The REX satellites are designed to study scintillation effects of the Earth's atmosphere on RF transmissions. Rex 2 was also the first spacecraft to successfully employ GPS navigation for full closed loop attitude control. The basic design features of both spacecraft are similar, though there are some obvious differences, i. e. no GPS equipment on the first REX. Specifications presented here are for Rex 2. Spacecraft: Three axis stabilised, nadir pointing to within 5 degrees. Passive attitude control provided by 21 foot gravity gradient boom with two 20 inch long nickel/iron magnetic hysteresis rods mounted at the tip for damping purposes (4 lbs. total). Active control achieved through pitch bias momentum system, including one wheel and three torque coils. Attitude determination is provided by GPS equipment, coarse sun sensors, and a magnetometer. Payload: The primary communications experiment built by the US Air Force Rome Laboratories advances research on electron density irregularities that cause disruptive scintillation effects on radio signals transmitted through the Earth's ionosphere. Rex II GPS receivers, supplied by Trimble Navigation, are able to resolve spacecraft position to 100m, velocity to 0.2 m/sec, and attitude to within 0.3 deg.

Design Life: 1 year, goal of three years. Total Length: 0.6 m. Maximum Diameter: 0.8 m. Total Mass: 85 kg.

Radiation Experiment; tested communications components in high radiation environment. Spacecraft engaged in research and exploration of the upper atmosphere or outer space (US Cat B).

LEO

• 4 - McDowell, Jonathan, Jonathan's Space Report (Internet Newsletter), Harvard University, Weekly, 1989 to Present. HTML when accessed: ftp://sao-ftp.harvard.edu/pub/jcm/space/news
• 6 - JPL Mission and Spacecraft Library, Jet Propulsion Laboratory, 1997. HTML when accessed: http://msl.jpl.nasa.gov/home.html