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Fundamentals and Special Problems of Synthetic Aperture Radar (SAR)

The basic principles of SAR will be explained.
Equations for geometric and radiometric reso—
lution and their inter—relations will be gi-
ven in addition to a range equation. The dif—
ference between focussed and unfocussed SAR
and the conception of beam sharpening will be
explained.
For remote sensing purposes the resolution of
the respective sensor is one of the main fac—
tors. Resolution in the wide sense is defined
as the degree to which a sensor can discrimina—
te two closely spaced targets, having similar
properties (geometry, colour, velocity, etch
However, angular resolution is a matter of do—
minant concern. It is defined as the minimum
angular separation between two items which can
be distinguished by a system.
Note that for all systems using elecromagnetic
waves, the laws of optics apply. The angular
resolution of an optical system is principally
limited by both the aperture diameter and the
turbulence of the wave propagation medium (i.e.
the earth's atmosphere). Quantitatively the an—
gular resolution of an aperture of a given size
l is found by the ratio of wavelength 1 over
this size.
The human eye, for example, is an optical sy—
steHn the resolution r ofwhich is limited by
the pupil diameter. under mean illumination
with a wavelength of about 5 x 10“1 mm the pu-
pil diameter varies between 1 mm and 8 mm and
an angular resolution power between about 2 arc
minutes and 10 arc seconds results. The mean
resolution power of l are minute corresponds to
a ground resolution of 3 m for a distance of 10
km. (Thisvalue can be experienced (approxima—
tely!) by looking down to the ground from an
aircraft). For optics with diameters roughly a
factor lOO larger and at altitude of about 100
km, a resolution power in the order of 10 cm
results. This corresponds excellently to values
known or guessed for military satellites which
are now operational.
Diffraction limiting laws of optics apply to
microwave remote sensing systems, too. Here,
the antenna is the lens of the System and the
antenna aperture, the diameter or length deter—
mines the aperture size.