Radar cross section (RCS) is a measurement of an object's radar reflectivity, usually in units of square meters. RCS data are integral to the development of radar stealth technology, particularly with aircraft. Exact values for current military aircraft are almost all highly classified.
Measurement of RCS is usually conducted in an anechoic chamber, though it is sometimes done outdoors.
RADAR CROSS-SECTION REDUCTION
- Purpose Shaping. Purpose shaping is an RCS reduction technique in which the shape of the target’s reflecting surfaces is designed such that they reflect energy away from the source. The aim is usually to create a “cone-of-silence” about the aircraft’s direction of flight. Purpose-shaping techniques can be seen in the design of surface faceting on the F-117A Nighthawk stealth fighter. This aircraft, designed in the late l970s though only revealed to the public in 1988, uses a multitude of flat surfaces to reflect incident radar energy away from the source. Yue suggests that limited available computing power for the design phase kept the number of surfaces to a minimum. The B-2 Spirit stealth bomber benefited from increased computing power, enabling its contoured shapes and further reduction in RCS. The F-22 Raptor and F-35 Joint Strike Fighter continue the trend in purpose shaping and promise to have even smaller monostatic RCS.
- Active Cancellation. In active cancellation techniques, the target aircraft generates a radar signal equal in intensity but opposite in phase to the predicted reflection of an incident radar signal. This creates destructive interference between the reflected and generated signals, resulting in reduced RCS. To incorporate active cancellation techniques, the precise characteristics of the waveform and angle of arrival of the illuminating radar signal must be known, since they define the nature of generated energy required for cancellation. The implementation of active cancellation techniques is extremely difficult due to the complex processing requirements and the difficulty of predicting the exact nature of the reflected radar signal over a broad aspect of an aircraft.
- RAM. The third RCS reduction technique for aircraft and missiles is the use of RAM either in the original construction or as an addition to highly reflective surfaces. There are two types of RAM: resonant; and non-resonant. Resonant or “lossy” materials are applied to the reflecting surfaces of the target. The thickness of the material corresponds to one-quarter wavelength of the expected illuminating radar-wave. The incident radar energy is reflected from the outside and inside surfaces of the RAM to create a destructive interference pattern. This results in the cancellation of the reflected energy. Non-resonant RAM uses ferrite particles suspended in epoxy or paint to reduce the reflectivity of the surface to incident radar waves. Because the non-resonant RAM dissipates incident radar energy over a larger surface area, it usually results in an increase in surface temperature, thus reducing RCS at the expense of an increase in infrared signature. A major advantage of non-resonant RAM is that it can be effective over a broad range of frequencies, whereas resonant RAM is limited to a narrow range of design frequencies.
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