ADS-B (xhtml w3c 01/10)

	ADS-B surveillance technology Rev. 6 — page content was last changed November 26 2009 consequent to editing by RA-Aus member Dave Gardiner www.redlettuce.com.au
Flight Planning and Navigation

Module content

13.1 ADS-B navigation and surveillance technology system concept
13.2 Aircraft and ground stations
13.3 Airservices Australia's ADS-B project
13.4 Options for airborne ADS-B avionics
13.5 Possible consequences for recreational aviation

13.1 ADS-B navigation and surveillance technology system concept

Satellite-based technologies — which 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 that has been trialled in Australia. The results show that ADS-B will contribute substantially to air traffic control [ATC] surveillance and separation capabilities at a comparatively low cost. It enables aircraft to be accurately tracked by ATC and pilots, without the need for conventional radar.

The concept for a total system is that all, or most, airborne aircraft 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 (the unique airframe identification), flight identification (call sign), GPS-derived latitude and longitude, barometric (or Mode C) altitude plus its three-dimensional velocity; i.e. rate of climb/descent, azimuth direction and speed.

The United States 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 data link standards.) For general aviation aircraft the FAA has specified 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. Airservices Australia has opted for 1090ES for regular passenger transport aircraft and possibly for general aviation.

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

13.2 Aircraft and ground stations

Airborne avionics

To achieve the full Airservices Australia ADS-B system concept the ADS-B avionics for general aviation and recreational aviation aircraft would have to include several functions or modes:

data transmission. This provides 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 must have this minimum function. The accuracy of the broadcast data is dependent on the positional/navigational 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.

In the first stage of Australian ADS-B operational implementation in December 2013, ADS-B Out units will be mandatory for aircraft operating at or above FL290
data reception. This refers to the capability to receive all data packets broadcast by all ADS-B Out units within an appropriate range
data processing. This entails using the received data to provide a real-time plot of own and other aircrafts' tracks, speeds and altitudes; it is also known as a 'cockpit display of traffic information' [CDTI]. Alternatively, processing provides the data to a separate CDTI unit, which might be a handheld PDA. Data reception plus CDTI is known as 'ADS-B In' capability, providing a pilot's own airborne surveillance and traffic alerting system — completely independent of ATC. ADS-B In is the function that would benefit VFR recreational pilots, through enhanced situation awareness information; but probably only if an audible alert is provided, and the traffic information and own aircraft's position is overlaid on an accurate topographical chart navigation display; i.e. a GPS moving map with terrain database.

ADS-B In capability is unlikely to be made mandatory.

Ground station networks

If all aircraft in Class A, C, D and E airspace (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 ATM centres, then a satellite surveillance network can 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. An ADS-B satellite network could replace the secondary surveillance radar [SSR] network. 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.

A fuller description of ADS-B is contained in an NPRM issued by the US FAA regarding their plans for ADS-B implementation.

13.3 Airservices Australia's ADS-B project

The upper airspace program

Airservices Australia states that ADS-B "is an air traffic surveillance technology that enables aircraft to be accurately tracked by air traffic controllers and other pilots without the need for conventional radar."

Airservices Australia is currently deploying ADS-B ground stations across Australia which, when combined with radar, will provide ATC surveillance capability over the entire continent above FL300 (30 000 feet) — 28 sites are anticipated to be operational.

The decision to implement the upper airspace 1090ES ADS-B program has been taken. From December 2013 carriage of ADS-B Out avionics will be mandatory for aircraft operating at or above FL290.

The first 28 ground stations have been ordered; 18 were operational by December 2008 and the balance is expected to be operational by December 2009. This establishes an Australian ADS-B network for ground-based air traffic management; i.e. an ADS-B Out system. These stations, each with a range up to 200 nm, are co-located at existing VHF communication relay sites and linked to surveillance displays at ATC centres, which allows Airservices Australia to provide an SSR-like traffic separation service across the current non-radar airways above FL290. 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. As at February 2009, about 700 ADS-B equipped aircraft, including one trial ultralight (a Jabiru from David Eyre's Bundy Flying School), have been approved for operations in Australian airspace.

Currently in Australia the main Mode S transponder function is to allow aircraft equipped with Traffic Alert and Collision Avoidance Systems to 'talk' directly with each other, thereby enabling mutual resolution of potential traffic conflicts. Mode S can also provide faster, more accurate ATC surveillance, provided the ground radars are of the fast single pulse interrogation type. Many (most?) of the Australian SSRs are not monopulse radars, so are to be replaced.

The ADS-B Out function is accomplished by upgrading an aircraft's existing Mode S transponder to 1090ES, and linking the navigation system and the transponder. This program won't affect 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 eventual decommissioning of a number of radar sites."

Initially an accelerated introduction of ADS-B surveillance into lower airspace was planned, but in the last quarter of 2008 it was agreed that a more gradual transition to satellite-based systems, harmonised with the North American and European transition plans, would be wiser. Airservices Australia will now "proceed with the replacement of its enroute radars and navaids as necessary to ensure the integrity of Australia's air traffic control system." (CASA's Notice of Final Rule Making – "Transition to Satellite Technology for Navigation and Surveillance"). The previously proposed cross-industry funding proposals will not go ahead.

"The timing and scope of future steps will be progressed through normal regulatory processes and will take into account ... outcomes of the Government's Aviation Policy Green Paper consultation." (NFRM – "Transition to Satellite Technology for Navigation and Surveillance").

So, it is likely that Australian recreational aviation and general aviation aircraft will be required to fit ADS-B Out equipment (probably at their own cost) for operations in controlled airspace and perhaps above 10 000 feet, but that is unlikely to be mandatory before 2020.

13.4 Options for airborne ADS-B avionics

Airservices Australia's ADS-B implementation team [ABIT] examined 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 an 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 has developed an Australian TSO to cover this unit.

Option 2. Add a low-cost, moving map navigation display (a 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. This is a separate 1090ES ADS-B In receiver connected to a more advanced moving map navigation display (a 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. This is the simplest solution — the same as Microair's projected solution — and CASA has issued an Australian TSO to cover it.

This is a very good option for 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. This is 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. It is a lower capability version of option 1 with no transponder: a 'cheap' solution that would satisfy a mandatory 'ADS-B Out' capability with minimum cost — and minimum usefulness to the aircraft owner. But its utility would be increased if an RS-232 serial data port is provided for output of GPS navigational data.

13.5 Possible consequences for recreational aviation

It is possible that all aircraft currently required to carry a VHF radio will have to fit ADS-B Out. RA-Aus registered aeroplanes operating OCTA below 5000 feet amsl are not required to carry a VHF radio — although perhaps 80% of RA-Aus certificated pilots carry a portable transceiver and perhaps 10% of aircraft are transponder-equipped.

The ADS-B Out only units 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, checking of electrical load, antennas, ongoing 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 very small airframe, of providing appropriate antenna positioning — and maybe a need to provide all antenna feed from a single bus.

There may be no real benefit for day VFR pilots in the ADS-B In facility to provide CDTI. However, the experience in the US Alaskan trials was a significant improvement in the general accident rate in the areas with ADS-B. This was probably due to improvements in situation 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.

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. Probably no more than 10% of the 15 000 registered aircraft are airborne at any time. So it is not surprising that the Australian history of recreational day VFR aircraft 'mid-airs' or 'near-misses' appears to be confined 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 CDTI traffic display.

For those ultralight aircraft which are equipped to operate in Class E (only in daylight hours) under existing regulations, 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, about 75% of RA-Aus pilots carry hand-held GPS receivers mostly with moving map display and it is likely that every RA-Aus aircraft now regularly operating in CTA is equipped with GPS. By the time low-cost ADS-B In, with CDTI on a terrain database is generally available those ultralights will already be equipped with off-the-shelf GPS moving maps with a very accurate terrain database. 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.

Groundschool – Flight Planning & Navigation Guide

| 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 2004–2010 [contact information]