A low probabililty of intercept radar has the characteristic that it is unlikely to be detected 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 you to find and track an opponent without them realising that you are doing so.
The different 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 minimal amount of power required for the task. Using pulse compression will also reduce 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 decrease the probability of interception, since this will make it harder to detect the radar when it's not pointing at the RWR. However, most radars will sweep a large volume of space in order to find the platform carrying the RWR, so this will ensure that the main lobe will repeatedly be pointing in that direction anyway. 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 may be enough for a RWR to not recognize the radar as a threat, even though the signal itself is detected. All military EM emitters, including fighter aircraft, naval ships and missile systems all desire reduced electro-magnetic profiles for improved stealth.
In addition to stealth considerations, reducing side and back lobes is desirable because it makes the radar harder to characterise, meaning not only is it more difficult once the radar is detected to tell which type it is (which may give hints about the carrying platform), but it is also much harder to jam. This is another reason why most modern radars which typically feature LPI also feature small side and back lobes.
Systems which feature LPI 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.
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