The Biomass satellite will be carrying a synthetic aperture radar (SAR) operating in P band (-435 MHz, with a -69cm wavelength). The satellite’s configuration is dictated by the need to accommodate its large deployable reflector antenna system under the Vega launcher’s fairing.
This very large antenna has to be folded for launch and unfurled in orbit to provide a stable aperture throughout the mission. The manufacturers looked at different types of antenna and various folding concepts before deciding on a reflector design.
Concepts A and B are designed around an antenna system composed of a boom and a large deployable reflector (LDR), with a planned aperture of approximately 12 metres. In both concepts, the LDR’s line of sight points so that the SAR is able to acquire images to the right of the satellite’s ground track.
The Biomass payload consists of a P-band synthetic aperture radar (-435 MHz, wavelength ~69 cm). Only a very narrow band of frequencies (6 MHz) is available for use. P band was chosen because it will enable forest biomass to be measured fairly precisely. The mission’s designers have taken advantage of a relatively narrow band (-438 MHz (A) freed up by the World Radiocommunication Conference (WRC) in 2003 and adapted it to Earth-observation requirements. The radar uses a large reflector (12 metres in diameter) that is unfurled in orbit. Two U.S. manufacturers have matured this technology, which is already used for mobile telecommunications satellites. Stowing the antenna inside the Vega launcher’s relatively cramped fairing nevertheless remains one of the challenges of the project. The signal is emitted by a small feed antenna onto the reflector, which receives the return signal. The whole instrument weighs around 200 kg. Several configurations are still being studied.
Vega has been chosen as the baseline launcher for:
- Its low margins with respect to the dynamic envelope
- Its better mass margin for the concepts based on the Northrop Grumman system compared to the Harris reflector
Antares (U.S.) and PSLV (India) have been selected as the back-up launchers. Both offer sufficient mass and volume margins for concepts A and B.