Aircraft geometric height computation using secondary surveillance radar range differences

A new system for accurate geometric height measurement based on secondary surveillance radar (SSR) equipment is presented. The system includes an interrogator and receiver station, and three receiving-only stations. The aircraft responds to interrogations, and the time differences of signal arrival (TDOA) at the four receiving stations are used to estimate the vertical position of the aircraft. The use of time differences eliminates the biases caused by the systematic errors in the aircraft SSR transponder delay