The GEODE concept
In order to operate a synthetic aperture radar from geostationary orbit (GEOSAR) such as G-CLASS, the orbital determination must be very accurate. Since the G-CLASS mission operates at C-Band, the orbit would have to be determined with a precision in the order of meters which is well above usual requirements for telecommunication satellites.
The GEostationary Orbital DEtermination (GEODE) project consists of the design and operation of a 3-channel ground interferometer able to process either the radar transmissions or an adhoc location low-power beacon. Given the high sensitivity of the signals phase to satellite-receiver range, phase differences obtained from well-separated antennas will provide high sensitivity observables allowing to determine the position and orbital dynamics of the spacecraft with the expected accuracy. Intrinsic problems such as the coherent operation of the system and phase ambiguitity resolution are being investigated and the proposed solutions will be tested using a campus interferometric system under development shown in the figure.
GEODE Inteferometer Geometry
Illuminators of opportunity
Since no GEOSAR satellite has been launched yet, the interferometer will use the Ku-Band DVB-S1/S2 telecommunications digital transmissions as signals of opportunity to carry out the proof of concept and performance evaluation. The inherent phase modulation in DVB-S1/S2 digital transmissions will have no impact on the interferometric observations since it will be removed by digital cross-channel correlation techniques in a similar way to VLBI systems in radioastronomy.