Editing Low-probability-of-intercept radar (section)

==Rationale==
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 the [[inverse 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 simple [[radio receiver]]. Military aircraft and ships have defensive receivers, called [[radar warning receiver]]s (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 is always at an advantage [neglecting disparity in antenna size] 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 using [[Phase-comparison monopulse|phase]] or [[Amplitude-Comparison Monopulse|amplitude comparison]]. Typically RWRs store the detected pulses for a short period of time, and compare their broadcast frequency and [[pulse 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. Agile radars like [[Active electronically scanned array|AESA]] (or [[Passive electronically scanned array|PESA]]) 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.<ref name=RWRIntegrationStudy >{{cite web|title=IEEE AESS November 2004|url=http://ieeetmc.net/r5/dallas/aes/IEEE-AESS-Nov04-Wiley.pdf |url-status=dead|date=March 2022}}</ref> 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)<ref name=RWRIntegrationStudy /> and add successive pulses through time-frequency processing to achieve useful detection rates.<ref>{{cite web
  |url=http://www.emrsdtc.co.uk/conferences/2004/downloads/pdf/tech_conf_papers/A14.pdf
  |accessdate=17 June 2015
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  |title=Archived copy
  |archive-url=https://web.archive.org/web/20150630090556/http://www.emrsdtc.co.uk/conferences/2004/downloads/pdf/tech_conf_papers/A14.pdf
  |archive-date=30 June 2015
  }}</ref>