Landing aids for bare bases: the war in Afghanistan has seen US military aircraft having to operate from overseas bases with minimal air-traffic control or blind-landing facilities, and US personnel having to renovate airfields that had been shattered by combat operations. Luckily, rapidly deployable landing aids were available, and newer systems are in development. (In Focus).

Master Sgt. Bart Decker of the US Air Force may have trained to be a combat controller in modern warfare, but earlier this year found him riding on horseback with the Northern Alliance forces in Afghanistan. In a world where roads may be primitive and the horse is the best form of personal transport, it is certain that the local airfields leave much to be desired in the way of facilities and equipment.

Decker had learned that in October 2001 when he became one of the first US personnel to arrive at an airfield in Uzbekistan. This airfield was to be used as forward-deployed base in support of Operation Enduring Freedom, the US campaign in Afghanistan against terrosism. The existing air-traffic control systems were antiquated.

"The tower and the runway [were] not in bad condition, and the host nation was actively using [them]," Decker recalls. "However, the control tower was set up with Cold War-era equipment with 1950's technology. It had fairly old systems that weren't in the best of shape."

"The in-place radar system was not up to today's technology standards and would limit the ability to bring American aircraft in bad weather and if visibility was not perfect," says Decker. This would have been a critical factor given that dust storms are common in the region, and winter was rapidly approaching.

After installing a US navigational system and placing portable lights along the runway and approach end, Decker and his fellow controllers set up a mobile microwave landing system (MMLS).

Though the MMLS was already being used to support peacekeeping operations in Bosnia, this first combat employment of the system allowed air traffic control operations to begin quickly. "We averaged about 15 sorties a day, around-the-clock airflow; bringing the aircraft in, getting them moved and unloaded, and flying them back out again."

The traditional system used for bad-weather landings is the Instrument Landing System (ILS). However, this is difficult to deploy because of topography limitations, and since only a small number of operating frequencies are available, the system suffers from frequency congestion. There are also frequency modulation interference problems in some areas. In the United States, the ILS is to be phased out by 2010.

Known to the US Air Force as AN/TRN-45, Textron Systems' MMLS is a mobile microwave landing system designed for rapid deployment. Because of its high operating frequency (5.03 to 5.09 GHz), it is less susceptible to the FM frequency and multi-path interference inherent in conventional Instrument Landing Systems.

The system is three-man portable and can be installed in under an hour. It can be used to provide landing guidance in remote, unprepared areas, or to restore landing services to military or civilian runways when such capability has been damaged or destroyed.

The system is trailer-based, and completely self-contained. It can be easily transported by virtually any vehicle with standard towing capacity, then be set up and fully operational in under 15 minutes. A shipboard variant (modified to compensate for the ship's motion) has been successfully demonstrated on aircraft carriers for shipboard aircraft approaches.

In a typical configuration, the elevation and azimuth antennas would be deployed at a location similar to that used for an ILS glide slope antenna, although a split-site configuration allows the azimuth antenna to be sited at the departure end of the runway. The system has an azimuth coverage of +/-40 degrees, elevation coverage of 0.9-15 degrees, and a 15 nm range under optimal conditions

MMLS has been operationally deployed at US Air Force bases in the United States and overseas for more than five years. While installations at bases such as Little Rock in Arkansas and Ramstein in Germany are long-term, other systems have been deployed around the world as required. For example, a system was installed at Tirana in Albania, allowing the airfield to act as a 24-hour a day landing site for US Air Force C-17 military and medical supply missions flown in support of Nato operations in Kosovo. It has also been deployed to the Antarctic, providing landing guidance to both the Ross Sea ice runway and the permafrost runway at McMurdo Station.

In July 2000, Italy became the first European Nato nation to adopt MMLS when the Italian Air Force accepted its first system. Two more were delivered that year for Air Force use, which were then followed by two systems for the Italian Army.

Between 1999 and 2000, the US Air Force equipped nine RC-135 reconnaissance aircraft with the Rockwell Collins Multi-Mode Receiver (MMR) precision navigation and landing system. Each aircraft received two GNLU-920 receivers, as replacements for the traditional ILS receivers, with a single approach guidance system that can be configured for compatibility with ILS, Microwave Landing System and Global Positioning System landing systems.

This was followed by an order for enough GNLU-945 receivers to equip each of 126 C-5 transport aircraft with two systems to provide high frequency omni-directional radio range, Instrument Landing System (ILS), marker beacon and Microwave Landing System (MLS) capabilities, enabling Cat III operations. Deliveries are due to begin in 2002 and continue through 2004.

Late last year Rockwell Collins successfully completed flight tests of a Multi-Mode Receiver (MMR) which combines MMLS, ILS, VHF Omni-directional Range (VOR), GPS, and GLS (GPS landing systems) into a single unit. Tests had included more than 100 MLS approaches at twelve airports in the United States and Europe equipped with military and commercial MLS ground stations from four manufacturers.

Conducted under the oversight of the Federal Aviation authority and the European Civil Aviation Authority, in co-operation with the National Aerospace Laboratory of the Netherlands, the trials demonstrated the Category IIIb capabilities of the MMR's integrated MLS module. Formal US Air Force approval for the new receiver is expected this year, paving the way for deliveries to begin.

Currently the United States depend on two types of mobile equipment for air-traffic control and ground controlled approach (GCA) at tactical airfields. The AN/TPN 19 Landing Control Central (Radar Set) can be used as a complete Radar Approach Control or as a Ground Controlled Approach (GCA) facility. It consists of an airport surveillance radar able to identify aircraft at ranges of up to 60 nm using primary radar and 200 nm with secondary radar, as well as of a precision approach radar (Par) which provides azimuth and elevation information from 20 nm to touchdown.

The AN/MPN-14K Landing Control Central is used by the US Air National Guard, reservists who would be responsible for 60 per cent of the American wartime air traffic control mission. The system can be configured as a complete Radar Approach Control (Rapcon) or Ground Controlled Approach facility. Deployment takes 26 hours and operation requires ten maintenance personnel and six controllers.

The system is based on a 60-mile range E-Band (2.7 to 2.9 GHz) Airport Surveillance Radar (ASR), and a 15-mile I-Band (9.0 to 9.6 GHz) Precision Approach Radar, the latter having a computer-controlled expanded scan antenna that enables coverage of multiple runway configurations, including parallel runways.

The AN/TPN-19 and AN/MPN-14K will be replaced by the new Mobile Approach Control System (Macs), a rapidly deployable, highly mobile air traffic control system. This consists of an ITT digital Air Surveillance Radar with associated tactical shelter and an ITT Operations Subsystem housed in separate shelters. These two subsystems can be deployed together or separately. The Macs also interfaces with a Par, and in early January, the US Air Force's Global Air Traffic Operations Mobility Command and Control Program Office based at Hanscom Air Force Base, Massachusetts awarded a $ 9.5 million contract to ITT-Gilfillan for the development of a suitable radar.

Testing of the first Par is due for completion in November 2004; ITT is expecting to deliver 18 Macs precision approach radars by June 2006. Eight will be operated by the Air Combat Command, and the other ten will be delivered to the Air National Guard.

The Macs can be easily deployed and requires a maximum of three C-130 Hercules aircraft for complete system transportation, including crew and spares. The TPN-19 System that is currently in use requires seven C-130s for deployment.

Improved capabilities provided by the Macs include a system operational availability of at least 99 percent. "Under the older systems we are able to operate for eight hours and then need eight hours of down time for maintenance," says 1st Lt. Patrick Widhelm, programme manager for the precision approach radar program office. "With the Macs, for every 100 days of operation only one day is needed for maintenance. This is definitely a big improvement."

ITT Gilfillan has already secured a significant share of the Par market with its GCA-2000 radar, a unit designed to provide both air surveillance and precision approach radar functions. Introduced in 1996, the GCA-2000 family is available in fixed, transportable and mobile versions for civil and military applications. The mobile version has been accepted for US Air Force service as the AN/MPN-25, while international customers for the GCA-200 include Brazil, Canada, Romania, Turkey and the United Kingdom.

The AN/MPN-25 system can be transported using a single C-130 Hercules, and set-up in less than 90 minutes, providing airport surveillance out to 30 nautical miles and a precision approach range of 20 nautical miles. Operating in secondary radar mode, it provides surveillance out to 100 miles. It is the only tactical ATC landing system capable of changing to any of six runways in less than one minute.

The crash on 3 April 1996 of a US military transport in Bosnia while performing a non-precision approach in adverse weather, with the resulting deaths of US Secretary of Commerce Ron Brown and 34 passengers, demonstrated the tactical need for a new rapidly-deployable precision approach system. To provide a near-term solution, a system which combined MMLS with a Precision Landing System Receiver was proposed. For the longer term, the US Department of Defense drew up plans to use a differential GPS system which should allow aircraft to land on any suitable land or sea-based surface worldwide even in conditions of low cloud ceiling or poor visibility.

In practice, the Department of Defense and the US Federal Aviation Administration both embarked on the development of GPS based precision approach and landing systems, known respectively as the Joint Precision Approach and Landing System (Jpals) and the Local Area Augmentation System (LAAS). LAAS in the designation used by the FAA--the International Civil Aviation Organization (ICAO) uses the term Ground Based Augmentation System.

The two systems will have features dictated by their missions, but are designed to be interoperable. Military aircraft equipped with Jpals will be able to use civil LAAS-equipped airfields, while civil LAAS-equipped aircraft tasked with transporting military troops and cargo will be able to make all-weather landings at Jpals equipped airfields.

Both are differential GPS systems in which a ground station monitors signals from GPS satellites, checks its GPS-derived location against its own surveyed location, computes the current error in the GPS information, and broadcasts corrections to suitably equipped aircraft. The main differences between LAAS and Jpals systems is that the military system can be rapidly deployed, and makes full use of military-standard P-code GPS functionality.

In January 2002, Honeywell announced the successful demonstration of a series of global positioning system (GPS)-guided autoland flight procedures to each of the four runways at Moses Lake Airfield in central Washington using a single Honeywell/Pelorus LAAS station. Full-scale engineering development of LAAS will begin this year, and the FAA will then begin the procurement of ground stations for installation at selected US airports.

The Jpals is "very much an R&D-type program," says Global Air Traffic Operations/Mobility Command and Control Program Office programme manager Eric Lekberg. "We're trying to reduce risk as much as possible before moving forward." So it is likely that it will be several years before the Jpals goes into production, the US Air Force explained.

If there are no useful landing aids in position at the destination airstrip, the only current way of flying in suitable systems is for the first transports to land when visual conditions are acceptable. In the future, some form of aircraft-mounted system, such as the BAE Systems Autonomous Landing Guidance (ALG), could be used to handle the final approach to the ground.

The ALG is based on a 94 GHz millimetric-wave radar which generates real-time radar imagery with a ten Hz refresh rate on a head-up display supplied by BAE Systems Avionics. The system has already been successfully demonstrated aboard a C-130H transport, the C-135C `Speckled Trout' trials aircraft, a United Airlines Boeing 727 and a Cessna light aircraft, and the company anticipates receiving a US Air Force contract to equip its fleet of C-130 and C-17 aircraft.

Regarding flight safety, a subject that became evident on recent quick but intensive military deployments in unknown areas, there is now a need to be able to share the airspace with airliners. There is also a growing requirement for military transports to be compliant with civil aviation authority rules, something that was recently exemplified with the Alenia/LM C-27J.

On the other hand, certain avionics companies now offer traffic alert and collision avoidance systems as well as enhanced ground proximity warning systems. A recent example is provided by the Hellenic Air Force which awarded L-3 Communications with a contract that includes certain systems as part of a major avionics upgrade of its fleet of 15 Hercules. The programme is underway and will first involve two aircraft modified by Spar in Canada (part of L-3). Hellenic Aerospace Industries will enter the scene on the third aircraft with final kit verification and will then carry out installation on the twelve remaining aircraft in Greece.