A high-resolution ground-mapping technique that effectively synthesizes a large receiving antenna by processing the phase of the reflected radar return. The along-track resolution is obtained by timing the radar return (time-gating) as for ordinary radar. The cross-track (azimuthal) resolution is obtained by processing the Doppler phase of the radar return. The cross-track 'dimension' of the antenna is a function of the length of time over which the Doppler phase is collected. 
https://earthobservatory.nasa.gov/glossary/q/s