Jumat, 09 Mei 2008

ADS B Concept Of Use

In the early 1990’s, ICAO approved the concept of Future Air Navigation System (FANS) based on satellite technology, which later evolved into CNS/ATM. The traditional ATC surveillance system has limitations that constrained its capabilities in the current and future ATM environment. These limitations include the following:
a) limited or no conventional surveillance, including non-equipped continental areas, low altitudes, non-continental areas, surface movements, silence cones, blind areas, antenna screening, etc. In some cases (e.g. oceanic areas), this will result in the need for procedural control, using voice position reports;
b) mechanical rotation of the classical radar antennas, leading to inefficient scanning periods and limitations to adaptation of the reporting rate to suit ATC needs.
(Note.— E-SCAN antennas may offer an alternative in this case);
c) garbling, fruit and splitting;
d) unavailability of aircraft derived data, beyond the Mode A/C identification and altitude data;
e) non-homogeneous operation, caused by the current existence of a diversity of systems with different performance and capabilities;
f) in some regions the shortage of Mode A codes (only 4 096 available) requiring frequent changes of code during the flight which may also create identification ambiguities;
g) lack of capability to support future airborne situation awareness applications, because the corresponding surveillance data are not available to the aircrew; and
h) lack of capabilities to support airport surface surveillance applications.

As the name implies, ADS can be considered to be a hybrid of “traditional” surveillance techniques, combining a dependence on position reports with the automation that is typical of SSR replies. ADS-C reports are made in accordance with an agreed contract between ATS and the aircraft. Similar to radar, these reports are not received by other aircraft and the reporting intervals are relatively long. Conversely, ADS-B reports are broadcast at more frequent intervals and, in addition to “radar-like” ground surveillance, can provide an airborne surveillance capability. One of the major advantages of ADS-B, ADS-C and Mode S is the ability to convey state vector and intent data.
Note.— Enhanced Mode S can be used to convey intent information to the ground system but not to other aircraft as can be done with ADS-B.

ADS-B functionality
ADS-B is a surveillance application that allows the periodic transmission of parameters, such as identification, position and position integrity, via a broadcast-mode data link. Any user, either airborne or ground-based, within range of this broadcast may choose to receive, process and display this information. ADS-B information is broadcast without any knowledge of which users may be receiving it and without the expectation of an acknowledgement or reply. In addition to aircraft and vehicles, ADS-B may be used to identify hazards, such as obstacles, skydivers, etc.
ADS-B is automatic in the sense that no flight crew or controller action is required for the information to be transmitted. 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 emitter.
An ADS-B system consists of the following components (see Figure 1): a transmitting subsystem that includes message generation and transmission functions at the source aircraft/vehicle/obstacle, the data link broadcast medium, and a receiving subsystem that includes message reception and report assembly functions at the receiving aircraft/vehicle or ground system. It should be noted that some ADS-B users may be able to transmit but not receive; some ground based users may be able to receive but not transmit.
The source of the transmitted information as well as the user applications are not considered to be part of the ADS-B system, but their performance needs to be included in defining overall ADS-B system performance.

ATM improvements and benefits

ADS-B and its applications are expected to provide important operational improvements by addressing some of the limitations of the current surveillance system, optimize the controller/flight crew workload and provide benefits in the areas of safety, capacity, efficiency and environmental impact, thus contributing to the overall CNS/ATM objectives. These benefits include the following:
a) extension of the surveillance coverage for low altitudes (below existing radar coverage) and areas where no radar coverage currently exists, leading to more efficient use of airspace;
b) enabling a seamless “gate-to-gate” surveillance service, not only to international civil aviation but should include general aviation and military operations;
c) use of aircraft-derived data in a variety of systems e.g. ground-based conflict alert, minimum safe altitude warning, danger area proximity warning, automated support tools, surveillance data processing and distribution, as well as enabling access by the controller to state vector parameters, (sometimes referred to as controller access parameters (CAP));
d) airborne surveillance capability that can improve flight crew situational awareness and enable the introduction of airborne separation assistance systems;
e) increasing airport safety and capacity, especially under low visibility conditions, by providing airport surface surveillance and, at the same time, protecting against runway incursions. ADS-B will enable the identification and monitoring of relevant airport vehicles as well as aircraft;
f) changes to airspace sectorization and route structure resulting from improved surveillance should provide more efficient routing;
g) reduced infrastructure costs. Especially, in airspace in which all aircraft are ADS-B equipped, it may be possible to decommission some radar equipment. Where multiple surveillance coverage is presently required, optimization of the surveillance infrastructure should be achieved by the implementation of the most efficient mix of radar sensors and ADS-B. Consequently, ADS-B coverage could reduce the required number of radar sensors; and
h) cost savings achieved from the implementation of an ADS-B based surveillance system rather than the life cycle expenses associated with installing, maintaining, and extending existing radar-based surveillance systems.

ADS-B applications
Ground-based surveillance applications
ATC surveillance for airspace with radar coverage
The improved accuracy and higher update rates of the ADS-B reports, in combination with other capabilities, may enhance surveillance services and allow the application of reduced separation standards.
This application will enhance ATC surveillance currently provided by radar, in en route airspace. An example is the case of surveillance in areas where single radar coverage is provided. Where SSR is used, ADS-B can provide a backup system and supplement radar position updates through additional position reports. Where PSR is used ADS-B can provide additional data, such as aircraft identification and barometric altitude.
In these environments ADS-B can also provide additional aircraft-derived data, which can enhance the surveillance data processing (e.g. intent data, state vector).
ATC surveillance in airspace without radar coverage
This application will provide ATC surveillance in non-radar areas, (e.g. remote continental areas, offshore operation areas or certain oceanic areas). The purpose is to enhance traffic information and separation services.
Even where ADS-C is used ADS-B can provide more frequent position updates facilitating a possible reduction in separation minima.
Airport surface surveillance
This application will provide a new source of airport surveillance information for safer and more efficient ground movement management at airports. Relevant airport ground vehicles can also be equipped with ADS-B and displayed, together with aircraft, on a situation display.
ADS-B will support ground conflict detection by providing frequent updates to aircraft and vehicle positions, enabling the monitoring of aircraft and vehicles to protect against runway incursions.
This application will provide additional aircraft-derived data such as state vector and intent data via ADS-B to be used by the ATC ground system for developing or enhancing automated support tools such as:
a) conformance monitoring;
b) conflict prediction;
c) conflict detection;
d) minimum safe altitude warning;
e) danger area proximity warning; and
f) traffic sequencing.

Aircraft-based surveillance applications
Situational awareness
Situational awareness applications are aimed at enhancing the flight crews’ knowledge of the surrounding traffic situation, both in the air and on the airport surface, and thus improving the flight crew’s decision process for the safe and efficient management of their flight. No changes in separation tasks or responsibility are envisaged through the implementation of these applications.
Appropriate training will be necessary to prevent inappropriate use of enhanced traffic situational awareness such as inadequate questioning, or unexpected manoeuvres, which could be disruptive to ATC.
Enhanced traffic situational awareness on the airport surface
This application provides the flight crews with an enhanced traffic situational awareness on the airport surface for both taxi and runway operations, at all times and particularly in low visibility conditions. The objectives are to improve safety on pushback, at taxiway crossings and before entering the runway.
Enhanced traffic situational awareness during flight operations
This application provides the flight crews with an enhanced traffic situational awareness during flight operations, irrespective of visual conditions. Additional data are provided to flight crews to supplement traffic information provided either by controllers or other flight crews. This could ease the aircrew comprehension of the ATC instructions, potentially decrease voice communications and, where CPDLC and/or high frequency (HF) is implemented, compensate for the loss of party line. Consequently, the objectives should result in improvements to safety and the efficiency of air traffic services.
Enhanced visual acquisition
This application is an aid for the flight crews to enhance their visual acquisition capability, particularly with respect to “see and avoid” procedures as they apply to VFR/VFR and IFR/VFR operations in airspace classes D, E and G. Indeed, we have reached the limits of the conventional “see and avoid” principle because of the increasing speed of aircraft, the poor external visibility in modern cockpits and pilots’ workload in some phases of flight.
Enhanced successive visual approaches
This application is an aid for the flight crews to perform successive visual approaches when they are responsible for maintaining visual separation from the aircraft they are following. The objectives are to perform successive visual approach procedures on a more regular basis to enhance the runway throughput, and to conduct safer operations.
Airborne spacing and separation applications
Enhanced sequencing and merging operations
The objective is to redistribute tasks between the controllers and the flight crews related to sequencing (e.g. in-trail following) and merging of traffic. The controllers will be provided with new procedures that would enable a flight crew to establish and maintain a given time or distance from a designated aircraft. The flight crews will perform these new tasks using an advanced human-machine interface. The expected benefits are enhanced controller task management, and a more consistent adherence to required spacing values.
In-trail procedure in oceanic airspace
The in-trail procedure in non-radar oceanic airspace would allow in-trail ADS-B equipped aircraft, which may not be longitudinally separated from each other by the existing distance-based separation minimum, to climb or descend through each other’s flight levels. The objective is to improve the utilization of oceanic airspace by facilitating a higher rate of flight level changes than is currently provided, yielding better flight efficiency (e.g. fuel savings, avoiding turbulent flight levels).
Enhanced crossing and passing operations
The objective is to redistribute tasks between the controllers and the flight crews related to crossing and passing designated traffic. The controllers will be provided with new procedures that would enable a flight crew to manoeuver, based on controller instructions, so as to achieve a given separation value from a designated aircraft. The flight crews will perform these new tasks using an advanced human-machine interface. The main expected benefit is increased controller availability through the redistribution of tasks.

Description of ADS-B and its environment
The operational environments in which ADS-B will be used may include any of the following characteristics:
a) varying infrastructure capabilities, ranging from the lack of any surveillance means up to the co-existence of ADS-B with different types of conventional data sources such as primary and secondary surveillance radars. It is expected that a variety of other technologies such as ADS-C and CPDLC will play a complementary role in the provision of ATC service;
b) mixed aircraft equipage levels, at least in the transition period;
c) varying airspace types (e.g. different traffic density levels);
d) varying flight phases, e.g. airport surface, TMA, en-route, non-continental, continental; and
e) varying types of application/services in different environments.

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