Synthetic Aperture Radar (SAR)

Synthetic Aperture Radars were developed as a means of overcoming the limitations of real aperture radars. These systems achieve good azimuth resolution that is independent of the slant range to the target, yet use small antennae and relatively long wavelengths to do it .
SAR Principle
A synthetic aperture is produced by using the forward motion of the radar. As it passes a given scatterer, many pulses are reflected in sequence. By recording and then combining these individuals signals, a "synthetic aperture" is created in the computer providing a much improved azimuth resolution
It is important to note that some details of the structure of the echoes produced by a given target change during the time the radar passes by. This change is explained also by the Doppler effect which among others is used to focus the signals in the azimuth processor. We will illustrate this point with an analogy.
Let us consider, as in the case of this figure ,a plunger going up and down in the water, producing circles of radiating waves, each with a constant frequency fZ .
These waves travel at a known speed. The plunger is a source of waves analogous to those from a radar. We are interested in the appearance of this wave field at a certain distance.
Consider a boat is moving along the line. At position B, a passenger on the boat would count the same wave number as emitted, since he is moving neither toward nor away from the waves (source).
However, at position A, the boat is moving towards the waves and the passenger will count a higher number of waves: the travelling speed of the waves is slightly increased by the speed of the ship.
On the contrary, at position C, the boat is moving away from the buoy and the apparent frequency is lower: the waves are moving in the same direction as the boat.
Doppler frequency is the difference between received and emitted frequencies where the difference is caused by relative motion between the source and the observer.
Equivalently, the relative spacing between crests of the wave field could be recorded along the line AC, measured as if the wave field were motionless.
This leads to a phase model of the signals that is equivalent to the Doppler model.
During the movement of the boat from position A to position C, the recording by the observer of the number of waves would look like the curve at the right of the figure.
Instead of a plunger, let us now consider an aircraft emitting a radar signal. The boat corresponds to a target appearing to move through the antenna beam as the radar moves past.
The record of the signals backscattered by the target and received would be similar to the record of the passenger in the boat. Such a record is called the Doppler history (or phase history) of the returned signals.
When the target is entering the beam, the Doppler shift is positive because the source to target distance is decreasing. The phase history is then stored to be used during the SAR processing.
By the time the antenna is abeam relative to the target, the received frequency is nominal, with the Doppler frequency being zero. Late it decreases as the satellite moves away.
The phase history is then stored to be used during the SAR processing.

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