The high resolution in range direction is obtained by the pulse compression of the initial chirp signal and the high resolution in azimuth
direction is derived from the movement of the platform--an airline or a satellite .
Genderen, "Improvement of target resolution in azimuth
by LMMSE technique," 2009 European Radar Conference, 230-233, Rome, September 2009.
A synthetic aperture can be derived through the movement of antenna, and the higher resolution in azimuth
will be obtained.
However, in conventional SAR systems, high geometric resolution in azimuth
and wide swath (HRWS)  coverage imposes contradicting requirements on system design: a high azimuth resolution requires a large Doppler bandwidth, which has to be sampled with a sufficiently high pulse repetition frequency (PRF).
This imaging mode results in higher Doppler bandwidth signals for a high resolution in azimuth
than that in stripmap SAR.