Measurements of energetic particles obtained with a set of three geosynchronous satellites (1977--007, 1981--025, and 1982--019) are used to investigate the substorm injection region and particle drifts for an event on Feb. 3, 1983. A technique has been developed which allows remote sensing of the boundaries of the substorm injection region and the injection time by using measured energy dispersion and modeling particle drifts within the semi-empirical magnetospheric field model of Tsyganenko and Usmanov [1982>. The injection region for this event was found to span 90¿ around local midnight. The presence of spacecraft on either side of the injection region limit possible errors. Good agreement between long and short drift paths is found for ions while electron measurements give less reliable results. ¿American Geophysical Union 1990