Editing Radar (section)

====Clutter====
[[File:Multipath propagation diagram en.svg|thumb|Radar multipath [[Light echo|echoes]] from a target cause ghosts to appear]]
{{main|Clutter (radar)}}
Clutter refers to radio frequency (RF) echoes returned from targets which are uninteresting to radar operators. Such targets include man-made objects such as buildings and — intentionally — by radar countermeasures such as [[Chaff (countermeasure)|chaff]]. Such targets also include natural objects such as ground, sea, and — when not being tasked for meteorological purposes — [[precipitation]], [[hail spike]], [[dust storm]]s, animals (especially birds), turbulence in the [[atmospheric circulation]], and [[Meteoroid|meteor]] trails. Radar clutter can also be caused by other atmospheric phenomena, such as disturbances in the [[ionosphere]] caused by [[geomagnetic storm]]s or other [[space weather]] events. This phenomenon is especially apparent near the [[geomagnetic pole]]s, where the action of the [[solar wind]] on the earth's [[magnetosphere]] produces convection patterns in the ionospheric [[Plasma (physics)|plasma]].<ref name="Riddolls-2006">{{Cite tech report|last=Riddolls|first=Ryan J|title=A Canadian Perspective on High-Frequency Over-the-Horizon Radar|number=DRDC Ottawa TM 2006-285|page=38|publisher=[[Defence Research and Development Canada]]|location=Ottawa, Ontario, Canada|date=December 2006|url=https://cradpdf.drdc-rddc.gc.ca/PDFS/unc81/p527279.pdf|access-date=2 December 2023}}</ref> Radar clutter can degrade the ability of [[over-the-horizon radar]] to detect targets.<ref name="Riddolls-2006"/><ref name="Elkins-1980">{{cite tech report|last=Elkins|first=TJ|author-link=Terence James Elkins|title=A model for high frequency radar auroral clutter|page=9|series=RADC Technical Reports|volume=1980|number=RADC-TR-80-122|publisher=[[Rome Laboratory|Rome Air Development Center]]|location=Rome, New York|date=March 1980|url=https://apps.dtic.mil/sti/pdfs/ADA091049.pdf|access-date=2 December 2023}}</ref>

Some clutter may also be caused by a long radar [[waveguide]] between the radar transceiver and the antenna. In a typical [[plan position indicator]] (PPI) radar with a rotating antenna, this will usually be seen as a "sun" or "sunburst" in the center of the display as the receiver responds to echoes from dust particles and misguided RF in the waveguide. Adjusting the timing between when the transmitter sends a pulse and when the receiver stage is enabled will generally reduce the sunburst without affecting the accuracy of the range since most sunburst is caused by a diffused transmit pulse reflected before it leaves the antenna. Clutter is considered a passive interference source since it only appears in response to radar signals sent by the radar.

Clutter is detected and neutralized in several ways. Clutter tends to appear static between radar scans; on subsequent scan echoes, desirable targets will appear to move, and all stationary echoes can be eliminated. Sea clutter can be reduced by using horizontal polarization, while rain is reduced with [[circular polarization]] (meteorological radars wish for the opposite effect, and therefore use [[linear polarization]] to detect precipitation). Other methods attempt to increase the signal-to-clutter ratio.

Clutter moves with the wind or is stationary. Two common strategies to improve [[Test and evaluation master plan#Performance evaluation|measures of performance]] in a clutter environment are:
:* Moving target indication, which integrates successive pulses
:* Doppler processing, which uses filters to separate clutter from desirable signals

The most effective clutter reduction technique is [[pulse-Doppler radar]]. Doppler separates clutter from aircraft and spacecraft using a [[Radar signal characteristics#In the frequency domain|frequency spectrum]], so individual signals can be separated from multiple reflectors located in the same volume using velocity differences. This requires a coherent transmitter. Another technique uses a [[Moving target indication|moving target indicator]] that subtracts the received signal from two successive pulses using phase to reduce signals from slow-moving objects. This can be adapted for systems that lack a coherent transmitter, such as [[Radar signal characteristics#Unambiguous range|time-domain pulse-amplitude radar]].

[[Constant false alarm rate]], a form of [[automatic gain control]] (AGC), is a method that relies on clutter returns far outnumbering echoes from targets of interest. The receiver's gain is automatically adjusted to maintain a constant level of overall visible clutter. While this does not help detect targets masked by stronger surrounding clutter, it does help to distinguish strong target sources. In the past, radar AGC was electronically controlled and affected the gain of the entire radar receiver. As radars evolved, AGC became computer-software-controlled and affected the gain with greater granularity in specific detection cells.

Clutter may also originate from multipath echoes from valid targets caused by ground reflection, [[atmospheric duct]]ing or [[Skywave|ionospheric reflection]]/[[refraction]] (e.g., [[anomalous propagation]]). This clutter type is especially bothersome since it appears to move and behave like other normal (point) targets of interest. In a typical scenario, an aircraft echo is reflected from the ground below, appearing to the receiver as an identical target below the correct one. The radar may try to unify the targets, reporting the target at an incorrect height, or eliminating it on the basis of [[jitter]] or a physical impossibility. Terrain bounce jamming exploits this response by amplifying the radar signal and directing it downward.<ref>{{Cite thesis|last=Strasser|first=Nancy C.|title=Investigation of Terrain Bounce Electronic Countermeasure|pages=1–104|publisher=[[Air Force Institute of Technology]]|location=Wright-Patterson AFB, Dayton, Ohio|date=December 1980|oclc=|url=https://apps.dtic.mil/sti/tr/pdf/ADA101208.pdf|access-date=2 December 2023|archive-date=7 October 2024|archive-url=https://web.archive.org/web/20241007062110/https://apps.dtic.mil/sti/tr/pdf/ADA101208.pdf|url-status=live}}</ref> These problems can be overcome by incorporating a ground map of the radar's surroundings and eliminating all echoes which appear to originate below ground or above a certain height. Monopulse can be improved by altering the elevation algorithm used at low elevation. In newer air traffic control radar equipment, algorithms are used to identify the false targets by comparing the current pulse returns to those adjacent, as well as calculating return improbabilities.