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Automatic Dependent Surveillance Broadcast (ADS-B)

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INTRODUCTION

ADS-B (Automatic Dependent Surveillance Broadcast is a means by which aircraft, aerodrome vehicles and other objects can automatically transmit and/or receive data such as identification, position and additional data as appropriate in a broadcast mode via a data-link. The transmission of key data via ADS-B is at a standard interval of twice every second, a considerable improvement on ground-based radars.

ADS-B is automatic because no external stimulus is required; it is dependent because it relies on on-board position sources and on-board broadcast transmission systems to provide surveillance information to other parties. Finally, the data is broadcast , the originating source has no knowledge of who receives and uses the data and there is no two-way ‘contract’ or interrogation.

ADS-B is increasingly being considered as a low cost alternative to SSR for both high level non-radar ATC and as a means to increase position and traffic awareness for low level GA traffic. A number of ANSP Regulators including CASA, Transport Canada, the FAA and EUROCONTROL have been leading ADS-B trials and moving towards full implementation using the developing ICAO guidance and taking full account of all relevant existing aspects of PANS-ATM. It now seems increasingly likely that ADS-B will have a much greater role than ADS-C in the provision of Future Air Navigation Services except on some low traffic density transoceanic or transcontinental corridors.


THE ADS-B PROCESS

The ADS-B system enables the automatic broadcast of an aircraft’s identity, position, altitude, speed, and other parameters at half-second intervals using inputs such as a barometric encoder and GNSS equipment The result is a functionality similar to SSR. ADS-B aircraft transmissions received by a network of ground stations can provide surveillance over a wider area. Referred to as ADS-B OUT, this provides ATC with the ability to accurately track participating aircraft. The benefits of ADS-B can also extend beyond air-to-ground to provide an air-to-air function. As transmissions are being broadcast by a participating aircraft, they may also be received and displayed to other aircraft equipped with an appropriate flight deck display facility. This air-to-air capability is called ‘ADS-B IN’ and in the USA it is also referred to as ‘TIS-B’ (Traffic Information Service - Broadcast).

ADS-B is not limited to one particular technology. Standards for ADS-B have currently matured for three processes: • 1090MHz Extended Squitter (1090ES) • Universal Access Transceiver (UAT) • VDL Mode 4 Currently only 1090ES, which is a fully compatible adaptation of Mode S Transponder systems, has gained universal acceptance and it is this version which is being promoted by both IATA and ICAO for extensions of ADS-B service. At the industry level, the Technical Standards for ADS-B are being jointly handled by EUROCAE and RTCA using the co-ordination mechanism provided by their ‘Requirements Focus Group’ (RFG)


AIRCRAFT EQUIPMENT

All new Boeing and Airbus aircraft are now routinely being fitted with ADS-B OUT capability. Some older aircraft of various types have also been selectively retro-fitted with ADS-B OUT and ADS-B IN capability. ADS-B can either be achieved by software upgrade to an existing Mode S Transponder fit or be installed as a freestanding unit. In the former case, the fight crew interface is included in the Transponder Control Box which will usually be provided with a separate ADS-B ON/OFF and will be configured so that selection of the IDENT button will activate both SSR and ADS-B. Occasionally, an ADS-B equipped aircraft with the system linked to the Transponder Control Box will not have a separate ON/OFF switch and the ADS-B function will be ON whenever the transponder is ON. If ADS-B is installed as a freestanding Unit, then it will have its own control box with IDENT button and ON/OFF switch. This last option is not yet very common.


GROUND EQUIPMENT

Since the ADS-B technology used by aircraft equipment can vary between individual aircraft and all three standards use different frequencies and message formats, the current planning objective is for ground stations to be able to receive and process data provided by all of them. If this is not the case, then the applicable AIP can be expected to include any restrictions. Display of ADS-B information to controllers does not involve any new methods since for the ATC user, the information available is similar to Transponder Mode C and Mode S, albeit with greater integrity


ADS-B DATA

ADS-B broadcast or ‘downlinked’ data will always include at least the following:

•Aircraft horizontal position (latitude/longitude derived from GNSS)

•Aircraft barometric altitude (will be the same as the SSR figure where SSR fitted and is not usually visible to the flight crew for verification)

•Aircraft position quality indicators (not usually visible to flight crew)

•Aircraft identification:

     −The unique 24-bit aircraft address with every transmission
     −A periodic identification utilising either the RTF Callsign or the aircraft registration  

•Emergency/urgency status indicators as selected by the flight crew on the appropriate/available control box

•IDENT / SPI (special position indicator) when selected

INITIAL ANSP APPLICATIONS OF ADS-B

Early non-ANSP trials of ADS-B have been made by two air cargo operators in the USA, FEDEX and UPS, who have busy hub operations at airports where they are the principal carrier. Both have reported the potential for significant efficiency gains with no loss of safety standards by using ADS-B OUT and IN to provide traffic awareness to inbound aircraft.

ANSP trials of ADS-B OUT were pioneered by Australia and the effort there has been directed at the en route surveillance of aircraft operating above FL300. The ANSP, Airservices Australia, has now begun implementing the ‘ADS-B Upper Airspace Program’. This is based upon the progressive introduction of a network of 28 ground stations which, when completed, will provide complete coverage of the country. The use of ADS-B IN as part of the input to moving map displays for low level GA traffic is now also being investigated.

Plans for widespread ANSP use of ADS-B in both Canada and the USA are well advanced, with major trials being implemented in the Hudson Bay and Gulf of Mexico areas respectively. In both cases there are now aggressive plans for the development of more comprehensive ground-based networks. In addition, early experience of the application of ADS-B to low level GA navigation safety was gained as part of the FAA ‘Capstone’ initiative to improve low level GA safety in Alaska, where CFIT rates were high.

In Europe, a number of trials in support of ADS-B are being run under the EUROCONTROL ‘CASCADE’ Programme and the Regulatory focus is directed not on a full scale ground network but on a the specific case of non-radar airspace (NRA) where ADS-B OUT is seen as the means to reduce traffic separation from procedural requirements to standards similar to radar. This case is specifically identified as ADS-B-NRA. Examples of this selective application are the areas around some smaller regional airports, off-shore operations in support of oil and gas production and remote airspace such as that around small islands where, owing to the level of traffic, the location of the island or the cost of the equipment, the installation of radar could nit be justified. Four airport based trials across Europe have involved the construction of ground receivers - at Alghero, Sardinia, at Kiruna, Sweden, at Pescara, Italy and at Trabzon, Turkey. Once the implementation phase gets fully under way, the European initiative is likely to be extended to include areas where existing radar is to be decommissioned and it is judged that equivalent or better service and safety can be provided at less cost by an ADS-B solution than by a replacement ground radar system.


PARTIAL OR TOTAL ADS-B SERVICE

Since the ability to participate in a traffic separation system based upon ADS-B depends on each individual aircraft having the necessary on-board equipment fitted and serviceable, one of the issues is whether designated airspace should be restricted to aircraft which are able to participate or whether a ‘mixed-traffic’ environment should be tolerated. The latter requires that non-equipped aircraft be procedurally separated using existing standards for that service whereas ADS-B equipped aircraft are separated at the radar-equivalent standard.

Aircraft with ADS-B capability are expected to ensure that this is indicated on their FPL or RPL. In the RPL case, a change in the filed status on a particular flight must be notified in the usual way.


ICAO SARPs

(1) Amendment 82 to Annex 10 (2007) introduces SARPs for the universal access transceiver (UAT) technology and updates SARPs and technical specifications supporting the latest version of 1090ES technology (ED-102 / DO 260A)

(2) Amendment 5 to Doc 4444 PANS-ATM (2007) includes facilitating the application of available ADS-B technology in the general context of ATS datalink applications. It also includes operational procedures and phraseology for the use of ADS-B.

(3) Circular 311 provides the necessary guidance to States on assessment of ADS-B options and implementation of ADS-B solutions

(4) More detail in respect of each of the three ADS-B broadcast technologies is provided in: • Doc 9871 - 1090 MHz Extended Squitter (1090 ES) • Doc 9861 - Universal Access Transceiver (UAT) • Doc 9816 - VDL Mode 4

Details of the four specific publications are included in ‘Further Reading’ below.


FURTHER READING

ICAO Circular 311 Assessment of ADS-B to support Air Traffic Services and guidelines for Implementation [1]

ICAO Doc 9871 ‘Technical Provisions for Mode S Services and Extended Squitter’ First Edition 2008

ICAO Doc 9861 ‘Manual on UAT’ First Edition 2008

ICAO Doc 9816 Manual on VHF Digital Link (VDL) Mode 4 First Edition 2004

EUROCAE ED126 Safety, Performance and Interoperability Requirements Document for ADS-B-NRA Application 2006

EUROCAE ED-102 / RTCA DO-260A Minimum Operational Performance Standards for 1090 MHz Automatic Dependent Surveillance - Broadcast (ADS-B) and Traffic Information Services (TIS-B)

EUROCONTROL CASCADE Programme see [2]