Low Probability of Intercept Radar (LPIR)

Low probability of intercept (LPI) is that property of an emitter that because of its low power, wide bandwidth, frequency variability, or other design attributes, makes it difficult to be detected or identified by means of passive intercept devices such as radar warning, electronic support and electronic intelligence receivers, In order to detect LPI radar waveforms new signal processing techniques are required.

In the past, many types of radar were designed to transmit short duration pulses having relatively high peak power, to reduce all the propagation losses of the electromagnetic waves and, at the same time, to guarantee a straightforward recovery of the reflected wave from the target in clutter. Later, for military applications, it became important to handle problems like chaff and jamming that decreased the capability of those initial radars limiting their use on the battlefield.

Today, radar designers are considering new waveforms that can provide the same capability of target detection but which are more difficult to be detected or intercepted; in other words, a Low Probability of Intercept (LPI) radar tries to provide detection of targets at longer ranges than intercept receivers can accomplish detection of the radar. The term LPI provides a collection of properties offering covertness to the radar signal making detection difficult with conventional receivers. LPI waveforms include Frequency Modulation Continuous Wave (FMCW), several polyphase-coded CW waveforms such as Frank, P1, P2, P3 and P4, frequency hopping, and combined frequency hopping-phase coding.

The LPI technique is based on the property of an emitter that due to its low power, wide bandwidth, frequency variability and other attributes makes radar difficult to intercept or identify by conventional passive intercept receiver devices. A LPI radar works to detect targets at a longer range than an intercept receiver. This concept is well-summarized in a statement "It tries to see and not be seen." This is a response to the increasing capability of modern intercept receivers to detect and locate radar emitters, possibly leading rapidly to an electronic attack or the physical destruction of the radar by guided munitions or Anti-Radiation Missiles.

The ability of an enemy receiver of a given sensitivity to detect a radar is a function of such factors as peak radiated power, radiated frequency, spectrum agility, and pulse repetition-frequency agility. Basic considerations of radar intercept resistance indicate that conventional low probability of intercept (LPI) techniques will not provide adequate intercept resistance. Conventional techniques such as high processing gain and spread spectrum, though useful for LPI communication, cannot overcome the 1/R.sup.4 radar signal decrease versus that the 1/R.sup.2 one-way intercept signal decrease. In addition, radar requirements limit the amount of signal processing gain which can be accommodated.

A LPI radar requires wideband signal modulations that reduce the signal's detectability. Wideband modulations spread the signal's energy in frequency, so that the frequency spectrum of the transmitted signal is wider than what is required to carry the signal's information (Information bandwidth). Spreading the signal energy reduces the signal-strength-per-information bandwidth. Since the noise in a receiver is a function of its bandwidth, the SNR in any receiver attempting to receive and process the signal will be greatly reduced by the signal spreading.

The most important antenna characteristic for reducing the possibility of an intercept receiver detecting the radio frequency (RF) emissions is a low-side-lobe-transmit pattern. Another antenna technique relates to the scan pattern, which must be precisely controlled to limit the intercept receiving time making it short and irregular. Antenna techniques can also achieve an LPI capability by using multiple simultaneous receive and transmit antenna beams to increase the target dwell time without compromising the target revisit time. Following the same criteria, a single wide beamwidth transmit antenna and many simultaneous receiving beams could also be employed.

Water Vapor Absorption Resonance

One approach to LPI radar designed to reduce probability of intercept is accomplished primarily by radiating frequencies which are attenuated rapidly in the atmosphere. For a radar that operates at or near the lower water vapor absorption resonance frequency of 22.235 GHz, the propagation media becomes non-linear so that the 1/R.sup.4 decrease in intercept signal and radar signal no longer applies. At ranges greater than twice the target range, the intercept receiver is "looking through" more absorbing medium than is the radar. Absorption is significant at several millimeter frequencies, the lowest being 22.235 GHz. Actual atmospheric mediums are not uniform but total absorption depends upon altitude, temperature, and humidity. When the radar is operated at or near the water-vapor absorption frequency of 22.235 GHz, the signal energy is attenuated rapidly by the water vapor in the atmosphere.

The use of continuous-wave (CW) rf energy, coupled with a very high radar receiver antenna gain at K-band further reduces the radiated peak power. Frequency hopping techniques, as well as variable PRF, are also combined with these concepts in order to force the interceptor to wide predetection bandwidths.

A K-band LPI radar can make use of atmospheric absorption to reduce energy at long ranges for decreased probability of intercept by enemy receivers while, at the same time, providing adequate energy at short ranges for navigation, air traffic control, and point defense - that is, a few dozen kilometers, but not the several hundred kilometers required for wide area battlefield surveillance.