The radars used during the SRTM mission were actually developed and flown on two shuttle missions that took place in 1994. The Shuttle Imaging Radar-C (SIR-C) and the X-Band Synthetic Aperture Radar (X-SAR) flew on board the space shuttle in April and October of 1994 to gather data about Earth's environment.
SIR-C was developed by NASA. X-SAR was developed through a joint collaboration with the space agencies of Germany and Italy. So SRTM was actually a mission that re-used proven flight hardware plus some additional equipment that was needed to get the job done.
The major innovation of SRTM was that additional antennas were deployed on a 60-meter mast so the system could operate as a single-pass interferometer to efficiently and accurately collect elevation data. The 60-meter mast is the longest structure ever flown in space.
Funding for SRTM came from the Defense Department's National Geospatial-Intelligence Agency (NGA), while NASA supplied the SIR-C hardware, shuttle launch, ground systems, mission operations, and data processing support. The German Aerospace Center (DLR) and the Italian Space Agency (ASI) supplied the X-SAR hardware, ground systems, mission operations, and data processing support.
NGA also provided ground control and worked with JPL to determine the errors in the SRTM data. NGA will also format, archive, and distribute the data for the Department of Defense.