It is one of the ironies of the future air-navigation system (FANS) that, although it is aimed primarily at airline operations, the general-aviation (GA) community is so far its major user. One FANS element - satellite navigation - is already commonplace in fixed-wing GA in the form of the global-positioning system (GPS). Now, the rotary world is adding the other two major FANS components - satellite communications (satcoms) and surveillance.
Like their fixed-wing cousins, helicopter operators are finding that satellite-based systems are initially driven by specific requirements, but then bring a host of potential applications with them. A classic example is emerging in the Norwegian sector of the North Sea's heavily used airspace. There, the driving force is a safety concern, but economic benefits are expected to follow.
In early 1998, the Norwegian Civil Aviation Authority (NCAA) hopes to declare its North Sea airspace sector under positive air-traffic control (ATC). Most of the area is outside radar range, however, and the NCAA is using a system called modified automatic dependent surveillance (M-ADS) to achieve its aims. As its name suggests, the programme is a modified version of the ADS concept intended to bring real-time surveillance to oceanic airways. As with ADS, airborne equipment is used to transmit aircraft positions and other data via a satellite link as requested by ATC, similarly via satellite (see diagram). The aircraft fit is a slimmed-down version of airliner equipment and is capable only of data communications - not voice links.
Operators in the area, primarily Helikopter Service of Stavanger, will have to install the equipment, but are happy to do so for safety reasons. The International Federation of Airline Pilots Associations (IFALPA) awarded the region one of its "black star" classifications in protest at the demands being put on crews in terms of separation. A race track pattern is used to separate inbound and outbound rig-flights - which are frequently in "real" instrument meteorological conditions - but conflicts, particularly during level changes, are common. Not long ago, two fully laden Eurocopter AS.322L Super Pumas fortunately broke cloud moments before their paths would have met, barely avoiding each other. Understandably enough, the Norwegian development programme is being paid for by the operators' primary customers, oil companies Statoil, Shell, Saga, Hydro, Phillips, Elf and Amoco.
Helikopter Service is buying its airborne equipment from Norsk Forsvarsteknologi (NFT) of Norway, which is using a slimmed-down Racal Avionics satcoms unit - dubbed a transceiver - to handle the M-ADS data. The NFT unit takes navigation data from the aircraft's GPS, packages it into the required ADS message-format and inputs it to the transceiver. The planned ADS format will include GPS position, universal timing mark, ground speed, track, altitude, and next-leg with way points.
Racal marketing manager Robbie Stewart explains that the satcoms unit is a re-packaged version of the Racal-Honeywell MCS-3000 airliner system. It is a 4MCU (modular concept unit) installation, compared with the 14MCU MCS-3000. The entire airborne installation weighs 9.5kg. Stewart comments that the success of the re-packaging is largely a reflection of "technology moving on". He also notes, however, that the high-power amplifier is required to support only single-channel data, rather than multi-channel on the airliner system, and that there is no need for voice capability - obviating the need for a voice codex and permitting use of a low-gain antenna. Helikopter Service will first equip its Sikorsky S-61Ns and its Super Pumas, on which the configuration is not actually so important, but then will add its Boeing Chinooks, Bell 214STs and Eurocopter Dauphins, on which it is more significant. Stewart notes that the northerly sphere of operations implies a low look-angle to the satellite, making antenna mounting critical.
SYSTEM COMPATIBILITY
Although the ADS data is not particularly complex to pass, the system is also compatible with the Data 3 communications format and ground-based X.25 electronic data interchange networks - laying the ground for operators such as Helikopter Service to pass other data for their own use. For ATC purposes, however, the data will be passed to the Stavanger and Trondheim centres, where it will be fused with radar data when available. The NCAA, "shocked" by off-the-shelf prices, developed its own ADS workstation for controllers, which Raytheon has since been trying to sell on to India.
NCAA programme manager Tor Helgesen explains that, at present, pilots make voice position reports every 15min when they are outside radar coverage. That is as much to narrow the search area in the event of a crash or ditching as for ATC purposes. ATC uses a personal-computer-based system to watch for overdue reports. M-ADS reporting intervals are selectable in real-time, but might typically be 30s, with a message being triggered automatically in the event of an emergency.
Helikopter Service chief test pilot Knut Lande stresses the importance of mandating M-ADS, saying: "We said that we would participate because we believe in the system - but only if it is mandatory. We don't believe in it if it is not compulsory because we see it as an alternative to radar and a means of establishing controlled airspace.
"The pilots have been responsible for their own safety and can blame nobody else - but they only have an advisory service. That has been complained about for many years by the pilots' union. IFALPA pressed the NCAA to do something, although [the Authority] had resisted as long as possible and it turns out that, if they want to do so, then they can do it."
Although primarily looking for the ADS function, Lande notes that eventually the datalink could be turned into a two-way, pilot-controller, link to send and receive clearances. Helgesen agrees, but notes that there is little room in current helicopter cockpits for an input and display screen. "It will need 'glass' cockpits to come," he says.
What could emerge sooner however - and not just in the North Sea - is the use of M-ADS to pass health-and-usage monitoring-system (HUMS) data in real time. Lande says that Helikopter Service would like to do that so that technicians could monitor the data and diagnose problems before an aircraft returns to base. Since the system is satcoms-based, it can be used globally to monitor helicopter fleets, which are increasingly being operated in different continents from those of their owners' headquarters.
In Malaysia, meanwhile, a mixture of economic and operational motivations similarly lies behind helicopter trials of a new navigation and tracking system. Asia presents one of the most demanding environments in the world for helicopter operators, consisting of vast areas of remote mountainous jungle and large expanses of water prone to typhoons. To improve navigational accuracy and provide a real-time tracking capability, Malaysian Helicopter Services (MHS) has turned to Sweden's Scandinavian Avionics (SA).
SA has been conducting a series of differential GPS (DGPS) trials in Malaysia, using a self-organising, time-synchronised, transponder. Developed by SA subsidiary GP&C Global Support, it promises a wide range of applications. Aside from providing satellite positioning and tracking, the system is expected to offer ADS-broadcast (ADS-B) precision approaches, collision avoidance and the transmission of special data, such as HUMS. The US Air Force has ordered an initial eight GP&C systems for its Lockheed C-5 transport aircraft.
VERSATILITYKEY
The key to the GP&C system's versatility is a time-division multiple-access (TDMA) datalink developed by Swedish engineer Haakan Lans. The system provides a two-way datalink of 25kHz bandwidth transmitted on one frequency in the VHF/UHF bandwidth. In place of conventional contention or squitter techniques, with TDMA multiple, digital, data-communications are time-synchronised using the global-navigational-satellite system (GNSS). Up to 2,250 transmissions can be accommodated in a single minute, each message transmitting at a rate of 9,600 bits/s.
The present TDMA system can handle 255 different types of messages, including satellite position reports and differential corrections. Other transmission messages could include text, ADS, flight information, systems status and secondary-surveillance-radar position data.
The system's applications extend beyond simply the relay of position information between aircraft and base stations. Data can also be transmitted between aircraft, providing a manual proximity warning and, as soon as the necessary software is certificated, automatic alert and collision avoidance.
"With our system, every user knows exactly where everyone else is," says GP&C managing director Erling Linneberg, "Unlike Mode S, GP&C provides real-time continuous monitoring of aircraft and we can see what their intentions are without depending on a ground infrastructure. According to Linneberg, the GP&C system has demonstrated an accuracy of "approximately 1m" during GNSS landing trials conducted by the Swedish Civil Aviation Administration in 1993. He says that the system achieved a lateral-plane accuracy better than that of Category 3 requirements. Improvements, however, are needed in the vertical mode.
Flight-certification requirements are likely to mean that the system's initial operational application will be on the ground. It has already been tested by the Danish, German and Swedish aviation authorities, for use by airport support vehicles to prevent runway incursions and aircraft collisions. A report prepared by Stanford Telecommunications for the US Department of Transport in October strongly recommended consideration of the GP&C system for ground-vehicle use. The report concluded that the system was relatively "...low cost and low risk, and was free of technical defects that could affect integrity or operational usefulness".
The Malaysian trials, under way since September, have concentrated primarily on demonstrating the systems' positioning, ADS-B and tracking capabilities for offshore helicopter operations. The tests have been conducted from MHS' main helicopter base at Kerteh, on the East Coast of Malaysia. In the first phase of testing, an MHS S-61N was fitted with a GP&C transponder and tracked on a digital moving map from the time it left Kerteh to its arrival at the Irong Barat-A oil rig, nearly 180km (100nm) offshore. The helicopter's transmission each second was picked up by a repeater station, north of Kerteh, and retransmitted to the GNSS transponder base station at the airport.
MHS is funding a second phase of trials to demonstrate full sea-level coverage up to a range of 185km. Transponders will be fitted to two S-61s, a Sikorsky S-76 and oil-support ships. Coverage has been extended by moving the repeater offshore to an oil rig and the base station to a 1,100ft (330m)-high hill, south of Kerteh Airport.
It is envisaged that the next phase will involve establishing a fully operational system - including two base stations with separate power supplies for added redundancy, each equipped with a DGPS transponder and a real-time moving-map display. The station could be a stand-alone operation, or form part of a cellular network covering a region or country. Seven have already been established in Sweden and Denmark and planned to be connected.
Proposed modifications to MHS' fleet of 24 helicopters would centre on the installation of a low-cost GP&C transponder, consisting of a GPS receiver, communications processor and transceiver, together with VHF and GPS antennae. There would also be the option to fit a cockpit display for traffic monitoring and integration of the transponder with the flight-management system, auto-pilot, satcoms, and other avionic systems.
The system's ability to provide a computer-generated localiser and glide-scope has also attracted the interest of the Malaysian Department of Civil Aviation. Precision approaches to oil rigs and airfields not equipped with instrument landing systems, are expected to form the focus of later trials in Malaysia.
OLYMPICS PROJECT
In the USA, the Federal Aviation Administration plans to conduct in July a small-scale demonstration of the low-level-airspace system, which it hopes to have in place over Atlanta, Georgia, for the 1996 Olympic Games. The Atlanta project is expected to involve more than 200 helicopters serving up to 13 sites via a network of low-altitude routes overlaying major roads. It will produce a prototype short-haul transport system, which will be left in place after the Games, to serve as a model for other cities.
The project will be an application of work already under way to develop standards for helicopter operations using the GPS and ADS. Aircraft participating will be required to be equipped to transmit and receive ADS reports, allowing a complete picture of airspace activity to be displayed on the ground and in the cockpit.
Atlanta's businesses, emergency services and security agencies see the project as a means to rise above the traffic congestion expected when an estimated 5.7 million people flock to the Games. The Helicopter Association International sees it as a chance to prove that helicopters can be a safe and effective means of urban transport. The FAA regards it as an opportunity to bring together work on the GPS and ADS, to demonstrate the satellite-based airspace-management system of the future.
FIRST GPS APPROACH
A US helicopter operation generated the first GPS-approach approved anywhere in the world - at the Erlanger Medical Center in Chattanooga, Tennessee. A GPS-only non-precision approach to the hospital's helipad was approved in June 1994. Subsequently, the FAA has approved GPS approaches for Petroleum Helicopters (PHI), in Morgan City, West Virginia; the University of Wisconsin, in Madison, Wisconsin; and the Mayo Clinic, in Rochester, Minnesota.
Before Erlanger's GPS approach was approved, the hospital's Life Force unit, flying an emergency-medical service (EMS) Bell 412, was required to divert to Chattanooga Airport, 19km (12 miles) away, if the helipad fell below visual-flight-rules (VFR) minima. The patient had then to be transferred to an ambulance, adding 20min to the trip. By early 1995, the GPS's only approach was estimated to have saved some 20 lives.
Life Force flew 50 test flights to validate the GPS approach. Its 412 is equipped not only with an approach-certified Garmin International GPS receiver, but also with an Arnav Systems GPS receiver and VHF datalink, which transmits position fixes to the unit's operations centre and to the local ATC tower. The datalink allows both Life Force and ATC to track the aircraft out to 155km, beyond radar range.
Erlanger is now awaiting FAA approval for a second GPS approach, into a neighboring valley, which was previously inaccessible except in VFR conditions. A second EMS 412 has been bought and is being equipped with the GPS and a datalink. Life Force plans to demonstrate precision-approach capability using DGPS by the end of 1995.
While the Arnav datalink is not being used for ATC, the tower display of helicopter is routinely monitored by Chattanooga ATC, Erlanger says, and can provide a backup to the primary-radar picture. The same datalink is also being used elsewhere, Arnav says: systems are being delivered to Quebec Hydro to monitor aircraft operations across the Canadian Province; and a GPS-based datalink demonstration is under way in the Gulf of Mexico.
The Gulf trial begun in March is sponsored by Oil Company Exxon and involves one helicopter operated by a subsidiary of PHI and a re-supply vessel. In the demonstration, GPS position reports are being datalinked to a master display at the operations base at Grand Isle, Louisiana, either directly, or, if either the helicopter or vessel is beyond line of sight, relayed automatically via the other datalink-equipped participant, or a repeater station on an offshore oil-platform.
Arnav says that Gulf operations pose significant problems, as the offshore platforms are served by large numbers of helicopters operating below 500-1,000ft (150-300ft) in a non-radar VFR, "see-and-avoid", environment. All ATC is procedural, resulting in extensive delays and "tremendous" communications problems as weather deteriorates. Adding to the problem is the proximity of the edge of US sovereign airspace, and the need for aircraft to be identified positively to avoid unnecessary interceptions.
Datalink offers an answer, the company believes, by providing a real-time interface with ATC and weather and operation centres, while potentially allowing helicopters and vessels to see each other on displays. The latter capability would assist in the rescue following a helicopter ditching in the Gulf or in coordinating air-sea operations against oil spills or platform fires, Arnav believes.
PHI is also evaluating an alternative solution based on satcoms, under an agreement with Newcomb Communications. Details are sparse, but a single helicopter operating in the Gulf has been equipped with a small satcom unit, with embedded GPS receiver, which is being used to transmit position reports via satellite to a shore station. In every case, much more development work is needed, but it is clear that FANS concepts have a critical role to play in battling the twin demons of the helicopter world, economics and safety.
Source: Flight International
