US Pat. 6,232,922 Passive three dimensional track of non-cooperative targets through opportunistic use of global positioning system (GPS) and GLONASS signals Abstract A method and apparatus for utilization of GPS, GLONASS or other existing RF signals is disclosed. These existing RF signals are scattered by targets, with a receiver of these scattered signals providing processing to extract three dimensional track of these objects. Angle-of-Arrival (AOA) information of a received signal may be used, but is not required. Modifications of standard GPS signal processing allows observables such as range-sum, range-difference, and bistatic Doppler frequency to be observed. These observables, when coupled with standard bistatic/multistatic location equations, provide unambiguous and even redundant information on target coordinates. The method/device employs a modified code (range)/carrier(Doppler) search routine for initial target search/acquisition, wherein a direct path signal is used as a reference from which chip delay and Doppler shift excursions are examined. In this manner, the range and Doppler components observed will correspond to [rang! e-sum-direct path] range, and true bistatic target Doppler irrespective of the satellite or receiver induced Doppler shifts.