ADS-B as simple as ABC-D

May 21, 2009

A better way to spell out ADS-B.

A couple of years ago the government realised it had to replace the ageing radar system. But it didn’t want to spend the money. Instead, they came up with an idea to force all of us into ADS-B.

A better system, sure, but it meant every aeroplane owner would have to fork out up to $25,000 for the equipment. Naturally we kicked up a fuss. It all got too hard. The government went ahead and bought its new radars anyway and the issue was quietly shelved. The problem with that, though, is we may have missed the chance to improve our own safety. And, as the recent increase in mid air accidents shows, it might be good to have a better system. Andrew Andersen has come up with a way it might be done at a price that won’t break the bank.

In 2007, the four Australian Government agencies with responsibility for aviation, DOTARS, Airservices Australia, CASA and the Department of Defence, set out a case for adopting ADS-B technology in GA aircraft and other users of low-level airspace in Australia. Much has been written about the outcomes, or rather lack of them, from that affair. But two things were learned: Mixing mandatory regulatory requirements with leading edge technology creates issues. The short timetable and extreme nature of the proposal created objections on almost every front, ranging from risk in equipment choices to basic concerns about unprecedented monitoring and restrictions on VFR flight, even in Class G airspace. The delivery of benefits must be balanced between all stakeholders in initiatives which require cooperation. Very few GA aircraft owners could identify any benefit from fitting ADS-B OUT, but other stakeholders stood to gain millions from reductions in airways charges and capital investment. Among the more responsible approaches was that of AOPA. AOPA identified the potential national benefit and supported the medium and long-term directions offered by the technology. But rejected the regulatory push which would have relied on a law to rope-in almost all GA aircraft, regardless of owners’ perceptions of need and value. AOPA’s representatives helped redraft the proposal, with the VFR mandate removed, but apparently too late. AOPA also offered remedial strategies, but little use appears to have been made of that work. It’s clear ADS-B is the way of the future; and even some of its most vocal critics are prepared to concede that. The issue is how to do it.

How does it work?

ADS-B OUT involves transmitting reliable position data to other aircraft and ground stations. The equipment provides no new aircraft capability and no information in the cockpit. However it does offer a way to keep aircraft under surveillance “in the ATC system” in areas where radar coverage is not available. ADS-B OUT might even replace terminal and enroute radar eventually , but not in the next ten or fifteen years. Even the government’s ambitious proposal would have meant aircraft would need both ADS-B OUT and a radar transponder to fly in the most commonly used airspace. Airbus and Boeing airliners are built with ADS-B OUT as part of their standard fit, but general aviation aircraft aren’t. For GA, ADS-B always involves at least one “extra box”. Aircraft owners will be much more amenable to installing this “extra box” if there is some benefit, but up to now there hasn’t been. The real benefit for aircraft owners and pilots is ADS-B IN. It involves receiving messages about other aircraft and presenting them to the pilot, usually graphically on a GNSS unit or multi-function display.

Traffic Information

Some people have argued GA aircraft don‘t need traffic information displays in the cockpit. In the past, the argument has been somewhat moot, since certified TCAS devices are too expensive while less expensive systems such as TAS and TCAD have never been particularly popular. But the simple fact mid-air collisions continue to occur in areas of high traffic density indicates not all pilots are seeing other aircraft. It also highlights flaws in the argument that traffic information in GA aircraft is an unnecessary luxury. Arguments that traffic information displays might distract the pilot ignore the fact there is already plenty of other technology in aircraft cockpits to do that. Also, many general aviation aircraft operate in IMC in busy areas near capital city Class C airspace, where awareness of traffic purely by sight may not be possible.

Is TIS-B the best for GA?

Australia has the unique situation of having relatively long geographic distances between high population concentrations in the coastal cities and favourable aviation weather almost all year round.

These factors give rise to a concentration of GA flying at or around secondary aerodromes at capital cities. The airspace at these aerodromes is managed, through GAAP procedures, separate to the terminal air traffic services at the adjacent primary airports. The concentration of mid-air collisions in recent years in the vicinity of these types of aerodromes points to the heightened risk at these aerodromes relative to others. The concentration of traffic is not just within the GAAP control zones. The surrounding areas including approach points also present hazards to both VFR and IFR operations as they pass outside the associated Class C airspace.

It is also significant all five capital city secondary aerodromes (Archerfield, Bankstown, Moorabbin, Parafield and Jandakot) are located right beside major airports with terminal radar services. All transponder-equipped aircraft within these secondary airport zones are visible on the major airport terminal radar. Air traffic controllers use this information to provide separation services in nearby Class C airspace and offer radar advisories to IFR aircraft. In the secondary aerodrome towers, the information is used by air traffic controllers using a TSAD.

What it means, though, is that in these high traffic areas it’s possible to deliver information to users of one air traffic surveillance system, such as ADS-B, even if it was collected using another system, such as radar. It’s called TIS-B. TIS-B receives information from a radar and uses ADS-B messages to send it to aircraft equipped with an ADS-B IN receiver. TIS-B is used at many US locations as well as in Europe. Its also been formally accepted as a standard by the FAA.

If TIS-B services were available in areas around the secondary GAAP aerodromes, aircraft owners considered to be Innovators or Early Adopters would move quickly to install ADS-B IN receivers. Many such aircraft would already have suitable displays by virtue of existing Garmin, Honeywell, Avidyne and other MFD or GNSS equipment installations.

Because TIS-B works by receiving ground broadcasts and does not involve transmitting GNSS-derived aircraft position data, the need to install expensive GNSS equipment would not be required. Also, the aircraft owner’s investment in TIS-B receiver and display avionics could continue to be used once full ADS-B IN and OUT is eventually introduced. Owners could install ADS-B IN equipment first to receive the TIS-B service, and add to that equipment for ADS-B OUT later. Introducing TIS-B this way would “future-proof” owners by allowing them to use the avionics they purchased for TIS-B, even after the ADS-B rollout program is eventually completed.

As the initial installed group of general aviation aircraft begins to benefit from TIS-B, word of mouth and competitive pressure on commercial and private operators, including flight schools, aero clubs and charter companies would drive further ADS-B IN installations.

Over time, the take-up of ADS-B IN would also encourage the installation in the same aircraft of ADS-B OUT equipment, as existing transponders are retired and replaced. This would probably be a gradual process, over a period of years, but one that would see most aircraft fitted out even sooner than if it was made mandatory.

TIS-B would only be a transitional service. Once enough GA aircraft were fitted with ADS-B OUT, the need for TIS-B would naturally expire and be replaced by air-to-air ADS-B messages, which would operate in all airspace and not just in the TIS-B service areas. The air-to-air ADS-B messages would be received and displayed on exactly the same equipment installed to use TIS-B.

When there is a critical mass of ADS-B in aircraft, it would be appropriate to regulate for mandatory ADS-B in areas of high traffic density and higher classes of controlled airspace.

This approach would be sympathetic to the need of many owners to stagger re-equipment costs over time, but with benefits available to many owners from day one.

TIS-B is intended as an aid to visual sightings. Like other systems it can’t provide traffic avoidance and does not guarantee there’s no unannounced traffic about. But given that Australian GA will be mostly starting from a base of no traffic information, TIS-B has the potential to warn the pilot of traffic which might otherwise go unnoticed.

The failure of the regulatory authorities to achieve consensus was linked to their heavy-handed ambit claim to restrict VFR flight, and the lack of benefits to GA to agreeing to adopt ADS-B.

The danger now is that with no one being encouraged to take-up ADS-B nothing will happen and Australia will again find itself far behind comparable countries.