Metrology, attitude, and orbit determination for spaceborne interferometric synthetic aperture radar

The Shuttle Radar Topography Mission (SRTM), scheduled for an 11 day Space Shuttle flight in 1999, will use an Interferometric Synthetic Aperture Radar (IFSAR) instrument to produce a near-global digital elevation map of the earth’s land surface with 16 m absolute vertical height accuracy at 30 meter postings. SRTM will achieve the required interferometric baseline by extending a receive-only radar antenna on a 60 meter deployable mast from the shuttle payload bay. Continuous measurement of the interferometric baseline length, attitude, and position is required at the 2 mm, 9 arcsec, and 1 m (1.6 sigma) levels, respectively, in order to obtain the desired height accuracy. The Attitude and Orbit Determination Avionics (AODA) subsystem will provide these functions for SRTM. The AODA flight sensor complement includes electro-optical metrology sensors, a star tracker, an inertial reference unit, GPS receivers, plus supporting electronics and computers. AODA ground processing computers will support SRTM system performance evaluation during the mission and baseline reconstruction after the mission. The final AODA data products will be combined with the radar data (range and interferometric phase) to reconstruct the height information necessary for topographic map generation. A description of the AODA system architecture, error budgets, and the major issues involved with measuring large space structures are presented.