4.3 Synthetic Aperture Radar

4.3 Synthetic Aperture Radar

Compared to real aperture radar, Synthetic Aperture Radar (SAR) synthetically increases the antenna's size or aperture to increase the azimuth resolution though the same pulse compression technique as adopted for range direction. Synthetic aperture processing is a complicated data processing of received signals and phases from moving targets with a small antenna, the effect of which is to should be theoretically convert to the effect of a large antenna, that is a synthetic aperture length, as shown as Figure 4.3.1.

The synthetic aperture length is the beam width by range which a real aperture radar of the same length, can project in the azimuth direction.

The resolution in the azimuth direction is given by half of real aperture radar as shown as follows.

Real beam width : = /D
Real resolution: L= R=Ls (synthetic aperture length)
Synthetic beam width : s = / 2Ls= D / 2R
Synthetic resolution : Ls = sR = D / 2

where :wavelength D: aperture of radar R: slant range

This is the reason why SAR has a high azimuth resolution with a small size of antenna regardless of the slant range, or very high altitude of a satellite.

Figure 4.3.2 shows the basic theory of SAR or synthetic aperture processing including the Doppler effect, matched filter and azimuth compression.

SAR continues to receive return pulses from a target during the time the radar projects the beam to the target. In the meanwhile the relative distance between the radar and the target changes with the movement of the platform, which produces a Doppler effect to modulate a chirp modulation of received pulse. A matched filter corresponding to the reverse characteristics of chirp modulation will increase the azimuth resolution of azimuth direction. This is called azimuth compression.

In the case of SAR, unsuitability of satellite velocity and attitude will reduce the effect of the Doppler effect. Therefore the satellite with SAR is required to be high, because the correction for synthetic aperture processing due to instability at lower altitudes is very difficult.