Constant motion, acceleration, vibration, and rotation of objects in SAR data

Synthetic aperture radar (SAR) provides high resolution images of static ground scenes, but processing of data containing moving objects results in varying phase and amplitude effects. The work at hand illustrates via theoretical considerations and concrete simulations what happens to SAR imagery when parts of a scene are not static. We differentiate between four types of motion. Objects moving with a constant velocity cause position errors in azimuth as well as target defocusing and smearing in azimuth and range. Accelerating objects are responsible for even stronger shift and defocusing effects since the position errors are now a function of time. Closely related are vibrations of an object. They may be interpreted as a regular and continuous de- and acceleration whose range component results in so-called paired echoes on each side of an object in azimuth. Finally, rotation as an extreme example of constant radial acceleration may disturb a SAR image over a wide area. Through a thorough motion analysis, we developed a flexible SAR raw data simulator. Our simulations of point scatterers in raw data are based on the radar radiation pattern as a function of the system carrier frequency and the relative positions between the radar and each scatterer. All four types of movement described above may be expressed as varying relative positions and Doppler frequency shifts due to instantaneous phase variations. The standard SAR processing steps of range and azimuth compression for the simulated data provide impressive results for freely adaptable system parameters of the movement and of the SAR system.