“Africa and remote sensing are not for sissies”

South African Synthetic Aperture Radar II

 

Introduction to SAR

A synthetic aperture radar system works very like a flash camera and allows to 'photograph' a two dimensional surface, but at radio wavelengths. Usual (optical) monitoring systems record the reflected light of the surface to generate the "photo". SAR systems are active radar systems providing their own ‘illumination’ (the radar pulses) scene, any time, in any weather conditions, from any distance. As the radar wavelengths are much longer than those of visible light, SAR can also "see" through cloudy and dusty conditions that conventional monitoring systems cannot. 

A SAR system is mounted on a moving platform (satellite or airplane) and illuminates an area of interest, perpendicular to the motion of the plane. Carl Wiley of Goodyear, discovered that two objects at slightly different angles to the track of the moving radar have different speeds at any instant relative to the platform. The two targets will then have two distinct Doppler frequencies.

 

 

 

 

 

 

 

 

 

 

 

 

                                                         SAR system in motion

Each scatterer in the illuminated scene has a distinct “phase history”. A SAR system resolves the phase differences from the returned signals to the phase of the transmitted waveforms.

The process behind a SAR system seems simple enough, however a fair bit of digital processing needs to be done to extract any sensible information from the “raw data”.

The image shows the Gaeta Bay on the west coast of Italy. The volcano of Roccamonfina is seen to the right. Due to the oblique illumination of the radar beam the morphology of the landscape is very much enhanced. The different tones of grey in the sea surface are due to local differences in the wind regime roughening the water in some places more than in others, typical of low wind speed weather conditions