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Automatic dependent surveillance-broadcast (ADS-B) is a cooperative surveillance technique for air traffic control and related applications. An ADS-B-out equipped aircraft determines its own position using a global navigation satellite system and periodically broadcasts this position and other relevant information to potential ground stations and other aircraft with ADS-B-in equipment. ADS-B can be used over several different data link technologies, including Mode-S Extended Squitter (1090 ES), VHF data link (VDL Mode 4), and Universal Access Transceivers (UAT).
ADS-B provides accurate information and frequent updates to airspace users and controllers, and hence supports improved use of airspace, reduced ceiling/visibility restrictions, improved surface surveillance, and enhanced safety, for example through conflict management.
Under ADS-B, a vehicle periodically broadcasts its own state vector and other information without knowing what other vehicles or entities might be receiving it, and without expectation of an acknowledgment 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.[1]
A similar solution is the Automatic Identification System (AIS), a system used by ships and Vessel Traffic Services.
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ADS-B consists of three components:
The source of the state vector and other transmitted information as well as user applications are not considered to be part of the ADS-B system.[1]
Radar directly measures the range and bearing of an aircraft. Bearing is measured by the position of the rotating radar antenna when it receives a response to its interrogation from the aircraft, and range is measured by the time it takes for the radar to receive the interrogation response.
The antenna beam becomes wider as the aircraft gets further away, making the position information less accurate. Additionally, detecting changes in aircraft velocity requires several radar sweeps that are spaced several seconds apart. In contrast, a system using ADS-B creates and listens for periodic position and intent reports from aircraft. These reports are generated based on e.g the global positioning system (GPS)and distributed via VHF radio or Mode S transponders, meaning integrity of the data is no longer susceptible to the position of the aircraft or the length of time between radar sweeps.[2]
Primary Surveillance Radar does not require any cooperation from the aircraft. It is robust in the sense that surveillance outage failure modes are limited to those associated with the ground radar system. Secondary Surveillance Radar depends on active replies from the aircraft. Its failure modes include the transponder aboard the aircraft. Typical ADS-B aircraft installations use the output of the navigation unit for navigation and for cooperative surveillance, introducing a common failure mode that must be accommodated in air traffic surveillance systems.[1]
| Type | Independent? | Cooperative? |
|---|---|---|
| Primary surveillance radar (PSR) | Yes: surveillance data derived by radar |
No: does not depend on aircraft equipment |
| Secondary surveillance radar (SSR) | Yes: surveillance data derived by radar |
Yes: requires aircraft to have a working ATCRBS transponder |
| Automatic dependent surveillance (ADS-B) | No: surveillance data provided by aircraft |
Yes: requires aircraft to have working ADS-B function |
| Source:DO-242A[1] | ||
Today's ATM systems does not rely on coverage by a single radar only. Instead a multiradar picture is presented via the ATM system's display to the controller ( ATCO). This improves the quality of the reported position of the airplane, provides a measure of redundancy, and makes it possible to verify the output of the different radars against others. This verification can also use sensor data from other technologies, such as ADS-B and multilateration.
There are two commonly recognized types of ADS for aircraft applications:
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. For example, ADS-A reports are employed in the Future Air Navigation System (FANS) using the Aircraft Communication Addressing and Reporting System (ACARS) as the communication protocol. During flight over areas without radar coverage (e.g., oceanic, polar), reports are periodically sent by an aircraft to the controlling air traffic region.[1]
The transmission delay caused by protocol, satellites, etc., is so long that it requires "huge" separation between airplanes. The cost using the satellite channel leads to less frequent updates. Another drawback is that no other aircraft can benefit from the transmitted information.
The ADS-B link can be used to provide other broadcast services, such as TIS-B and FIS-B (see below).
Another potential aircraft-based broadcast capability is to transmit aircraft measurements of meteorological data.
ADS-B is intended to increase safety and efficiency. Safety benefits include:[3]
ADS-B enables increased capacity and efficiency by supporting:
However, although ADS-B is suitable for surveillance of remote areas where the siting of radars is difficult, some ATC providers are not yet convinced that it is currently suitable for use in high traffic volume areas, such as in UK and Northern European airspace. Changing from conventional SSR to ADS-B would also require investment in ATC infrastructure, something which many European providers may be unwilling to sanction. Furthermore, ADS-B provides no ground verification of the accuracy of the information provided by aircraft and this could have adverse security implications. [4]
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 TIS-B station transmits surveillance target information on the ADS-B data link for unequipped targets or targets transmitting only on another ADS-B link.
TIS-B uplinks are derived from the best available ground surveillance sources:
The multilink gateway service is a companion to TIS-B for achieving interoperability in low altitude terminal airspace. In some airspaces, aircraft that primarily operate in high altitude airspace are equipped with 1090ES, and aircraft operating primarily in low altitude airspace are equipped with UAT. These aircraft cannot directly share air-to-air ADS-B data. In terminal areas, where both types of ADS-B link are in use, ADS-B/TIS-B ground stations use ground-to-air broadcasts to relay ADS-B reports received on one link to aircraft using the other link.[3]
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.[1]
In the US, FIS-B services will be provided over the UAT link in areas that have a ground surveillance infrastructure.[3]
Three link solutions are being proposed as the physical layer for relaying the ADS-B position reports:
A comparison of these link solutions was made in Gatwick/Heathrow in year 2002 for the Eurocontrol ADS programme. [5]
In 2002, 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.[2]
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.[6]
With 1090ES, the existing Mode S transponder (TSO C-112 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. The basic ES does not offer intent since current flight management systems do not provide such data – called trajectory change points. To enable an aircraft to send an extended squitter message, the transponder is modified (TSO C-166A) and aircraft position and other status information is routed to the transponder. 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.[citation needed]
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.
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, invented by Swedish Håkan Lans in 1988) that allows it to be self-organizing, meaning no master ground station is required.
In November 2001 this protocol was published by ICAO as a global standard.
This medium is best used for short message transmissions between a large number of users.
VDL Mode 4 systems required by airlines in Europe as a mean for more efficient ATC service [7].
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.
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.[1]
ADS-B is seen as a valuable technology to enhance ACAS operation. Incorporation of ADS-B can provide benefits such as:
Eventually, the ACAS function may be provided based solely on ADS-B, without requiring active interrogations of other aircraft transponders.[1]
Other applications that may benefit from ADS-B include:
The U.S. FAA ADS-B implementation is broken into three segments each with a corresponding time line. Ground segment implementation and deployment is expected to begin in 2009 and be completed by 2013 throughout the National Airspace System. Airborne equipage is user driven and is expected to be completed both voluntarily based on perceived benefits and through regulatory actions (Rulemaking) by the FAA. The cost to equip with ADS-B Out capability is relatively small and would benefit the airspace with surveillance in areas not currently served by radar. The FAA intends to provide similar service within the NAS to what radar is currently providing (5NM en route and 3NM terminal radar standards) as a first step to implementation. However, ADS-B In capability is viewed as the most likely way to improve NAS throughput and enhance capacity.
ADS-B deployment and voluntary equipage, along with rule making activities. Pockets of development will exploit equipment deployment in the areas that will provide proof of concept for integration to ATC automation systems deployed in the NAS.[8]
ADS-B ground stations will be deployed throughout the NAS, with an In-Service Decision due in the 2012-13 time frame. Completed deployment will occur in the 2013-2014 time frame. Equipage is expected to begin after the proposed rule is finalized in around 2010:[8]
ADS-B In equipage will be based on user perceived benefit, but is expected to be providing increased situational awareness and efficiency benefits within this segment. Those aircraft who choose to equip in advance of any mandate will see benefits associated with preferential routes and specific applications. Limited radar decommissioning will begin in the time frame with an ultimate goal of a 50% reduction in the Secondary Surveillance Radar infrastructure.[citation needed]
A concern for any ADS-B protocol is the capacity for carrying ADS-B messages from aircraft, as well as allowing the radio channel to continue to support any legacy services. For 1090ES, each ADS-B message is composed of a pair of data packets. The greater the number of packets transmitted from one aircraft, the lesser the number of aircraft that can participate in the system, due to the fixed and limited channel data bandwidth.
System capacity is defined by establishing a criterion for what the worst environment is likely to be, then making that a minimum requirement for system capacity. For 1090ES, both TCAS and ATCRBS/MSSR are existing users of the channel. 1090ES ADS-B must not reduce capacity of these existing systems.
The FAA national program office and other International aviation regulators are addressing concerns about ADS-B non-secure nature of ADS-B transmissions. ADS-B messages can be used to know the location of an aircraft, and there is no means to guarantee that this information is not used inappropriately. Additionally, there are some concerns about the integrity of ADS-B transmissions. ADS-B messages can be produced, with simple low cost measures, which spoof the locations of multiple phantom aircraft to disrupt safe air travel. There is no foolproof means to guarantee integrity, but there are means to monitor for this type of activity. This problem is however similar to the usage of ATCRBS/MSSR where false signals also are potentialy dangerous (uncorrelated secondary tracks).
There are some concerns about ADS-B dependence on other systems. This paper from 2001 mentions some of the potential riscs.[18] The riscs mentioned can be mitigated by using other sources of information, e.g. GLONASS, Galileo or multilateration.
There are some General Aviation concerns that ADS-B removes anonymity of the VFR aircraft operations.[citation needed] The ICAO 24-bit transponder code specifically assigned to each aircraft will allow monitoring of that aircraft when within the service volumes of the Mode-S/ADS-B system. Unlike the Mode A/C transponders, there is no code "1200"/"7000", which offers casual anonymity. Mode-S/ADS-B identifies the aircraft uniquely among all in the world, sort of a MAC number in an ethernet card or the IMEI (International Mobile Equipment Identity) of a GSM phone.
Currently there are no laws preventing anyone from listening to and decoding ADS-B transmissions. Like Cellular Phone however, laws can easily be implemented to make reception a crime. The ongoing debate amongst hobbyists is to display real-time activity on personal screens and then delay five minutes on networked displays. Others feel that, like GPS data, it should be freely available.
Two receivers are currently popular, although both are at the expensive end for consumer devices. The first on the market was Kinetic Avionics with their SBS-1. The second was the AirNav RadarBox. Both of these devices are designed to be used for portable operation, although many have used them in base operation. They are both supplied with a short USB cable for interface to a Windows PC, and a short coax to a small omni antenna.
Both products have specialized single-use receivers in them. Unlike most radios there is no Intermediate Frequency (IF). The ADS-B data flows from the supplied antenna through an LNA pre-amp. It is bandwidth filtered, and then the pulses are extracted using a logarithmic amplifier chip. This chip can operate directly at the ADS-B frequency of 1090 MHz. These analog pulses are then applied to a high speed analog to digital (A/D) converter (40 MHz for Kinetic, 8 MHz for AirNav) and then on to the Field-Programmable Gate Array (FPGA), where the Mode-S frames are detected. The detected Mode-S packets are then sent to the PC via a USB interface.
MASPS = Minimum Aviation System Performance Standards
MOPS = Minimum Operational Performance Standards