Radar is an acronym for radio detection and ranging. And it simply does what the name states out.
The aim of a radar system, in case of air traffic control, is to detect aircraft in the air and on the ground. May we can say the radar are an air traffic controllers eyes and gives her/him the ability and the required support to control air traffic in a save and efficient way.
The Radar Principle
Like written above radar must have to do something with radio, detection and ranging. And this is truly the principle how it works. There are three fundamental key items to understand radar basics:
1. Electromagnetic energy reflects off electrically leading surfaces.
2. Electromagnetic energy travels through air at constant speed (c = 300000000 m/s).
3. Electromagnetic energy travels in a straight line.
The radar sends out electromagnetic pulses. When the electromagnetic energy of the sending pulse encounter an aircraft, it's reflected and scattered in all directions. A very small portion of the sent signal is returned to the radar-site. Through measuring the span of time between sending and receiving it's possible to estimate the distance to the target. Because of a turning directional antenna and azimuth encoders, which determine the position of the antenna related to north, the direction of the target is well known. This data is sent to a target processor which produces tracks to trace an aircraft.
There are several different radar systems used in air traffic control. The principle above relates to the primary radar. Primary radars are used to detect targets even without transponder. It's called an Non-Cooperative Independent Surveillance system.
The secondary radar is a Co-operative Independent Surveillance system. It's based on the same principles like the primary radar. The difference between primary and secondary radar consists in using the aircraft built in transponder to get more information like identification, altitude, selected flight level, indicated air speed, magnetic heading etc.. For this purpose the ground station interrogates the transponders database.
A further advantage is the reduced transmission power at the ground station. This as a matter of fact, that the transponder equipped in the aircraft sends back a much stronger signal then the reflections at a primary system.
Multilateration systems are based on hyperbolic positioning. It locates the position of an aircraft by accurately computing the time difference of arrival (TDOA) of a signal emitted from the aircraft built in transponder to various receivers. The transponder of an aircraft sends out an acquisition squitter every second and an extended squitter every four to six seconds. The acquisition squitter contains the ICAO 24 bit technical identification address. This address should be unique and leads to the unambiguous identification of an aircraft. The extended squitter contains more information like positioning and velocity data and it's part of the ADS-B system used additional to multilateration. ADS-B means Automatic Depended Surveilance-Broadcast and it's an Co-operative Dependent Surveillance system
The data gained from the MLAT system is directed to the Advanced Surface Movement Guidance and Control System.
The Advanced Surface Movement Guidance and Control System (A-SMGCS) utilizes multiple Surveillance data sources such as Primary and Secondary Surveillance Radar (PSR, SSR) and Surface Movement Radar (SMR).
Many other data like flight plan data, environemental data (wind, visibility, etc.), airport ground lighting data (stop bars lights, taxiway centerline lights, etc.) are then added to an advanced multi-sensor processor which then provides the air traffic controller a comprehensive overview of the traffic situation at and around the airport.
A-SMGCS Level 1
At the first level, the air traffic controller will be assisted by a surveillance service which completes its visual observation by displaying on a screen:
- The airport traffic context (airport map)
- The position of all vehicles in the movement area
- The position of all aircraft in the movement area
- The identity of all aircraft in the movement area
- The identity of all cooperative vehicles
A-SMGCS Level 2
At the second level, a control function dedicated to runway incursion alerting (RIMCAS) is added. The function detects runway conflicts and alerts the air traffic controller in due time:
A-SMGCS Higher Levels
The higher levels are guidance and routing functions dedicated to support in a first step the air traffic controller to optimze the traffic flow of the airport. These functions are then in a second step down-linked to pilots and vehicle drivers: