Spaceborne synthetic aperture radar (SAR) systems are used to measure geo- and biophysical parameters of the Earth's surface, e.g. for agriculture, forestry and land subsidence investigations. Upcoming SAR sensors such as the Japanese Phased Array L-band Synthetic Aperture Radar (PALSAR) onboard the Advanced Land Observing Satellite (ALOS) exemplify a trend towards lower frequencies and higher range chirp bandwidth in order to obtain additional information with higher geometric resolution. However, the use of large bandwidths causes signal degradation within a dispersive medium such as the ionosphere. Under high solar activity conditions at L-band frequencies, ionosphere-induced path delays and Faraday rotation become significant for SAR applications. Due to ionospheric effects, blind use of a generic matched filter causes inaccuracy when correlating the transmitted with the received signal. Maximum correlation occurs where the length of the matched filter, based on a synthetic chirp model of the transmitted signal, is adjusted to correspond to that of the received signal. By searching for the proper adjustment necessary to reach this maximum, the change in length can be estimated and used to derive variations in the total electron content (TEC) and degree of Faraday rotation within the ionosphere from all range lines in a SAR image.
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.
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