Precision Approach Radars

Issue: 6 / 2012By Lt General (Retd) Naresh Chand

Flying from an aircraft carrier is very tricky, precise and complex. While landing on the deck of a carrier, the indicated air speed (IAC) of the aircraft is reduced by sailing the carrier as fast as possible in the opposite direction of landing aircraft. The differential of the speed of the carrier and the aircraft then helps in reducing the IAC, thus making it safe for landing. There can also be adverse flying conditions due to weather, terrain, operational environment or a fault developed in the aircraft. Radars can assist the pilot to land safely in such complex and difficult conditions by providing an electronic glide path. There are two types of radar approaches, i.e. approach surveillance radar (ASR) and precision approach radar (PAR). A radar approach may be given to any pilot on request or when the aircraft is in distress or to expedite traffic. Acceptance of an ASR or a PAR by a pilot is only possible when the weather conditions are within the prescribed limit. However, the decision to make a radar approach when the weather is below the prescribed minimum limits rests with the pilot.

ASR

ASR is a type of radar approach carried out with the active assistance of air traffic control (ATC). While landing, the pilot communicates with the air traffic controller who vectors or guides the aircraft to align it with the runway centreline. The controller continues to guide the aircraft until the pilot can complete the approach and carry out landing by visual reference to the surface of the runway. An ASR is used only when there is an ATC operational requirement or in an unusual or emergency situation.

Precision Approach

In a precision approach, the traffic controller provides highly accurate navigational guidance in azimuth and elevation to a pilot so that he can keep his aircraft aligned with the extended centreline of the runway. The accuracy of the radar permits lower minimum descent than a non-precision approach. Thus the pilot has a better chance of seeing the ship or airfield in bad weather conditions. Accurate navigational assistance includes information of glide-path interception approximately 10-30 seconds before it occurs, and when to start descent. The published decision height will be given only if the pilot makes a request for it. Corrections for any deviation from the glide-path is, communicated to the pilot. Trend information is also issued with respect to the elevation of the aircraft. Countdown for range to touchdown is communicated every mile/km. If an aircraft flies outside the specified safety zone limits in azimuth and/or elevation, then the pilot will be guided to execute a missed approach or to fly a specified course unless the pilot has the runway in sight. Precision is automatically terminated upon completion of the approach.

Some Examples of PARs

MPN1: The first PAR known as MPN1 was developed by the Bendix Corporation of the US and used operationally in January 1945 in France. It was called ground control approach (GCA) which is the original air traffic control technique to use radar for landing. The first PAR used in Britain was CPN4/MPN11 which was acquired from the US under the lend/lease arrangement by the Royal Air Force and later on transferred to the Royal Navy (RN). The first GCA operators were pilots till ground controllers were trained. The radar picked up the aircraft along with the weather clutter which had to be suppressed. The precision beams were also very narrow thus it was necessary to servo the antennas so that they remained pointed towards the aircraft. Later on the PARs acquired sophisticated clutter suppression technology and auto-tracking.

Talkdown Technique

The navigation data is passed to the pilot by the controller through a set of continuous radio transmissions. The technique used by the Royal Navy was called “continual talkdown”. The pilot was guided by a set of directions as he approached the decent point. The transmit switch was then left open and there were continuous talks, advising the pilot of their position in both azimuth and elevation, and also of necessary correction. This became known as a ‘Talkdown’.

Carrier-controlled approach (CCA) and GCA radars: CCA and GCA systems can help in safe landing of aircraft, even under zero visibility conditions. The primary approach systems in the US Navy are the AN/SPS-46(V) precision approach landing system (PALS) for CCA and the AN/FPN-63 PAR for CGA.

AN/SPN-46(V) PALS: The AN/SPN-46(V)1 system replaced the AN/SPN-42A automatic carrier landing system. Textron Systems’ SPN 46 (V)1 and (V)2 PALS is meant for aircraft carriers and amphibious assault ships. AN/SPN-46 PALS is installed on all the US Navy aircraft carriers and provides safe and reliable final approach and landing for PALS-equipped carrier-based aircraft, during daylight or darkness. It is rarely affected by severe weather and sea state conditions, and is not affected by low ceiling and visibility problems. It can also provide safe and reliable final approach and landing guidance for Marine Corps helicopters and AV-8B Harrier VSTOL attack aircraft. The AN/SPN-46 is capable of controlling up to two aircraft simultaneously in a “leapfrog” pattern because of having two dual-band radar antennas/transmitters. The AN/SPN-46 employs low-probability-of-intercept technology to decrease the probability of passive detection by hostile forces. The AN/SPN-46 employs an X-band coherent transmitter and receiver, utilising monopulse tracking and doppler processing for clutter rejection and rain attenuation at an operating range of 8 nm (about 14.8 km). Designed primarily as an automatic landing system, it can also be operated in manual control. The source of information can be display or voice.

Other US Naval Radar Systems used with CCA: Other radars, such as the AN/SPN-35, the CCA —AN/SPN-43 and the AN/SPN-44, are also used in conjunction with the precision CCA system for landing operations. The AN/SPN-35 radar set provides both azimuth and elevation data for precision approaches to aircraft carriers during adverse weather conditions. The AN/SPN-43 is a surveillance and ATC radar used on carriers and amphibious-type ships. It provides air navigational data for control and identification of aircraft in the area of the ship. With a range of 50 nm (about 92.6 km), it tracks low-flying aircraft to a minimum of 250 yards and covers 360° at altitudes from radar horizon to 30,000 feet (9,144 m). The AN/SPN-44 is a range-rate radar that computes, indicates and records the true and relative air speed of aircraft, making a landing approach to the carrier.

AN/FPN-63(V) PAR: The AN/FPN-63(V) PAR is used by the Navy and Marine Corps for providing safe, orderly and expeditious recovery of aircraft in instrument meteorological condition (IMC). The AN/FPN-63(V) provides talk-down capabilities to land military and civil aircraft during reduced meteorological conditions. The AN/FPN-63(V) is also used in conjunction with an airport surveillance radar system to provide a complete GCA capability for Navy, Marine Corps, and other military and civilian aircraft as required. Information of azimuth, elevation and range to touchdown is provided and displayed on the operator’s console. This information of glide slope and centreline deviation is then used by the air traffic controller to guide the pilot for a precision instrument landing.

Selex Gallileo’s SPN-720 Shipborne Precision Approach Radar: SPN-720 is a naval precision radar designed for fast takeoff and recovery of aircraft. The radar is able to provide safe and reliable final approach and deck landing guidance for aircraft during day/night and in adverse weather conditions. The SPN-720 offers CV Naval Air Training and Operating Procedures Standardisation (NATOPS) Programme Mode III landing, manual approach, during which the radar controller relays continuous updates to the pilot on his position and direction via a secure VHF channel. The SPN-720 employs an I-band Doppler radar with coherent solid state transceiver, utilising frequency agile monopulse tracking at an operating range of 12 nm (about 22 km). The antenna is fitted on a stabilised gimbal which automatically locks onto the landing aircraft. The SPN-720 can be operated as a stand-alone system or it can be integrated with the ship combat management system as it is capable of automatically correcting the parallax error between the radar location and landing path. The SPN-720 can provide simultaneous control of two aircraft. Low probability of interception is ensured by minimal radiated power. The radar features two consoles (master and slave), each having a PAR display and an air search display. The PAR display indicates:

  • Conventional azimuth vs elevation (Az-El) display format: The Az-El display tracks the aircraft indicating its position with respect to the touchdown point, horizon sea level and runway centreline. The tracking data is updated every second.
  • Width/height indicator with the error data set: Width/height indicator is centred on the glidepath to indicate aircraft offset/error in azimuth vs elevation, within ± 6 deg azimuth and ± 600 feet elevation from the centreline.
  • Messages and information area: The message and information area displays commands, tracks status and information, system status, operational status, operational mode, range, scale and general information.