In the past, geostationary telecommunication satellites used Earth and/or Sun sensors, along with gyroscopes, as the main instruments for measuring spacecraft attitude. Now, with the availability of compact autonomous star trackers, more and more platforms are deploying these devices because of the superior performance and the greater flexibility they offer, namely better pointing accuracy and the capability to use them in any orbit, independent of the distance from Earth. Their improved accuracy and their continuous availability throughout the orbit reduce the cost and complexity of other high-cost sensors, such as gyroscopes, and remove the need for direct Earth and Sun sensing. The sensor can also be used on satellites in inclined orbit, and, more importantly, during the transfer from injection orbit to geostationary orbit when using electric propulsion, known as Electric Orbit Raising, or EOR.
Sodern, a French company which specialises in space instrumentation and optics, was one of the first suppliers of compact autonomous star trackers, taking advantage of state-of-the-art Active Pixel Sensor, or APS, detector technology. Didier Vilaire, Director of Space Programs at Sodern, says, “With HYDRA, the company is offering a multi-head design which separates the optical head from the centralized processing unit. This eases the thermal control of the optical heads and allows continuous attitude measurement, whether the satellite is facing Earth, Sun or other bright objects.”
The generic HYDRA with four heads
Photo credit: Sodern
With support of ESA, under ARTES 3-4, and CNES, Sodern developed a version of HYDRA tailored for the geostationary telecommunication satellite market (HYDRA-TC). Because price-competiveness is critical in this market segment, the company simplified the electronics to drive only two optical heads, thereby significantly reducing costs, as well as mass and power consumption.
The unit was ground qualified in 2013, leading to a number of sales. The first commercial flight will be on the forthcoming DirectTV-15 mission, based on the Eurostar 3000 platform of Airbus Defense and Space, which will be launched in December 2014. It will also be deployed on ESA's EDRS-A mission whose launch is planned in 2015.
“With this version of HYDRA, Sodern will continue to maintain its position as a leading supplier in this key market,” says ESA technical officer Steeve Kowaltschek.
The new Hydra-TC electrical unit, internally redundant and able to pilot two optical heads
Photo credit: Sodern