Commercial Global Positioning System hardware is handling the primary attitude-determination task on the International Space Station, sparing the program the cost of an expensive star tracker. The Space Integrated GPS/Inertial Navigation System (SIGI) uses four GPS antennas mounted in a 1.5 X 3-meter rectangle on the S-Zero truss segment (see photo) installed during last month's ISS assembly mission. Since different phases of the GPS signal strike the different antennas, SIGI software can use interferometry to calculate the station attitude, accord- ing to Susan Gomez of Johnson Space Center, the ISS SIGI technical lead. The system is "incredibly noisy," Gomez says, but it works when rate gyro data are used as a filter. The Honeywell system, based on Trimble GPS receivers, also delivers position and velocity determination, with star trackers and other gear on the Russian side of the station providing redundancy.