Эффективность отражения радиолокационного сигнала

Strength of an object's radar echo

Radar cross section (RCS), denoted σ, also called radar signature, is a measure of how detectable an object is by radar. A larger RCS indicates that an object is more easily detected. An object reflects a limited amount of radar energy back to the source. The factors that influence this include:

As a rule, the larger an object, the stronger its radar reflection and thus the greater its RCS. Also, radar of one band may not even detect certain size objects. For example, 10 cm (S band radar) can detect rain drops but not clouds whose droplets are too small.

Materials such as metal are strongly radar reflective and tend to produce strong signals. Wood and cloth (such as portions of airplanes and balloons used to be commonly made) or plastic and fibreglass are less reflective or indeed transparent to radar making them suitable for radomes. Even a very thin layer of metal can make an object strongly radar reflective. Chaff is often made from metallised plastic or glass (in a similar manner to metallised foils on food stuffs) with microscopically thin layers of metal. Also, some devices are designed to be Radar active, such as radar antennas and this will increase RCS.

The SR 71 Blackbird and other aircraft were painted with a special "iron ball paint" that consisted of small metallic coated balls. Radar energy received is converted to heat rather than being reflected.

Quantitatively, RCS is calculated in three dimensions as Therefore, in order to cut the detection distance to one tenth, the RCS should be reduced by a factor of 10,000. While this degree of improvement is challenging, it is often possible when influencing platforms during the concept/design stage and using experts and advanced computer code simulations to implement the control options described below.

With purpose shaping, 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 target's direction of motion. Due to the energy reflection, this method is defeated by using passive (multistatic) radars. Purpose shaping can be seen in the design of surface faceting on the F 117A Nighthawk stealth attack aircraft. This aircraft, designed in the late 1970s 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 Lightning II continue the trend in purpose shaping and promise to have even smaller monostatic RCS.

This technique is relatively new compared to other techniques chiefly after the invention of metasurfaces. As mentioned earlier, the primary objective in geometry alteration is to redirect scattered waves away from the backscattered direction (or the source). However, it may compromise performance in terms of aerodynamics. One feasible solution, which has extensively been explored in recent time, is to utilize metasurfaces which can redirect scattered waves without altering the geometry of the target.

For the bistatic radar configuration—transmitter and receiver separated (not co located) the bistatic radar cross section (BRCS) is a function of both the transmitter target orientation and the receiver target orientation. A normalized bistatic radar cross section (NBRCS) or bistatic normalized radar cross section (BNRCS) may also be defined, similar to the monostatic NRCS.