SAR AS A TOOL FOR REMOTE SENSING 
Andrew D. Goldfinger 
Johns Hopkins University Applied Physics Laboratory 
Originally, radar was developed to detect discrete targets, 
such as ships or aircraft, But problems soon arose. As radars 
became more sensitive, returns were noticed from land and sea. 
This annoyance came to be called clutter, and techniques were developed 
to lessen its effect, 
In keeping with the old adage, however, one man's noise is 
another man's signal. The clutter turned out to be interesting. 
Sea return showed the patterns of ocean waves, and land return showed 
geologic features. Radar was reborn as a tool for remote sensing. 
The principal problem lay in achieving the desired resolution, To 
understand the nature of this problem we must first consider an 
ordinary, real aperture, search radar. 
Figure 1 shows the geometry. The radar antenna has a horizontal 
aperture size L and it rotates to view the surrounding space, Using 
polar coordinates, there are two spatial variables: radial distance 
(range) and angle (azimuth). The radar determines range by measuring 
the time it takes pulses to make a round trip to and from a target. It 
determines azimuth by knowing the direction the antenna is pointing. 
The resolution of the range measurement is determined by the 
precision of the time measurement. Usually, this varies inverse- 
ly with the band-width of the electronics, The resolution of the 
azimuth measurement is determined by the beam-width of the physical 
antenna. The angular beam width is of order 024/L, where ) is the 
wavelength of the transmitted radiation. 
It is possible to achieve fine resolution in the range direction 
by using electronics that have a large band-width, Resolutions of tens 
of centimeters or less are possible if gigahertz frequencies are used. 
To achieve comparable resolution in the azimuth direction is more of a 
problem. There are two difficulties: 
l. Since beamwidth varies inversely with aperture size, a large 
physical antenna aperture is needed, and 
2, Since beam width is an angle, the beam tends to spread out with 
increasing range, and resolution is degraded as distance from the 
radar increases. 
Synthetic aperture radar is a technique for solving these prob- 
lems. It solves the first by mathematically synthesizing a huge 
imaginary aperture rather than using a real physical one, and it solves 
the second as a serendipitous consequence of the geometry. 
292