This proposal addresses Phase I of a multi-phase effort to develop advanced frequency-measuring equipment for use in ESM and ECM systems. Wide instantaneous bandwidth, fast response, and ability to handle overlapping signals are receiver requirements that reflect the increasing sophistication of hostile radars as well as today's dense signal environments. Currently available Instantaneous Frequency Measurement Receivers (IFMR) have the desired instantaneous bandwidth and fast response, and are relatively small and lightweight. However, they lack one key characteristic: they cannot determine the frequencies of two or more simultaneous signals. The proposed effort is designed to eliminate this restriction and to provide IFMRs with multi-signal measuring capabilities. Phase I will be based on the MUltiple SIgnal Classification (MUSIC) algorithm developed by the Chief Investigator and on his US patent that showed how the algorithm can be used with an IFMR of novel design to measure the frequencies of simultaneous input signals. An experimental IFMR with appropriate characteristics is available for this project. During Phase I we propose to do extensive testing to validate our analytical work and to investigate the effects of parameter variations and of RF or IF limiters.