Until recently primary radar and secondary radar was the only surveillance system used in ATM ( Air Traffic Management )
Primary surveillance radar (PSR). The PSR system provides information on the bearing and distance of the aircraft. PSR does not require carriage of any equipment by aircraft and is capable of detecting almost any moving target. With increasing usage of more advanced surveillance systems, the use of PSR for ATM has diminished.
Secondary surveillance radar (SSR). The SSR interrogates transponder equipment installed in the aircraft. In Mode-A, the aircraft transponder provides identification information, aircraft bearing and distance and in Mode-C, it provides pressure-altitude information. The SSR is in widespread use in many parts of the world. The accuracy, resolution and over-all performance of range and azimuth information is significantly improved by the application of monopulse (MSSR) The advantage of SSR for surveillance purposes can be further enhanced through the use of Mode-S which is a technique that permits selective interrogation of Mode S transponder-equipped aircraft.
Automatic dependent surveillance-broadcast (ADS-B) is a function on a properly equipped aircraft or surface vehicle that periodically broadcasts its state vector (horizontal and vertical position, horizontal and vertical velocity) and other information.
Under ADS-B, a vehicle periodically broadcasts its own position and other information without knowing if other stations might be receiving it, and without expectation of an acknowledgement or reply. ADS-B is automatic in the sense that no pilot or controller action is required for the information to be issued. It is dependent surveillance in the sense that the surveillance-type information so obtained depends on the suitable navigation and broadcast capability in the source vehicle.
Theory of operation
ADS-B consists of three components:
A transmitting subsystem that includes message generation and transmission functions at the source.
The propagation medium.
A receiving subsystem that includes message reception and report assembly functions at the receiving vehicle or ground system.
The source of the state data and other transmitted information as well as user applications are not considered to be part of the ADS-B system.
Relationship to addressed ADS
There are two commonly recognized types of ADS for aircraft applications:
ADS-Addressed (ADS-A), also known as ADS-Contract (ADS-C)
ADS-B is inherently different from ADS-A, in that ADS-A is based on a negotiated one-to-one peer relationship between an aircraft providing ADS information and a ground facility requiring receipt of ADS messages.
Traffic information services-broadcast (TIS-B)
TIS-B supplements ADS-B air-to-air services to provide complete situational awareness in the cockpit of all traffic known to the ATC system. TIS-B is an important service for an ADS-B link in airspace where not all aircraft are transmitting ADS-B information. The ground ADS-B station transmits surveillance target information on the ADS-B data link for unequipped aircraft or aircraft transmitting only on another ADS-B link. TIS-B uplinks are derived from the best available ground surveillance source:
Ground radars for primary and secondary targets
Multi-lateration systems for targets on the airport surface
ADS-B systems for targets equipped with a different ADS-B link
Flight information services-broadcast (FIS-B)
FIS-B provides weather text, weather graphics, NOTAMs, ATIS, and similar information. FIS-B is inherently different from ADS-B in that it requires sources of data external to the aircraft or broadcasting unit, and has different performance requirements such as periodicity of broadcast.
ADS-B physical layer
Three link solutions are being proposed as the physical layer for relaying the ADS-B position reports:
1090 MHz Mode S Extended Squitter (ES)
Universal Access Transceiver (UAT)
VHF Digital Link (VDL) Mode 4.
In addition, FLARM is a simple but highly effective low-cost and low-range implementation of an ADS-B concept which has spread rapidly in general Aviation, especially gliders and helicopters. As a consequence of the low-range and non-certification, there is no ATC downlink.
Europe has not officially chosen a physical layer for ADS-B. A number of technologies are in use.
However, the influential Eurocontrol CASCADE program uses 1090ES exclusively. With 1090ES, the existing Mode S transponder or a stand alone 1090 MHz transmitter supports a message type known as the extended squitter (ES) message. It is a periodic message that provides position, velocity, heading, time, and, in the future, intent. To enable an aircraft to send an extended squitter message, the transponder is modified and aircraft position and other status information is routed to the transponder. Some ATC ground stations and TCAS-equipped aircraft already have the necessary 1090 MHz (Mode-S) receivers to receive these signals, and would only require enhancements to accept and process the additional Extended Squitter information. 1090ES does not support FIS-B service.
In USA the FAA has announced a dual link decision using 1090 MHz ES and UAT as mediums for the ADS-B system in the United States. The 1090 MHz extended squitter ADS-B link for air carrier and private/commercial operators of high performance aircraft, and Universal Access Transceiver (UAT) ADS-B link for the typical general aviation user.
Universal access transceiver
The UAT system is specifically designed for ADS-B operation. UAT has lower cost and greater uplink capacity than 1090ES. Although 978 MHz resides in the TACAN assigned portion of the aeronautical spectrum, in the US 978 is used for transmission of airborne ADS-B reports and for broadcast of ground-based aeronautical information. UAT users have access to ground-based aeronautical data and can receive reports from proximate traffic (FIS-B and TIS-B). TIS-B provides reports for proximate aircraft through a multilink gateway service that provides ADS-B reports for 1090ES equipped aircraft and non-ADS-B equipped Radar traffic.
VDL mode 4
The VDL Mode 4 system could utilize one or more of the existing aeronautical VHF frequencies as the radio frequency physical layer for ADS-B transmissions. VDL Mode 4 uses a protocol (STDMA) that allows it to be self-organizing, meaning no master ground station is required. This medium is best used for short message transmissions from a large number of users. VDL Mode 4 systems are under consideration in Northern Europe.
ADS-B supported applications
The ADS-B data link supports a number of airborne and ground applications. Each application has its own operational concepts, algorithms, procedures, standards, and user training.
Cockpit display of traffic information
A Cockpit Display of Traffic Information (CDTI) is a generic display that provides the flight crew with surveillance information about other aircraft, including their position. Traffic information for a CDTI may be obtained from one or multiple sources, including ADS-B, TCAS, and TIS-B. Direct air-to-air transmission of ADS-B messages supports display of proximate aircraft on a CDTI. In addition to traffic based on ADS-B reports, a CDTI function might also display current weather conditions, terrain, airspace structure, obstructions, detailed airport maps, and other information relevant to the particular phase of flight.