With this concept active and passive microwave imaging techniques are combined to record electromagnetic waves in an active (sending & receiving) and a passive (only receving) mode either simultaneously or with negigible time lag.
The active sensor is normally a Real Aperture Radar (RAR) or Synthetic Aperture Radar (SAR), while the passive sensor is a radiometer or synthetic aperture radiometer. Both acquisition modes can be operated on a single platform or on different platforms.
Satellite missions with active-passive imaging capabilities are the NASA missions AQUARIUS and SMAP.