (18 000 to 22 000 km), they have a similar view of GNSS constellations to the one that we have from Earth and hence they can use GNSS receivers similar to those used terrestrially.
Deploying GNSS for positioning on satellites in geostationary orbit is a much more daunting challenge. Spacecraft in GEO are orbiting above the GNSS constellations, whose antennas naturally point downwards, so the GEO satellites would need to catch the very weak signals that come from GNSS satellites orbiting on the other side of the Earth at distances ranging from 18 000 km to 36 000 km. However, using highly sensitive antennas and specialised receivers, it nonetheless appears possible to use GNSS to help guide GEO satellites from their geostationary transfer orbits (GTO) to their permanent orbits and then subsequently for autonomous station-keeping.
For conventional, chemically-propelled telecom satellites, the GTO-to-GEO process typically takes one to two weeks. During the launch and early operation phases, orbit determination of the spacecraft is performed by using tracking stations at various locations on Earth, which requires some planning and coordination.
With the introduction of all-electric propulsion (EP) satellites, such as OHB’s Electra, the need for on-board GNSS positioning becomes even more compelling. All-EP satellites will have GTO-to-GEO times in the order of 200 days, with continuous thrusting and continuously changing attitude, requiring significant effort for ground-based orbit determination.
“By using on-board, GNSS-based, autonomous orbit determination, the ground station traffic could be reduced to just telemetry and telecommand traffic, eliminating the need for a global network of tracking stations,” says Torsten Vogel, ESA Spacecraft Engineer. “For GTO-to-GEO using electrical propulsion, this should lead to a considerable reduction of operational complexity and cost, and pave the way for fully autonomous orbit slot acquisition and station keeping in the future.”
ESA is supporting this important breakthrough in geostationary satellite technology by means of a number of activities undertaken within the framework of the ARTES programme.
In one on-going ARTES 5.1 activity, prime contractor OHB is evaluating the performance of GNSS receivers in the GTO and HEO trajectories that will be encountered during orbit raising of future all-EP telecommunication satellites. Since they are heavily optimized to minimise flight times, these trajectories present specific challenges to the use of on-board GNSS, including large variations in visibility patterns and RF power levels, a high dynamic range of GNSS signals, and varying spacecraft attitude, which results in masking effects that affect antenna gain. This study will:
Evaluate the performance of a GNSS receiver in both low-thrust GTO and then in GEO;
Define the GNSS architecture, including optimum antenna placements on the spacecraft; and
Establish the cost-effectiveness of such a system versus current ground-based orbit determination systems.
In another on-going ARTES 5.2 activity, prime contractor Airbus Defence and Space is enhancing its LION Navigator, a multi-constellation, multi-frequency receiver product line. The LION is already space qualified, and Airbus DS will now make it suitable for GTO-to-GEO use. The main challenge here is making the transition from below to above the GNSS orbit.
A third recently completed ARTES 5.1 activity comprised the development by prime contractor Fraunhofer IIS of an engineering model for a new high-performance GNSS antenna for GEO telecom satellites. It is suitable for both GPS and Galileo satellite constellations.
In an earlier ARTES 5.1 activity completed in 2014, prime contractor Thales Alenia Space, developed a new compact GPS receiver family (dual or single frequency) able to deal with the new civil L2CS GPS signal, suitable for both LEO and GEO missions.
The first European telecommunications satellite to use GNSS for positioning will be the Hispasat AG1, which is currently being built by German satellite manufacturer OHB using the new SmallGEO platform, the development of which ESA has been supporting under ARTES 11. This mission, planned for 2016, should validate the application of GNSS for GEO.
“Using GNSS for satellites in geostationary orbit has tremendous potential,” Torsten Vogel says. “The new technology being developed with the support of ARTES should substantially increase the competitiveness of European satellite primes, particularly as we enter the era of all-EP spacecraft.”