Over the oceans

Without surveillance coverage, procedural control is applied by air traffic controllers. Taking account of the possible navigation errors by aircraft, which determine their position without the availability of ground-based navigation aids, as well as communication difficulties through the use of HF, meant that large procedural separations standards had to be applied.

These separation standards required up to 20 minutes spacing between aircraft of similar performance on the same track at the same flight level. As onboard navigation systems improved, these procedural distances reduced to 10 minutes separation on the same track. For modern jet aircraft this spacing, still often amounting to 80 miles or more, consumes large blocks of airspace and limits the capability of air navigation service providers (ANSPs) to accommodate aircraft at their optimum cruising level to achieve performance efficiencies.

The sophistication of aircraft flying across the Pacific has significantly increased in the past 20 years. A trial in the South Pacific during the 1990s was instrumental in the development of future air navigation systems (FANS). This trial assessed the use of satellites by aircraft to determine their position and undertake communication by controller pilot data link communication (CPDLC). Boeing used outcomes from this trial in its subsequent avionics development, termed FANS-1. The Airbus development of these avionics was termed FANS-A. Common protocols have been agreed for these avionics, now termed FANS-1/A.

The very long route length across the Pacific Ocean is an ideal setting to implement initiatives to substantively reduce the operating costs of airlines. Initially flex tracks provided an alternative to fixed routes. Flex track routes are generally planned a day in advance of the flight and take into account the forecast weather conditions, in particular the upper winds, expected on the route. They are offered by the ANSP with the route availability and details published via NOTAM. Flex tracks provide an optimised route between two waypoints taking into account the forecast winds and can change daily with changes to upper wind conditions.

However, flex tracks have always been seen as a somewhat intermediate step in the route availability for long-haul aircraft. User-preferred routes (UPRs) enable an aircraft to select its desired route, altitude and speed. The relatively high level of sophistication of aircraft flying across the Pacific has promoted the use of UPRs.

Doug Scott, Airservices manager of upper airspace services explains: “Most aircraft flying across the Pacific now use UPRs. These UPRs extend from a gateway clear of the domestic traffic on the west coast of the United States to a gateway on the east coast of Australia. A number of different operational criteria can be used by an airline in determining a UPR. For example, an airline might want to select a route that provides the lowest operating cost. On another occasion, the airline may need to choose a route with the shortest flight time to overcome crew time limitation or reduce downstream service delays. UPRs cater for these varying airline requirements,” Scott said.

In addition to the Pacific, UPRs are used between Japan and northern Australia and across the Indian Ocean from north of Mali to the west coast of Australia. At present, only flex tracks are available over continental Australia.

Ironically the efficiencies provided by UPRs have led to other issues to manage. Scott noted: “Taking advantage of upper winds can significantly reduce flight times, sometimes resulting in aircraft arriving ahead of schedule. At Sydney this means that some aircraft can arrive before curfew ends, or more significantly, aircraft adjust their flight parameters to arrive as soon as possible after the end of the curfew at 6am. Airservices has had to engage airlines to smooth out the number of aircraft arrivals occurring in the first hour after the end of curfew at Sydney. ” UPRs require sophisticated enabling technology by both airlines and the ANSPs providing these services. With aircraft no longer flying on fixed routes, new technology is needed for both the aircraft and ANSP to determine position, identify when aircraft are on conflicting flight paths and to effect separation.

Aircraft require FANS 1/A as their avionics foundation in order to operate UPRs. This involves the use of ADS-C (automatic dependent surveillance-contract) for position reporting. ADS-C involves a one-to-one relationship between the aircraft, ANSP and satellite communication provider ie, a contract. The aircraft determines its position from the GNSS satellite constellation and transmits this information, again using satellite, via the onboard ACARS (aircraft communication address and reporting system) datalink, to the ANSP. Under ADS-C, position information is sent by aircraft to the ANSP, typically every 14 minutes. Communication between the ANSP and aircraft in these oceanic areas utilises CPDLC, in the place of voice over HF radio.

Although UPRs were initially only available to aircraft equipped with FANS-1/A, as their use has become common, fitment constraints have been eased allowing more aircraft to participate. Within Australian-administered airspace, aircraft are now only required to have required navigation performance approval (RNP)10 plus HF communications.

For aircraft not equipped with FANS-1/A, the air traffic management (ATM) system calculates the position of an aircraft based on its flightplan track and presents this to controllers on a plan view display. In essence a symbol on the controller display shows the estimated position of the aircraft. It is based on aircraft performance and weather information parameters that are listed in the flightplan. This position information is updated during the course of the flight each time an aircraft makes a position report. Even though the symbol representing an aircraft’s position is presented on an air traffic control display, this remains a procedural control environment with large, 10-minute separation standards applied. It also requires aircraft to be flying on fixed routes and communication with aircraft is still conducted via voice over HF radio.

Modern oceanic ATM systems now incorporate FANS-1/A capability into their processing and display functions. The ADS-C position of an aircraft is depicted on the controller display and data link communication takes place directly between the controller and aircraft, without an HF air-ground operator being an intermediary. Greater accuracy in determining the aircraft position and improved communication has enabled the separation distance between aircraft, both equipped with FANS-1A, to be reduced. Instead of 10-minute spacing, this distance can be reduced to 30nm same track and 30nm between tracks.

It is commonly considered that around 70 per cent of aircraft flying across the Pacific are now equipped with FANS-1/A and able to undertake UPRs. This proportion is expected to increase as older 747-400s are replaced by newer generation airframes.

The use of UPRs require ATM systems to dynamically update aircraft route information, help controllers determine when aircraft are in conflict and be integrated with the ATM systems of adjoining flight information regions (FIRs). Air traffic control centres located in Oakland, Nadi, Auckland and Brisbane manage the oceanic airspace across the Pacific. Each system has the capability to accommodate aircraft flying on UPRs. Data is exchanged about aircraft through standardised system interfaces, significantly reducing the need for verbal coordination between controllers.

One important feature of UPRs is enabling an aircraft to request a flight plan amendment to the remainder of its route while it is airborne, called the dynamic airborne reroute procedure (DARP). As an example, an aircraft passing Hawaii en route from Los Angeles to Sydney may identify that the upper wind conditions are different from those forecast, resulting in a desire to change in its UPR. Subject to ATC approval, DARP enables the new route to be filed into and accepted by each of the Pacific oceanic ATM systems.

UPRs present considerable complexity for air traffic controllers in determining whether aircraft are, or will be, in conflict. Software upgrades to enable conflict detection are being introduced in the Australian ATM system to support manual calculations by air traffic controllers. At present, this conflict detection is based on flightplan track calculations, enhanced by ADS-C position information. This functionality is adequate for relatively low density operations.

Airservices is currently trialling enhanced conflict detection capability. Scott highlighted: “For the past few months we have successfully trialled flightplan conflict detection functionality west of 163 degrees. Next we intend to introduce probing capability.

“As a first step it will enable controllers to look ahead for 30 minutes to identify potential aircraft conflictions. Controllers will also be able to undertake ‘what-if’ scenarios to assess the impact of requested route and level changes by an aircraft and evaluate potential conflict resolution alternatives. This functionality will be expanded in time to enable a two-hour look ahead. ” Applications relating to ADS-B hold promise for oceanic capacity enhancements as well as being the key enabler for ATM development programs in the United States (NextGen) and in Europe (SESAR), directed at domestic airspace and airport operations.

At present, the Iridium satellite constellation provides communications for a wide range of users, and is one of the satellite systems supporting ADS-C and CPDLC (FANS-1/A) data exchange between aircraft and ANSPs. The current Iridium satellite constellation has been operational since 1999 and is coming the end of its operational life.

A replacement constellation of Iridium satellites has been planned with a target operational commencement date of 2017, named Iridium NEXT. The replacement will involve 66 cross-linked low earth orbit (LEO) satellites.

The Iridium constellation will provide uninterrupted communi-cations coverage of the entire earth’s surface.

This constellation will provide uninterrupted communications coverage of the entire earth’s surface. The Iridium NEXT satellites will also include an ADS-B receiver payload. This will enable ADS-B surveillance to be continuously available over oceanic and other low traffic density airspace across the globe, without the need for ADS-B ground stations. It is not, however, being designed with the capacity to provide ADS-B data from high traffic density airspace.

The ADS-B component of the Iridium constellation is being developed by Aireon, a joint venture company between Iridium and Nav Canada, Canada’s ANSP. Harris Communications is providing ADS-B receivers for the satellites under contract to Aireon. ITT Exelis will provide systems engineering for the project. ITT Exelis is also the lead integrator of the ADS-B ground station program currently being undertaken in the United States.

Aireon presents new possibilities for the use of ADS-B particularly for remote and oceanic airspace.

Airservices senior advisor CNS-ATM, Bob Peake, explained: “ADS-B provided by satellite is targeted at oceanic and low density airspace where ADS-B data is difficult to receive. No additional avionics fitment onboard aircraft is intended, although further validation work is still needed to quantify the system performance, particularly when a satellite is positioned directly above the aircraft. In this position the aircraft’s ADS-B antenna has much lower gain than in other directions. However, the satellites are orbiting rather than geostationary, so any coverage gap will be transitory. ” Peake added: “Australia, the US and Europe will all have fitment of ADS-B on a high percentage of aircraft within their airspace by 2020. Complete ADS-B coverage in low traffic density areas, provided through satellites, combined with a high percentage of ADS-B-equipped aircraft, presents a number of new possibilities for improving the operational performance of the ATM system in oceanic airspace. Airservices is cautiously optimistic about the opportunities presented by Aireon. ”

Implementation of ADS-B (Out), involving an aircraft emitting a transponder signal that is used as a surveillance source by air traffic controllers, is well advanced in a number of countries. Once a large percentage of aircraft are equipped with ADS-B (Out), the possibilities offered by ADS-B (In) become a vital enabler to increase traffic capacity, provide more efficient flight profiles, and reduce ATM-related costs.

ADS-B (In) involves an aircraft receiving ADS-B information from other aircraft and processing this data to support various onboard “applications” to assist with air traffic management and overall situational awareness for the flightcrew.

Within oceanic airspace the use of ADS-B (In) is being trialled for an In Trail Procedures (ITP) application. ITP can be applied when there are ADS-B (Out)- equipped aircraft on the same track and one aircraft is seeking to climb or descend through the level of other aircraft.

Instead of air traffic control needing to apply large procedural separations between the aircraft, the ADS-B (In) ITP application tells the pilots the distances and rates of closure with respect to other aircraft. If they fall within specified limits (which are smaller than the procedural separation which would otherwise apply) then ATC can approve the change in flight level.

Peake explained this procedure: “At present if an aircraft wishes to climb to a higher flight level, through the one occupied by a preceding aircraft, it could require anywhere between 30nm up to 100nm separation to be established. With ITP, if the aircraft are 15nm or more apart and the rate of closure is less than a specified rate, then air traffic control can clear the following aircraft through the level of the one leading. ” It is important to note that air traffic control still orchestrates the entire traffic situation, since there could be other aircraft that are not ADS-B-equipped in the vicinity. ITP can only be applied between an ITP-equipped aircraft and other aircraft equipped with ADS-B (Out).

The benefits of ITP are not limited to the fuel saved by increased access to more efficient flight levels on a given flight route. Since Because ITP decreases the probability of an aircraft being held down at inefficient flight levels, it allows airline operators to reduce the amount of contingency fuel carried on every flight, reducing operating costs and in some instances allowing an increased payload to be carried. It also holds environmental benefits.

Trials of ITP are currently taking place across the Pacific and over the North Atlantic Ocean. The Pacific trial is being undertaken by United Airlines, which has equipped 12 747-400s with an ITP ADS-B (In) application. The FAA Oceanic Control centre at Oakland is leading the initiative and Airways New Zealand has recently entered into an agreement with the FAA and United Airlines to participate in the trial for three months, with an option to extend for a further three, commencing on April 1.

Australia, the US and Europe will all have fitment of ADS-B on a high percentage of aircraft within their airspace by 2020. PEAKE – AIRSERVICES

Geoff Hounsell, Airways’ manager oceanic, said: “We are seeking to validate the benefits of ITP as another means of enhancing efficiency. The trials will enable us to fine-tune our operational procedures which can then be used in the development of international standards. ”

Another ITP trial is taking place across the North Atlantic Ocean involving five airlines and 25 aircraft comprising A330s, 767s and 777s. The trial is a multi-party arrangement between Eurocontrol, FAA, avionics manufactures and airlines. It is intended to confirm whether the expected operational benefits can be realised and to finalise technical standards for the ADS-B (In) ITP application avionics. Meaningful results from this trial are likely to be 12 months or more away. Assuming results are positive, it is likely to lead to the widespread future fitment of the ITP ADS-B (In) application for operations in oceanic airspace. The additional steps required to be completed before ITP enters full operational use include the validation and finalisation of pilot and air traffic controller procedures and the acceptance and publication by ICAO of globally standardised ATC procedures.

Other ADS-B (In) applications in development have applicability for terminal area operations where ADS-B (In) applications are being evaluated to improve traffic situational awareness, to enhance visual separations on approach.

At high density airports, the aircraft landing order sequence is arranged early in the approach phase. From the initial approach until the ground air traffic controllers issue direction, level and speed control instructions to aircraft. An ADS-B (In) application called Interval Management is being trialled to dramatically reduce this need for controller intervention.

With Interval Management, a following aircraft will then be required to maintain a specified interval (which may be time or distance) behind the leading aircraft. This application may also be used for departing aircraft, as they route clear of high density terminal airspace. The ADS-B ground network that is currently being installed throughout the United States is a pre-requisite for these operational uses.

In the airspace around an airport ADS-B (In) applications have the potential to improve runway safety. Considerable technology and procedures are applied to prevent runway incursions such as microwave barrier detection and safety alerts on ATM surveillance systems. These warnings predominantly alert air traffic controllers to a runway incursion.

An ADS-B (In) application developed by ACSS incorporates a moving map display of the airport surface and the position of other traffic (aircraft or vehicles) that are nearby on the ground or in the air. The tracks of this traffic can be processed to automatically alert the flight crew to potential conflicts. A trial of this technology was conducted at Philadelphia International Airport in 2009. Analysis of past incidents indicated that this type of moving map display with local traffic could reduce runway incursions by 95 per cent, and the inclusion of visual and aural alerts could reduce incursions by 99 per cent.

At major airports one of the constraints to improving the runway landing rate is wake turbulence protection. Future enhancements to the ADS-B system are being considered to support more dynamic wake vortex protection procedures, under which the impact of wake turbulence is visually presented in the cockpit.

The system involves a following aircraft receiving data about wind, temperature, aircraft type, eddy dissipation rate and flap setting of a leading aircraft through an ADS-B (Out) broadcast from that aircraft. An ADS-B (In) application would enable the pilot to select the designated aircraft from which to receive this wake turbulence data and see the impact of wake turbulence vortex in the prevailing meteorological conditions, and use this to maintain a safe separation distance behind the leading aircraft.

ASD-B technology is the backbone for next generation ATM systems. Within Australia, the use of ADS-B (In) is likely to be developed in the years to come to provide surveillance information to a support range of flightdeck applications. This will inevitably lead to greater pilot involvement in the separation process and increased situational awareness, both in flight and on the airport surface, contributing to improved safety.

ADS-B applications have the potential to improve runway safety.