Air Traffic Control System

PURPOSE:

The purpose of the air traffic Control (ATC) system is to enable ground controllers to maintain safe separation of aircraft,both on the ground and in the air.

OVERVIEW:

ATC uses ground based radars are used to detect the target this is done by a phenomenon of transmission and reception. PSR (Primary surveillance radars) are used and it provides the range and azimuth of the detected thing. For accurate detection and to avoid the clutter SSR (Secondary Surveillance Radars). are used in the combination with PSR. This is used because PSR has some disadvantages, one of which is that the amount of energy being transmitted is very large compared with the amount of energy reflected from the target,small targets ,or those with poor reflecting surfaces,could further reduce the reflected energy. The SSR transmits a specific low energy signal (the interrogation) to a known target. This signal is analysed by a transponder.

OPERATING FREQUENCIES:

The ATC system operates on two frequencies

within the L-band of radar:

      • Interrogation codes on a 1030 MHz
        carrier wave

• Reply codes on a 1090 Mhz carrier wave.

ATC TRANSPONDER MODES:

MODE A: If the controller needs to distinguish between two aircraft in close proximity an identity code will be requested; the pilot pushes a switch on his ATC control panel, and this highlights the icon on the controller’s screen. Since each aircraft is allocated with a unique code, only one icon per aircraft will be highlighted; this unique identification is referred to as a squawk code.

MODE S:Aircraft equipped with Mode S transponders allow specific aircraft to be interrogated; this increases the efficiency of the ATC resources.

The Mode S system has a number of advantages:

•  Increased traffic densities
•  Higher data integrity
•  Efficient use of the RF spectrum
•  Reduced RF congestion
•  Alleviation of Mode A and C code shortages
•  Reduced workload for ground controllers
•  Additional aircraft parameters available to the ground controller.

 Emergency ATC transponder codes

Code            Meaning

7700           General air emergency

7600           Loss of radio

7500           Hijacking

An aircraft can not be detected by a side lobe interrogation for that a side lobe suppression technique is used

MODE S Operation: Individual interrogations are sent to specific aircraft; only the transponder on this aircraft sends a reply. This reply contains additional information, e.g. selected altitude and

flight number.Directional and omni directional beam patterns are transmitted  Unlike ATCRBS, Mode S uses amonopulse SSR; this reduces the number of interrogations required to track a target. In theory, monopulse radar only requires one reply to determine the target’s azimuth (direction and

range). Two interrogation uplink formats (UF) are transmitted these are the all-call and roll-call interrogations.

Automatic dependent surveillance broadcast (ADS-B):

Automatic dependent surveillance-broadcast (ADS-B) is intended eventually to replace

conventional ground-based ATC radar systems The system also provides surveillance in remote

areas where ground radar coverage is not possible, e g over oceans ADS-B forms part of the FAA’s next generation air transportation system (NGATS) It will revolutionize how pilots

obtain traffic and weather information The intention is to increase air navigation safety by

providing crews with real-time information about other traffic, this makes it possible for the crew to

be responsible for their own aircraft’s separation and collision avoidance The system is automatic

in that no interrogation is required to initiate a transponder broadcast from the aircraft, this type

of unsolicited transmission is known as a squitter

In other words ADS-B provides real-time data for both flight crews and air traffic controllers; data is exchanged between aircraft and is independent of ground equipment. The system allows operations in

remote and/or mountainous areas not covered by ground radar.

Typical ADS-B architecture is given below

ADS-B used in remote areas is given below

References:

• Aircraft Communications and Navigation systems: Principles operations and Maintenance by Mike Tooley and David Wyatt
• Google images.