Precision approach radar

Precision approach radar (PAR) is a type of radar guidance system designed to provide lateral and vertical guidance to an aircraft pilot for landing, until the landing threshold is reached. After the aircraft reaches the decision height (DH) or decision altitude (DA), guidance is advisory only. Controllers monitoring the PAR displays observe each aircraft's position and issue instructions to the pilot that keep the aircraft on course and glidepath during final approach. It is similar to an instrument landing system (ILS) but requires control instructions. One type of instrument approach that can make use of PAR is the ground-controlled approach (GCA). Air traffic controllers must transmit a minimum of every 5 seconds to the pilot their relation to the azimuth portion and, once intercepting the glidepath, their elevation. The approach is terminated when the aircraft reaches the OCA/H (Obstacle Clearance Altitude/Height). Nevertheless, information is provided till threshold and aircraft may be monitored by controller till touchdown. Controller in charge of PAR should not be responsible for any duty other than the PAR approach concerned.

The upper portion of the display indicates elevation, the lower portion azimuth. Controllers must be able to interpret radar returns for the azimuth as a "top view" to inform them if the aircraft is left or right of course.

Precision approach radars are most frequently used at military air traffic control 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. The OJ-333 Radar scope is the indicator which the air traffic controller uses to provide instructions to the pilots.

In the United States PAR are used mostly by The Navy. This is because they present a more covert type of precision approach for use on Aircraft carriers. An ILS installed on a ship could provide guidance to enemy missiles but a PAR does not provide accurate guidance without controller instruction.

There are systems that provide PAR functionality without using primary radar. These non-traditional PAR systems use transponder multilateration, triangulation and/or trilateration.

...
Wikipedia