Low probability of intercept
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A low-probability-of-intercept radar (LPIR) is designed to be difficult to detect by passive radar-detection equipment (such as a radar warning receiver - RWR) while it is searching for or tracking a target. This characteristic is desirable because it allows finding and tracking an opponent without alerting them to the radar's presence.
Ways of reducing the profile of a radar include using wider-frequency bandwidth (wideband), frequency hopping, using a frequency-modulated, continuous-wave signal, and using only the minimum power required for the task. Using pulse 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. Modern phased-array radars not only control their side 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. All military EM emitters, including fighter aircraft, naval ships, and missile systems are designed for reduced electromagnetic profiles for improved stealth.
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 to jam.
Systems which feature LPIR include modern AESA radars such as that on the F/A-18E/F Super Hornet and the electronically steered phased array on the S-300PMU-2.
Thales Defence Deutschland GmbH in Kiel has designed an LPI radar system for naval vessels that operates in combination with a pulse radar. For open operation, the vessel is running the pulse radar. The boat will be identified by ESM systems (fingerprint of boat). For secret missions, it is switched to LPI radar. It works in FMCW mode with min. 1mW in 24 nautical miles .The boat remains invisible for ESM systems. The so-called SPHINX radar systems is supported by a pressure tight Microstrip Antenna (MSA) with CsC feature for simultaneous sea and air observation. References: Greece Navy, South Korean Navy
