Historically imaging in the microwave frequency domain was done either using passive imaging techniques (with solely recording capacities of the sensor) or using active imaging techniques (with transmitting and recording capacities of the sensor). Both imaging modi were developed in parallel for a long time in electrical engineering of microwave sensors for space-borne missions, but are combined in more recently launched missions.
With the concept of active and passive microwave imaging, both techniques are fused to record electromagnetic waves in an active (sending & receiving) and a passive (only receiving) mode either simultaneously on one carrier platform or with negligible time lag on different platforms.
The active sensor is normally a Real Aperture Radar (RAR, scatterometer) 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 depending on monolithic or distributed platform systems. The benefit of fusing both modi is in the higher spatial resolution of the active imaging modes combined with the higher sensitivity of the passive modes for intrinsic (non-structural) media properities, like permittivity or salinity.
Satellite missions with active-passive imaging capabilities are the NASA missions AQUARIUS (operation started in 2011 terminated in 2015) and SMAP (operation started in April 2015 and ceased for active sensor in July 2015). Currently (2021), no dedicated active-passive microwave satellite mission is operating in orbit.