Traffic monitoring using SRTM along-track interferometry

In February 2000, the shuttle radar topography mission SRTM was flown as the first single-pass SAR interferometer in space. The goal was to obtain a global digital elevation model. Therefore, a secondary receive-only antenna on the top of a 60 m long deployable mast supplemented the primary transmit and receive antenna mounted in the shuttle cargo bay. Due to mechanical constraints, the interferometric baseline did not only consist of the terrain height sensitive across track component but also contained an along track component of 7 m. The resulting time lag between the data acquisitions by the two antennas causes phase differences, which are proportional to the line-of-sight velocity of moving targets. Since this system ability has been expected prior to the SRTM launch, a traffic monitoring experiment has been carried out during the mission, which showed a surprisingly good agreement between the GPS velocity measurements on ground and the measurement from space. A strategy to exploit the along track interferometric (ATI) phase and the coherence together with the azimuth displacement of moving vehicles in focused SAR images has been developed and applied on sample test sides for traffic monitoring on highways.