The RCS of a target
is the projected area that would intercept the transmitted signal
and reflect isotropically an amount that produces the returned signal
at the receiver. In other words, RCS provides an indication of how well a given target
reflects radar energy. With these ideas in mind, it is
not surprising that the physical area of a target is
normally greater than the RCS because
some the incident energy is scattered and absorbed by the
target. Use of radar absorbent material (RAM) and specific shapes
and angles helps to minimise the RCS. Target aspect also changes the RCS markedly and needs to be allowed for in the
design of a radar system. Radar designers normally use extremely
pessimistic (low) values for RCS during
radar design and performance calculations to ensure their systems meet
minimum requirements when introduced into service.
The RCS of a target is not constant with operating frequency.
There are three broad areas of interest with respect to
physical target size, operating frequency and resulting RCS. These areas are:
Raleigh region. If the target is
a lot smaller than the wavelength of the radar system,
the target is said to be in the Raleigh region.
If the target is in the Raleigh region, the radar
cross section of the target tends to be
smaller than the target's physical size.
Resonance region. If the target
is of similar dimension to that of the wavelength, the
target is said to be in the resonance region. In
the resonance region, the RCS of
the target may vary a great deal but tends to
be larger than the physical size of the target.
The optical region occurs when the target is much larger
than the operating wavelength of the radar. This is quite
often the case with operational radar systems whose wavelengths are
normally in the order of centimetres in length. When operating
in this region, the RCS of
the target is similar to its physical size.
The implication of
these three regions is that the operating wavelength should not
be selected in total isolation from target considerations.
Other topics in our resources on Radar related to RCS include: