What is an Airport Surveillance Radar (ASR) System?

An Airport Surveillance Radar (ASR) system is used at airports to detect the presence and position of aircrafts flying in the vicinity of airports. It also provides other information (such as aircraft's identity code, level, an emergency status code, etc.). The information about the aircrafts is displayed on the workplace screen of air traffic controller (ATC). In a large airport, multiple screens are used in the air traffic control room. 

Figure 1: Air traffic control room

Air traffic controllers continuously monitor the information of aircrafts on the display screen and give directions to the pilots by radio to maintain a safe and orderly flow of air traffic in the airspace. Hence, air surveillance radar (ASR) system is the main air traffic control system for the airspace around airports. Typically, air surveillance radar (ASR) systems are capable of reliably detecting and tracking aircraft at altitudes below 25,000 feet (7,620 meters) and within 40 to 60 nautical miles (75 to 110 km) of the airport. The ASR is sometimes referred to as terminal area radar (TAR). 

The new generation Airport Surveillance Radar (ASR-11) system is a fully digital radar system. ASR-11 is a Digital Airport Surveillance Radar (DASR) that can detect & display aircraft position and weather conditions simultaneously in the vicinity of civilian and military airfields. It allows the ATC to maintain a safe and orderly flow of air traffic in the airspace, as well as helps to provide weather advisories to the pilots.  

Construction and working principle of Air surveillance (ASR) system

Radar stands for radio detection and ranging. As its name implies, the radar system uses electromagnetic wave/radio wave to determine the object's/ target's parameters such as location, speed, angle, and size. Typically, a radar system consists of a transmitter that produces electromagnetic waves in the radio or microwave domain, a transmitting antenna, receiving antenna (often the same antenna is used for transmitting & receiving), a duplexer, amplifier & mixer circuits, filters, signal processor, and other supporting systems to determine the object(s) parameters, and display system to show the captured information in visual form.

Figure 2: Air surveillance (ASR) system

The air surveillance (ASR) system is an integrated primary and secondary radar system (figure 2). It is located in the airport and connected to the air traffic control center (ATCC) and remotely controlled. 

Primary Surveillance Radar (PSR)

The primary radar is known as primary surveillance radar (PSR). Typically, the primary radar consists of a large rotating parabolic antenna dish mounted on a tower (figure 2). The PSR antenna rotates (usually at 5-12 rpm) and emits high power pulses of microwave radio waves in a narrow vertical fan-shaped beam around the airspace surrounding the airport. The radar pulses propagate at the speed of light (300 000 000 m/s).  In the US, the primary radar operates in S-band frequency (2.7 - 2.9 GHz) with a peak radiated power of 25 kW and average power of 2.1 kW. 

Since the antenna of the PSR rotates at a constant rate about a vertical axis, the radar beam scans the entire surrounding airspace about every 5 seconds. When the microwave beam strikes an aircraft, some of the energy of microwaves is reflected back (called the "echo") to the antenna and is detected by the radar receiver. The radar receiving system determines the distance of the aircraft from the ASR radar system by calculating the time delay between the transmitted and received echo pulses. The main function of primary radar is to determine the range and bearing (azimuth) of the aircraft in respect of the antenna position. Bearing (azimuth) is the horizontal angle between the aircraft and the true north. This calculated information is displayed on the workplace screen of the air traffic controller (ATC).

A key benefit of the primary surveillance radar (PSR) is that it does not require any on-board equipment to locate aircraft and provides good range & bearing information. The limitations of the PSR system are that it only determines the aircraft position (it does not identify the aircraft code), it cannot determine the altitude of the aircraft and also has clutter issues (reflections from terrain, buildings, clouds.). The clutter can be removed by the use of suppressors. The need for a secondary radar system is generated because of the limitations of primary radar and the need for more information by air traffic controllers due to the increasing air traffic. 

Secondary Surveillance Radar (SSR)

A secondary radar system is known as secondary surveillance radar (SSR). The secondary surveillance radar (SSR) consists of a second rotating antenna often mounted on the primary radar antenna. The SSR looks like a horizontal metal sheet (figure 2). Unlike PSR, the SSR requires an aircraft to be fitted with an onboard radar transponder (transmitter/receiver). A transponder is an electronic device in the aircraft that produces a reply when it receives a radio-frequency interrogation.

The secondary surveillance radar (SSR) operates in sync with the primary radar. The SSR antenna rotates (usually at 5-12 rpm) and emits high power pulses of microwave radio waves in a narrow vertical fan-shaped beam. The narrow vertical fan-shaped beam has a frequency of 1030 MHz in the L band with a peak power typically 160 - 1500 W. This beam interrogates the onboard equipment (transponder).

Figure 3: Secondary radar signal interrogation

For each interrogation signal, the transponder emits a coded reply signal with a frequency of 1090 MHz to the secondary radar antenna. The secondary radar antenna uses this signal for aircraft's identification, altitude of the aircraft, and knowing other information (for example, emergency status code, etc.) about the aircraft. This calculated information is displayed on the workplace screen of the air traffic controller (ATC). The SSR’s purpose is to improve the ability to detect and identify aircraft while automatically providing the Flight Level (altitude) of an aircraft and other required information. 

Typically, the information from both primary (PSR) & secondary radar (SSR) systems is displayed on the same screen. The air traffic controllers continuously monitor both PSR and SSR information of all the aircraft on the display screen.

There are several modes of interrogation that include Mode A, Mode B, Mode C, Mode D, & Mode S for civilian use and Mode 1 to 5 for military use. Each mode is indicated by the time spacing between two transmitter pulses, known as P1 and P3 (figure 5).  For example, if operated under mode A, the time interval between the P1 & P3 pulses is 8 microseconds, and if done under mode C, the interval is around 21 microseconds (figure 5). For each mode, the aircraft produces a different response.   Based on the chosen Mode, the transponder emits a coded reply signal containing the requested information of the aircraft.

For instance, for Mode A requests from the SSR, the transponder reply signal contains a code (called transponder code) for aircraft identification purposes. For Mode C requests, the transponder reply signal contains pressure altitude information of the aircraft. Often, Mode A is combined with Mode C. The transponder code (four-digit number) is assigned to the aircraft (for aircraft identification purposes) by the air traffic controller before takeoff. The air traffic Controllers use the term "squawk" when they are assigning a transponder code, e.g., "Squawk 7421". 

The Mode A and C responses are used to help air traffic controllers to identify a particular aircraft's position and altitude on a radar screen; in order to maintain separation. Mode S allows identification with selective addressing and improves data handling. In Mode S, the required information can be sent in text form between the aircraft and the ATC. So, it can very much reduce the radio transmissions, making the information much clearer and easier to understand for both the air traffic controllers & pilots.

Advanced airport surveillance radar (ASR) system

The new generation of Airport Surveillance Radars (ASR-11) are fully digital radar systems and an upgraded version of the previous ASR-9. The ASR-11 is an advanced radar system utilized by the United States as the next generation of terminal air traffic control. The DASR has been deployed around airport terminals across the United States to meet the requirements of a digital, automated air traffic monitoring system. The ASR-11 system can detect & display the aircraft position and weather conditions simultaneously in the vicinity of civilian and military airfields. The ASR-11 is replacing older radars (20 years old) to improve reliability, provide additional weather data, reduce maintenance costs, improve performance, and provide digital data to new digital automation systems for presentation on air traffic control displays.

Automatic Dependent surveillance-broadcast (ADS-B)

Automatic Dependent surveillance-broadcast (ADS-B) is a GPS-based surveillance technology that allows aircraft to transmit its identification, position, altitude, speed, and other information to the air traffic control center (ATC). The ATC uses the information of aircraft to observe, separate, and direct the aircraft more accurately and more efficiently; in the coverage area of the facility used. The ADS-B services are now being used in areas where currently there isn’t, or there is very little radar coverage, backup of radar surveillance systems.