SAR processing

The objective of SAR processing is to reconstruct the imaged scene from the many pulses reflected by each single target, received by the antenna and registered in memory.
Resolution describes the minimal discernable spacing between two similar point responses (A and B), but often is applied to the width of one response (C). A weaker response (D) requires a larger separation for detection.
Pixels refer to the discrete sample positions used for digital imagery. There must be at least two pixels within a resolution distance.
SAR processing is a simple process although it requires much computation. It can be considered as a two-dimensional focussing operation.
The first of these is the relatively straightforward one of range focussing, requiring the de-chirping of the received echoes.
Azimuth focussing depends upon the Doppler histories produced by each point in the target field and is similar to the de-chirping operation used to focus in the range direction.
This is complicated however by the fact that these Doppler histories are range dependent, so azimuth compression must have the same range dependancy.
It is necessary also to make various corrections to the data for sensor motion and Earth rotation for example, as well as for the changes in target range as the sensor flies past it.

It is important to note (see figure) that the pixel of the final SAR image does not have the same dimensions as the resolution cell during the data acquisition, due to the variation of range resolution with incidence angle. Thus it is necessary to perform a pixel resampling with a uniform grid.
Even more fundamental, at least two pixels are required to represent each resolution cell, which is a consequence of digital sampling rules. By convention, pixel spacing in SAR imagery is chosen to conform to standard map scales, hence must be a discrete multiple (or divisor) of 100 meters.
For example, ERS-1 data, having nominal resolution of 28 meters in range and azimuth, is delivered with 12.5 meter pixel spacings..

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry