Precision approach radar (PAR) is a type of

radar Radar is a detection system that uses radio waves to determine the distance ('' ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, we ...

guidance system designed to provide lateral and vertical guidance to an aircraft

pilot An aircraft pilot or aviator is a person who controls the flight of an aircraft by operating its directional flight controls. Some other aircrew members, such as navigators or flight engineers, are also considered aviators, because they a ...

for landing, until the landing threshold is reached. Controllers monitoring the PAR displays observe each

aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engine ...

's position and issue instructions to the pilot that keep the aircraft on course and glidepath during

final approach In aeronautics, the final approach (also called the final leg and final approach leg) is the last leg in an aircraft's approach to landing, when the aircraft is lined up with the runway and descending for landing.Crane, Dale: ''Dictionary of ...

. After the aircraft reaches the

decision height In aviation, an instrument approach or instrument approach procedure (IAP) is a series of predetermined maneuvers for the orderly transfer of an aircraft operating under instrument flight rules from the beginning of the initial approach to a landi ...

(DH) or decision altitude (DA), further guidance is advisory only. The overall concept is known as ground-controlled approach (GCA), and this name was also used to refer to the radar systems in the early days of its development. PAR Scope

PAR radars use a unique type of

radar display A radar display is an electronic device to present radar data to the operator. The radar system transmits pulses or continuous waves of electromagnetic radiation, a small portion of which backscatter off targets (intended or otherwise) and retu ...

with two separate "traces", separated vertically. The upper trace shows the elevation of a selected aircraft compared to a line displaying the ideal

glideslope Instrument landing system glide path, commonly referred to as a glide path (G/P) or glide slope (G/S), is "a system of vertical guidance embodied in the instrument landing system which indicates the vertical deviation of the aircraft from its o ...

, while the lower shows the aircraft's horizontal position relative to the runway midline. GCA approaches normally start with the controller relaying instructions to bring the aircraft into the glidepath and then begin any corrections needed to bring it onto the centerline. Precision approach radars are most frequently used at military

air traffic control Air traffic control (ATC) is a service provided by ground-based air traffic controllers who direct aircraft on the ground and through a given section of controlled airspace, and can provide advisory services to aircraft in non-controlled airs ...

facilities. Many of these facilities use the AN/FPN-63, AN/MPN, or AN/TPN-22. These radars can provide precision guidance to a distance of 10 to 20 miles. PAR is mostly used by the

Navy A navy, naval force, or maritime force is the branch of a nation's armed forces principally designated for naval and amphibious warfare; namely, lake-borne, riverine, littoral, or ocean-borne combat operations and related functions. It in ...

, as it does not broadcast directional signals which might be used by an enemy to locate an aircraft carrier.

Non-traditional PAR using SSR transponder reply

There are systems that provide PAR functionality without using primary

. These non-traditional PAR systems use

transponder In telecommunications, a transponder is a device that, upon receiving a signal, emits a different signal in response. The term is a blend of ''transmitter'' and ''responder''. In air navigation or radio frequency identification, a flight trans ...

multilateration Trilateration is the use of distances (or "ranges") for determining the unknown position coordinates of a point of interest, often around Earth (geopositioning). When more than three distances are involved, it may be called multilateration, for e ...

, triangulation and/or

trilateration Trilateration is the use of distances (or "ranges") for determining the unknown position coordinates of a point of interest, often around Earth (geopositioning). When more than three distances are involved, it may be called multilateration, for e ...

. One such system, Transponder Landing System (TLS) precisely tracks aircraft using the mode 3/A transponder response received by antenna arrays located near the runway. These antennas are part of a measurement subsystem that is used to precisely determine the aircraft 3-dimensional position using TOA, DTOA and AOA measurement techniques. The aircraft position is then displayed on a high-resolution color graphics terminal that also shows the approach centerline and the glide path. A GCA controller is then able to use this screen for reference to issue GCA instructions to the pilot. The signal strength for the

secondary surveillance radar Secondary surveillance radar (SSR)''Secondary Surveillance Radar'', Stevens M.C. Artech House, is a radar system used in air traffic control (ATC), that unlike primary radar systems that measure the bearing and distance of targets using the d ...

subsystem of a non-traditional PAR is not attenuated by rain since the frequency is within the long range band,

L-band The L band is the Institute of Electrical and Electronics Engineers (IEEE) designation for the range of frequencies in the radio spectrum from 1 to 2 gigahertz (GHz). This is at the top end of the ultra high frequency (UHF) band, at the lower ...

. Therefore, a non-traditional PAR does not experience noticeable rain fade and in the case of the TLS has an operational range of 60 nm. This system is co-operative depending, it means that in the case of transponder failure no aircraft detection will be provided.

Flight inspection of the PAR

A traditional PAR flight inspection procedure is performed without a navigation signal available to compare directly to a truth reference. A traditional PAR is flight inspected by comparing written notes between two observers, one taking notes at a truth reference system such as a theodolite and the other observer taking notes while observing the radar console, see ICAO Document 8071. The Transponder Landing System (TLS) non-traditional PAR can transmit an ILS signal that corresponds to the aircraft position relative to the precision approach. Therefore, the graphical depiction can be directly verified using Instrument Landing System (ILS) flight inspection techniques. This direct measurement removes some ambiguity from the PAR flight inspection process.

External links

{{commonscat, Precision approach radar * C. Wolff, '' Radartutorial'
Precision Approach Radar
Aeronautical navigation systems Aircraft landing systems Air traffic control Ground radars Types of final approach (aviation)

Non-traditional PAR using SSR transponder reply

Flight inspection of the PAR

See also

External links