Air Traffic Control Radar Beacon System
(Redirected from Air traffic control radar beacon system)
The Air Traffic Control Radar Beacon System (ATCRBS) is a system used in air traffic control (ATC) to enhance radar monitoring and separation of air traffic. ATCRBS assists ATC radars by acquiring information about the aircraft being monitored, and providing this information to the radar controllers. The controllers can use the information to identify returns from aircraft (known as targets) and to distinguish those returns from ground clutter.
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Parts of the system
The system consists of transponders, installed in aircraft, and secondary surveillance radars (SSRs), installed at air traffic control facilities. The SSR transmits interrogations and listens for any replies. Transponders that receive an interrogation decode it, decide whether to reply, and then respond with the requested information when appropriate. Note that in common informal usage, the term "SSR" is sometimes used to refer to the entire ATCRBS system, however this term (as found in technical publications) properly refers only to the ground radar itself.
Ground equipment
An ATC ground station consists of two radar systems and their associated support components. The most prominent component is the primary surveillance radar, or PSR. It is also sometimes referred to as skin paint radar because it operates using traditional radar principles, transmitting radio pulses and listening for and timing the reflections from the skin or other metal components of aircraft.
The second system is the secondary surveillance radar, or SSR. The SSR utilizes a pair of antenna systems, one with an omnidirectional pattern, and the other with a highly directional pattern. The directional antenna is typically fitted to the PSR antenna, so that they point in the same direction as the antennas rotate. The omnidirectional antenna need not rotate, but must be mounted close by.
The SSR repetitively transmits interrogations as the rotating radar scans the sky. The interrogation specifies what type of information a replying transponder should send by using a system of modes. There have been a number of modes used historically, but three are in common use today: mode A (also called mode 3/A), mode C, and mode 2. Mode 3/A is used to identify the aircraft amongst the other aircraft in the radar's coverage area. Mode C is used to request an aircraft's altitude, and mode 2 is used to identify military aircraft.
Neither Mode 4 nor mode S are part of the ATCRBS system but use the same transmit and receive hardware. Mode 4 is used by military aircraft for the Identification Friend or Foe (IFF) system. Mode S is a discrete selective interrogation rather than a general broadcast, that facilitates TCAS for civil aircraft. Mode S transponders ignore interrogations not addressed with their unique identity code, reducing channel congestion. At a typical SSR radar installation, ATCRBS, IFF, and mode S interrogations will all be transmitted in an interlaced fashion.
Returns from both radars at the ground station are transmitted to the ATC facility using a microwave link, a coaxial link, or (with newer radars) a digitizer and a modem. Once received at the ATC facility, a computer system known as a radar data processor associates the reply information with the proper primary target and displays it next to the target on the radar scope.
Airbourne equipment
The equipment installed in the aircraft is considerably simpler, consisting of the transponder itself, usually mounted in the instrument panel or avionics rack, and a small L band UHF antenna, mounted on the bottom of the fuselage.
Typical installations also include an altitude encoder, which is a small device connected to both the transponder and the aircraft's static system. It provides the aircraft's pressure altitude to the transponder, so that it may relay the information to the ATC facility.
The transponder has a small required set of controls and is simple to operate. It has a method to enter the four-digit transponder code, also known as a beacon code or squawk code, and a control to transmit an ident, which is done at the controller's request. Transponders typically have 4 operating modes: Off, Standby, On (Mode-A), and Alt (Mode-C). On and Alt mode differ only in that the On mode inhibits transmitting any altitude information. Standby mode allows the unit to remain powered and warmed up but inhibits any replies, since many transponders incorporate transmitters which must be warmed up before they will function.
Theory of operation
The steps involved in performing an ATCRBS interrogation are as follows: First, the ATCRBS interrogator periodically interrogates aircraft on a frequency of 1,030 MHz. This is done through a rotating or scanning antenna at the radar's assigned Pulse Repetition Frequency (PRF). Interrogations are typically performed at 450 - 120 interrogations/second. Once an interrogation has been transmitted, it travels through space in the direction the antenna is pointing at the speed of light until an aircraft is reached. When the aircraft receives the interrogation, the aircraft transponder will send a reply after a 3.0μs delay indicating the requested information. The interrogator's processor will then decode the reply and identify the aircraft. The range of the aircraft is determined from the delay between the reply and the interrogation. The azimuth of the aircraft is determined from the direction the antenna is pointing when the reply was received.
The interrogation
Interrogations consist of two pulses, 0.8μs in duration, referred to as P1 and P3. The timing between these pulses determines the mode of the interrogation, and thus what the nature of the reply should be.
Mode 3/A uses a spacing of 8.0μs, and is used to request the beacon code, which was assigned to the aircraft by the controller to identify it. Mode C uses a spacing of 21μs, and requests the aircraft's pressure altitude, provided by the altitude encoder. Mode 2 uses a spacing of 5μs and requests the aircraft to transmit its Military identification code. The latter is only assigned to Military aircraft and so only a small percentage of aircraft actually reply to a mode 2 interrogation.
The reply
Replies to interrogations consist of 15 time slots, each 1.45μs in width. The reply is encoded by the presence or absence of a 0.45μs pulse in each slot. These are labeled as follows:
F1 C1 A1 C2 A2 C4 A4 X B1 D1 B2 D2 B4 D4 F2 SPI
The F1 and F2 pulses are framing pulses, and are always transmitted by the aircraft transponder. They are used by the interrogator to identify legitimate replies. These are spaced 20.3μs apart.
The A4, A2, A1, B4, B2, B1, C4, C2, C1, D4, D2, D1 pulses constitute the "information" contained in the reply. These bits are used in different ways for each interrogation mode.
For mode A, each digit in the transponder code (A, B, C, or D) may be a number from zero to seven. These octal digits are transmitted as groups of three pulses each, the A slots reserved for the first digit, B for the second, and so on.
In a mode C reply, the altitude is encoded by a Gillham interface, which uses gray code. The Gillham interface is capable of representing a wide range of altitudes, in 100-foot increments. The altitude transmitted is pressure altitude, and corrected for altimeter setting at the ATC facility.
In a mode 2 reply, the information is similar to the mode A reply in that there are 4 digits transmitted between 0 and 7. The mode 2 reply differs from the mode A reply in that the transmitted code is assigned by a military air traffic controller, not a civilian air traffic controller.
The X bit is currently only used for test targets. This bit was originally transmitted by BOMARC missiles that were used as air launched test targets.
The SPI pulse is positioned 4.35μs past the F2 pulse and is used as a "Special Identification Pulse". The SPI pulse is turned on by the ident control on the transponder in the aircraft cockpit when requested by air traffic control. If there are two aircraft with the same mode A code in the same vicinity, the air traffic controller can request the SPI bit be turned on (by requesting that the pilot "squawk ident") so that the controller can separate them. This can occasionally occur when an aircraft moves from one air traffic control area to another.
Side lobe suppression
The SSR's directional antenna is never perfect; inevitably it will "leak" lower levels of RF energy in off-axis directions. These are known as side lobes. When aircraft are close to the ground station, the side lobe signals are often strong enough to elicit a reply from their transponders when the antenna is not pointing at them. This can cause ghosting, where an aircraft's target may appear in more than one location on the radar scope. In extreme cases, an effect known as ring-around occurs, where the transponder replies in such excess that the target is distorted into an arc or circle centered on the radar site.
To combat these effects, side lobe suppression (SLS) is used. SLS employs a third pulse, P2, spaced 2μs after P1. This pulse is transmitted from the omnidirectional antenna at the ground station, rather than from the directional antenna. The power output from this antenna is calibrated so that, when received from an aircraft, the P2 pulse is stronger than either P1 or P3, except when the directional antenna is pointing directly at the aircraft. By comparing the relative strengths of P2 and P1, transponders can determine whether or not the antenna was pointing at the aircraft when the interrogation was received.
Radar display
The beacon code and altitude were historically displayed verbatim on the radar scope next to the target, however modernization has extended the radar data processor with a flight data processor, or FDP. The FDP automatically assigns beacon codes to flight plans, and when that beacon code is received from an aircraft, the computer can associate it with flight plan information to display immediately useful data, such as aircraft callsign, the aircraft's next navigational fix, assigned and current altitude, etc. near the target in a data block.
Mode S
Mode S, or mode select, despite also being called a mode, is actually a radically improved system intended to replace ATCRBS altogether. A few countries have mandated mode S, and many other countries, including the United States, have begun phasing out ATCRBS in favor of this system. Mode S is designed to fully interface with ATCRBS systems: mode S SSRs can interrogate ATCRBS transponders, and mode S transponders will reply to older ATCRBS interrogations.
Frequency Congestion, FRUIT
Mode S was developed as a solution to frequency congestion on 1,090MHz. The high coverage of radar service available today means that some radar sites receive transponder replies from interrogations that were initiated by other nearby radar sites. This results in FRUIT, or False Replies Uncorrelated In Time, which is the reception of replies at a ground station that do not correspond with an interrogation. This problem has worsened with the increasing prevalence of technologies like TCAS, in which individual aircraft interrogate one other to avoid collisions.Finally, technology improvements have made transponders increasingly affordable such that today almost all aircraft are equipped with them. As a result, the sheer number of aircraft replying to SSRs has increased.
Mode S as a Congestion Solution
Mode S attempts to reduce these problems by assigning aircraft a permanent mode S address, derived from the aircraft's internationally assigned registration number. It then provides a mechanism by which an aircraft can be selected, or interrogated such that no other aircraft reply.
The system also has provisions for transferring arbitrary data both to and from a transponder. This aspect of mode S makes it a building block for many other technologies, such as TCAS 2, Traffic Information Service (TIS), and Automatic Dependent Surveillance-Broadcast.
References
- The Story of Mode S: An Air Traffic Control Data Link Technology - Story of the development of Mode S at MIT's Lincoln Laboratory
- EUROCONTROL Mode S & ACAS Programme - Home page for the European Mode S & ACAS implementation coordination program
- FAA TSO C74c - Minimum performance standards for ATCRBS transponders in the US
- AlliedSignal Aerospace (1996) Bendix/King KT76A/78A ATCRBS Transponder Maintenance Manual. (Rev. 6)
Categories
Air traffic control