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ADS-B surveillance technology

Rev. 4a — page content was last changed May 22, 2005 Flight Planning and Navigation

Module content

13.1 Design concept for ADS-B surveillance technology 13.2 Aircraft and ground stations 13.3 Airservices Australia's ADS-B project 13.4 Options for airborne ADS-B avionics 13.5 Consequences for light recreational aviation

13.1 Design concept for ADS-B surveillance technology

GPS based technologies, that promise a substantial change in airborne communication, navigation and surveillance techniques, are nearing fruition; among these is Automatic Dependent Surveillance – Broadcast [ADS-B].

ADS-B is an air traffic surveillance technology currently [October, 2004] on trial in the USA and in Australia with the expectation that it can substantially contribute to ATC surveillance and separation capabilities at a comparatively low cost. The system concept is that all, or most, aircraft in an area automatically and continually (i.e. roughly once per second) broadcast, or squitter, several digital data packets which together contain the aircraft's 24 bit address [unique airframe identification], flight identification [call sign], GPS derived latitude and longitude, barometric [or Mode C] altitude plus 3 dimensional velocity, ie. rate of climb/descent, direction and speed

The FAA has decided that ADS-B transmission in the USA will be via a Mode S 1090 MHz Extended Squitter [1090ES] surveillance link for air transport aircraft. (The term 'extended squitter' refers to the additional [112 bit] ADS-B data packet which is part of the Mode S transponder standards.) For general aviation aircraft the FAA has chosen a Universal Access Transceiver [UAT] surveillance link using 978 MHz in the DME band. UAT was developed in the USA specifically for ADS-B operation. (The Europeans seem to be favouring a third technology – there is no interoperability between the three technologies.)

The broadcast data packets from aircraft are received by ATC ground stations which can feed the data to air traffic management systems, for even more precise tracking than with radar. The broadcast ADS-B packets can also be received by aircraft equipped with an ADS-B data receiver. The derived data provides a real time cockpit display of traffic information; similar to the ground air traffic management systems except that the traffic is shown in relation to own aircraft's intended track.

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13.2 Aircraft and ground stations

Airborne avionics

To achieve the full Australian ADS-B system concept the ADS-B electronic units for recreational aircraft should include several functions or modes: Data transmission – data broadcast capability so that positional data, provided by an inbuilt GPS engine and an altitude encoding altimeter or a pressure altitude encoder, is continually broadcast. This is known as 'ADS-B Out' capability and all aircraft ADS-B units have this minimum function. Obviously the accuracy of the broadcast data is 'dependent' on the positional/navigation data from the GPS engine which, in turn, is dependent on the availability of GPS signals and the capability of the GPS software. A high performance TSO GPS engine is part of the system.

Data reception – capability to receive all data packets broadcast by all other units within an appropriate range.

Data processing – to provide, using the received data, a real time plot of own and other aircrafts' tracks, speeds and altitudes; known as a 'cockpit display of traffic information' – CDTI. Alternatively provide the data to a separate CDTI unit – which might be just a handheld PDA. Data reception plus CDTI is known as 'ADS-B In' capability.This airborne surveillance and traffic alerting capability – completely independent of ATC – is the function which might benefit VFR recreational pilots, through enhanced situational awareness information; but probably only if the traffic information and own aircraft position is overlaid on an accurate topographical chart navigation display; i.e. a GPS moving map with terrain data base. See topographical moving map software Ground station networks

If all aircraft using Class E [for example] are fitted with 'ADS-B Out' avionics and a network of ground stations – capable of receiving the 'ADS-B Out' aircraft transmissions – is ground or satellite linked to feed surveillance display screens at ATC centres, then a 'virtual' secondary surveillance radar [SSR] network could be created on the 1090 MHz transponder reply frequency. Such ground stations, with their fixed omni-directional antennas, are much more cost effective than radars to acquire, install and maintain; particularly so in remote areas. ADS-B also provides improvements in surveillance accuracy over that achievable by SSR with rotating directional antennas.

Other ADS-B ground stations and antenna systems have applications in terminal areas. Back to top

13.3 Airservices Australia's ADS-B project

Airservices Australia is promoting the advantages of ADS-B particularly for its 'ADS-B Out' function and its consequent economic benefits to Airservices and its customers (as an alternative to SSR) for air traffic separation services in that part of Australian airspace currently lacking SSR coverage – the pink tinged area in the image.

ADS-B is also suggested as a direct replacement for existing enroute SSR (the brownish area), when the current radars reach the end of their economic life – probably before 2010. Airservices proposes a readily implemented 1090 MHz frequency network. Airservices also believes that CDTI will contribute a significant improvement in pilot situational awareness – and not just traffic related.

The project comprises several programs: The Burnett Basin operational trial

"A pilot program to gain technical and operational knowledge and experience in ADS-B technology. Includes installation of a single ADS-B ground station, integration of ADS-B functions in Australia’s air traffic management system and equipping a number of aircraft with ADS-B avionics."

This trial is in the operational stage with the small number of aircraft involved (including a Jabiru from David Eyre's Bundy Flying School) using transponders and GPS navigators to provide just the 'ADS-B Out' function using 1090ES, so there is no Australian trial of the 'ADS-B In' function in the current program. However there is such a trial in progress in Alaska – funded by the FAA – involving some 250 aircraft fitted with full function ADS-B avionics using the UAT technology.

The Burnett Basin operational trial is not of much direct interest to Australian recreational aviation. The upper airspace program

"A program aimed at providing near-term safety and operational benefits in high level, non-radar airspace. Includes installation of 28 ADS-B ground stations, strategically located across Australia to provide air traffic surveillance above 30 000 feet in continental airspace outside of radar coverage."

The decision to implement the upper airspace 1090ES ADS-B program has been taken by the ordering of 28 ground stations for installation by December, 2005; thus establishing an Australian ADS-B network for ground-based air traffic management. These stations, each with a range exceeding 200 nm, will be co-located at existing VHF communication relay sites and linked to surveillance displays at ATC centres, allowing Airservices Australia to provide an SSR-like traffic separation service across the current non-radar airspace above 30 000 feet. Of course these same ground stations will also have the capacity for air traffic management at much lower altitudes – but shorter range, being line-of-sight dependent.

The 'ADS-B Out' function is accomplished by upgrading the aircrafts' existing Mode S transponders to 1090ES and linking the navigation system and the transponder. This program is unlikely to impact recreational aviation in the short term but advances in avionics will eventually flow down to recreational aircraft. The lower airspace program

"A major, longer term program designed to make ADS-B the primary means of ground to air and air to air surveillance in Australian enroute airspace. Includes installation of additional ADS-B ground stations to provide air traffic surveillance in airspace currently covered by enroute radar facilities. Intended to lead to the decommissioning of a number of radar sites. Mandatory aircraft ADS-B equipment requirements will apply – funding options to support general aviation operators will be explored." It is likely that all aircraft currently required to carry a VHF radio will have to fit 'ADS-B Out' and that includes any ultralight operating in Class E airspace or operating above 5000 feet amsl.

This program is in the initial planning and development stage, with implementation possible in 2009, and is extremely important to recreational aviation because of the possible proliferation of Class E controlled airspace and the potential for heavy compliance costs for those ultralights operating in controlled airspace. The mandatory ADS-B avionics requirement refers only to the 1090ES 'ADS-B Out' function and the 'funding options' indicate that Airservices hopes to ensure that the cost of the new technology doesn't preclude many aircraft from operating in the airspace affected by the 1090ES 'ADS-B Out' requirement.

Surveillance in lower airspace may be both 'air to air' based – a form of 'alerted see-and-avoid' – for VFR aircraft and 'air to ground' for an IFR separation service. At this stage the lower airspace program only includes a very few additional ADS-B ground stations – hence at low level normally aircraft will only have air to air. However one could speculate that if proven to provide a reasonable cost/benefit the lower level of Class E might be reduced – perhaps to 8500 feet throughout much of Australia – which would require additional ADS-B ground stations.

There are a number of open questions relating to the compulsory installation of ADS-B equipment in aircraft: If the mandate applies to all aeroplanes, including ultralights, operating in Class E airspace will such aircraft also be required to have an operating Mode A/C transponder?

Might the mandate also apply to all aeroplanes, including ultralights, operating in Class G airspace above 5000 feet amsl?

Legally all ultralights are not required to carry VHF radio (the same as other aircraft operating OCTA below 5 000 feet). Will the ADS-B funding subsidy options also apply to RAAus or HGFA registered ultralights should the aircraft owner apply for it?

Although the lower airspace program, as stated, is confined to Australian enroute airspace will the mandate apply to all aircraft operating in MBZs/CTAFs?

Aircraft operating in GAAPs [future Class D?] are currently required to switch transponders to standby; will they then be required to switch off ADS-B?

If an equipment subsidy is provided for aircraft existing at the time of implementation, will any subsidy be provided for aircraft built after that date?

The ADS-B project proposed by Airservices Australia will improve services and – compared to the alternative of replacing aging radars and NDBs – save industry and public monies; even with the subsidies of maybe $55 million to 11 000 GA and, by that time, maybe $4–5 million to 2600 ultralight aircraft owners. (Presuming that the subsidy covers one third of the hardware, installation and testing costs.) But will the Federal Government, and the public, really find it acceptable to provide such funding to aircraft owners? And will 2600 ultralight aircraft owners be prepared to contribute $8–10 million as their share? The research and development programs

"R & D activities to facilitate the Lower Airspace ADS-B Programs. Activities include development of low-cost ADS-B transmitters for general aviation aircraft, air to air ADS-B receivers and cockpit displays to provide airborne traffic information to pilots."

With the invitation to tender for the Bundaberg ground station was a request to provide budgetary estimates for the supply, to Airservices Australia, of 10 000 low-cost 'ADS-B Out' units for [subsidised?] distribution to general aviation. Whether such units will also include the transponder Mode A/C function to allow swap-out of existing Mode A/C transponders is not known.

Airservices have also entered an agreement with Microair to trial the addition of 1090ES 'ADS-B Out' capability to the Microair T2000 mode A/C transponder. The unit will respond normally to SSR and TCAS interrogations but will also broadcast the GPS derived position, velocity and altitude data.

Airservices Australia has purchased two cockpit display systems from Euro Telematik that will be used to demonstrate and evaluate CDTI concepts. One system is a certified, in-panel mounted multifunction display. The second system operates on a PDA with Euro Telematik's “Flight Companion CDTI” software. Both systems provide ADS-B traffic information on a moving map display and incorporate a terrain database and are expected to be “flying” in early 2005. Back to top

13.4 Options for airborne ADS-B avionics

The Airservices Australia ADS-B implementation team [ABIT] are examining options for airborne units suitable for the low end of general aviation and have suggested the following possibilities. The images below are based on images provided by Greg Dunstone, the ABIT program manager.

Option 1. Install a subsidised 'ADS-B Out' 1090ES transmitter (separate from any existing SSR transponder) with its own TSO'd GPS engine, external GPS antenna and an additional transmission antenna; probably bus-connected to avoid mutual interference. There must also be a connection to an altitude encoding altimeter or a blind encoder to supply pressure altitude. CASA have developed an Australian TSO to cover this unit.

Option 2. Add a subsidised, low cost, basic moving map navigation display [PDA?] to the GPS engine of the preceding unit – using an NMEA 0183 serial port. There is no traffic surveillance, the display just utilises the positional data from the integral GPS and the aviation data stored in the PDA.

It may be a reasonable solution for light recreational aircraft if something like the AirNav VFR / OziExplorer topographic moving map display were utilised.

Option 3. The aircraft owner provides a separate 1090ES 'ADS-B In' receiver connected to a more advanced moving map navigation display [PDA?] deriving the position data from the 'ADS-B Out' GPS engine – probably using a NMEA 0183 serial port. The overlaid CDTI data is supplied by the 1090ES 'ADS-B In' unit. The difficulty with this solution is the complexity, the cost to the owner, the availability of a receiver only ADS-B unit, and the availability of software to add a surveillance overlay to the moving map navigation display. There are possibilities to use a common antenna with the 'ADS-B Out'.

Option 4. Upgrade an existing Mode A/C transponder – or design a new unit – with 1090ES 'ADS-B Out' capability so that the existing aircraft Mode A/C transponder can be swapped out. The GPS could be either integrated or external. The simplest solution – same as Microair's projected solution – and CASA have issued an Australian TSO to cover it.

A very good option for the recreational aircraft using Class E but only if an RS-232 serial data port was provided for output of navigation data from an integrated GPS, in NMEA 0183 format, to something like an AirNav VFR or OziExplorer topographic moving map PDA display – or anything else that the aircraft owner may consider desirable at any time after the unit is installed.

Option 5. A low power consumption, lower GPS integrity 1090ES 'ADS-B Out' unit for aircraft that will never require ATC separation, but which facilitates adequate situation awareness for other aircraft in VFR operations. A lower capability version of option 1 with no transponder: a 'cheap' solution which would satisfy a mandatory 'ADS-B Out' capability with minimum subsidy cost – and minimum usefulness to the aircraft owner. But its utility would be increased if an RS-232 serial data port was provided for output of GPS navigation data. 13.5 Consequences for light recreational aviation

It is likely that all aircraft currently required to carry a VHF radio will have to fit ADS-B OUT. RAAus registered aeroplanes operating OCTA below 5000 feet amsl are not required to carry a VHF radio – although in 2004 75% of RAAus certificated pilots do carry a portable transceiver and about 10% are transponder equipped.

The 'ADS-B Out' only units, even if "low/subsidised cost" are of no benefit to those VFR recreational aviators who never operate in the currently defined controlled airspace.

Individual perception of cost/benefit will vary greatly; one person's "low cost" is another's "arm and leg"; and that cost also includes panel installation, cabling, power supply, antennas, on-going maintenance and the requirement for biennial test and inspection of the altitude reporting facility by a CASA approved technician. There is also the problem of shoe-horning it all into a small airframe – and of providing appropriate antenna positioning – and maybe a need to provide all antenna feed from a single bus.

The Australian land area is about 7.5 million km² and the airspace included from ground level to 5000 feet agl is about 12 million km³, and probably no more than 10% of the 15 000 registered aircraft airborne at any time. So it not surprising that the Australian history of recreational day VFR aircraft 'mid-airs' or 'near-misses' appears to be confined either to the circuit area, to aircraft flying formation or to gliders sharing a lift source. In all these circumstances the likelihood of collision with another aircraft (or with the ground in a stall/spin incident) would seem to be increased if the VFR pilot's eyes are in the cockpit checking a small screen.

There may be no real benefit for day VFR pilots if an 'ADS-B In' facility is also provided as an optional low cost add-on for CDTI, however the experience in Alaska was a significant improvement in the general accident rate in the areas with ADS-B, probably due to improvements in situational awareness flowing from CDTI associated with a moving terrain map display, thus reducing controlled flight into terrain [CFIT] accidents generally resulting from VFR incursion into IMC.

For those ultralights which are equipped to operate in Class E (only in daylight hours) under existing regulations then 'ADS-B Out' only units incorporating the Mode A/C transponder function [option 4] may be cost beneficial – if navigational use can be made of the integral GPS via an RS-232 serial port. However in 2004 about 75% of RAAus pilots carried hand held GPS receivers mostly with moving map display and it is likely that every RAAus aircraft regularly operating in CTA is equipped with GPS. By the time low cost 'ADS-B In', with CDTI on a terrain data base, is generally available those ultralights will already be equipped with off-the-shelf GPS moving maps with a very accurate terrain data base. At that time such units will be considerably cheaper than they are now.

This concludes the Flight Planning and Navigation Guide which I hope you have found useful. There are also two supplementary documents which should be read: "Operations at non-controlled airfields" and "Safety during take-off & landing".

If you have corrections or suggestions for improvement or expansion – please contact the author.

I have written other Guides which you may find useful and informative. These are: | Aviation Meteorology Guide | Flight Theory Guide | VHF Radiocommunication Guide |

| Coping with Emergencies | Learning to Fly Guide | Scratch Building Guide |

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Groundschool – Flight Planning & Navigation Guide

| Guide content | 1. Australian airspace regulations | 2. Charts & compass | 3. Route planning | 4. Effect of wind |

| 5. Flight plan completion | 6. Safety audit | 7. Airmanship & flight discipline | 8. Enroute adjustments |

| 9. Supplementary navigation techniques | 10. Global Positioning System | 11. Using the ADF |

| 12. Electronic planning & navigation | 13. ADS-B surveillance technology |

Supplementary documents

| Operations at non-controlled airfields | Safety during take-off & landing |

John Brandon 2004 [contact information]