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Alow-probability-of-intercept radar (LPIR) is aradar employing measures to avoid detection by passive radar detection equipment (such as aradar warning receiver (RWR), orelectronic support receiver) while it issearching for a target or engaged intarget tracking. This characteristic is desirable in aradar because it allows finding and tracking an opponent without alerting them to the radar's presence. This also protects the radar installation fromanti-radiation missiles (ARMs).
LPI measures include:
Radar systems work by sending out a signal and then listening for its echo off distant objects. Each of these paths, to and from the target, is subject to theinverse square law of propagation in both the transmitted signal and the signal reflected back. That means that a radar's received energy drops with the fourth power of the distance, which is why radar systems require high powers, often in the megawatt range, to be effective at long range.
The radar signal being sent out is a simple radio signal, and can be received with a simpleradio receiver. Military aircraft and ships have defensive receivers, calledradar warning receivers (RWR), which detect when an enemy radar beam is on them, thus revealing the position of the enemy. Unlike the radar unit, which must send the pulse out and then receive its reflection, the target's receiver does not need the reflection and thus the signal drops off only as the square of distance. This means that the receiver, assuming no disparity in antenna size, is always at an advantage over the radar in terms of range - it will always be able to detect the signal long before the radar can see the target's echo. Since the position of the radar is extremely useful information in an attack on that platform, this means that radars generally must be turned off for lengthy periods if they are subject to attack; this is common on ships, for instance.
Unlike the radar, which knows in which direction it is sending its signal, the receiver simply gets a pulse of energy and has to interpret it. Since the radio spectrum is filled with noise, the receiver's signal is integrated over a short period of time, making periodic sources like a radar add up and stand out over the random background. The rough direction can be calculated using a rotating antenna, or similar passive array usingphase oramplitude comparison. Typically RWRs store the detected pulses for a short period of time, and compare their broadcast frequency andpulse repetition frequency against a database of known radars. The direction to the source is normally combined with symbology indicating the likely purpose of the radar –Airborne early warning and control,surface-to-air missile, etc.
This technique is much less useful against a radar with a frequency-agile (solid state) transmitter. Because agile radars likeAESA (orPESA) can change their frequency with every pulse (except when using doppler filtering), and generally do so using a random sequence, integrating over time does not help pull the signal out of the background noise. Moreover, a radar may be designed to extend the duration of the pulse and lower its peak power. An AESA or modern PESA will often have the capability to alter these parameters during operation. This makes no difference to the total energy reflected by the target but makes the detection of the pulse by an RWR system less likely.[1] Nor does the AESA have any sort of fixed pulse repetition frequency, which can also be varied and thus hide any periodic brightening across the entire spectrum. Older generation RWRs are essentially useless against AESA radars, which is why AESAs are also known as "low probability of intercept radars". Modern RWRs must be made highly sensitive (small angles and bandwidths for individual antennas, low transmission loss and noise)[1] and add successive pulses through time-frequency processing to achieve useful detection rates.[2]
Ways of reducing the profile of a radar include using wider bandwidth (wideband,Ultra-wideband),frequency hopping, usingFMCW, and using only the minimum power required for the task. Usingpulse compression also reduces the probability of detection, since the peak transmitted power is lower while the range and resolution is the same.
Constructing a radar so as to emit minimal side and back lobes may also reduce the probability of interception when it is not pointing at the radar warning receiver. However, when the radar is sweeping a large volume of space for targets, it is likely that the main lobe will repeatedly be pointing at the RWR. Modernphased-array radars not only control theirside lobes, they also use very thin, fast-moving beams of energy in complicated search patterns. This technique may be enough to confuse the RWR so it does not recognize the radar as a threat, even if the signal itself is detected.
In addition to stealth considerations, reducing side and back lobes is desirable as it makes the radar more difficult to characterise. This can increase the difficulty in determining which type it is (concealing information about the carrying platform) and make it much harder tojam.
Systems that feature LPIR include modernactive electronically scanned array (AESA) radars such as that on theF/A-18E/F Super Hornet and thepassive electronically scanned array (PESA) radar on theS-300PMU-2surface-to-air missile system.
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