The self-mixing heterodyne concept has been used in a highly coherent 140GHz radar, which places minimal stability requirements on the source signal itself. Two-dimensional images of simple rotating targets were generated with the CW system through focussed synthetic aperture processing.
U. H. W. Lammers,
"Doppler radar studies at 140GHz", Proc. SPIE 1514, 15th International Conference on Infrared and Millimeter Waves, 15140S (1 December 1990); doi: 10.1117/12.2301438; https://doi.org/10.1117/12.2301438