By Air Marshal Anil Chopra (r)
Helicopters in low-level flight are endangered by power lines or telephone wires, especially when flying at night and under poor visibility conditions. Military helicopters have to fly ultra-low at high-speed in what is called Nap of the Earth (NOE) flight. Often helicopters have to do rescue missions in the mountains under low visibility or adverse weather conditions. In India, helicopters fly in the mountains carrying VVIPs.
On 8 December 2021, India’s first Chief of Defence Staff (CDS), General Bipin Rawat, died in a helicopter crash in the hills. There have been other similar helicopter accidents. Most night vision equipment cannot safely detect many obstacles that can be dangerous for flight. There is, thus, a widely accepted need for a reliable means of detecting such obstacles. Various solutions have been studied and tested, including millimetre wave technology and other active systems.
Background Information
Low-altitude, night-time helicopter operations provide significant advantages in many battlefield settings to avoid detection, but they also greatly increase the risk of collision with wires, cables, towers, antennae, trees, and terrain features. Nowadays, helicopters have also to fly in urban warfare conditions. To augment pilot vision, the helicopter commonly includes imaging devices such as forward-looking infrared scanners (FLIRs) and image intensifiers (night-vision goggles). These are alright for normal night approach and landing. However, resolution often proves inadequate at normal flight speeds for timely detection. That limitation significantly impairs operations and forces a speed reduction. Daytime operations also have obstacle-related problems. Thus, helicopter operations need a better way to detect obstacles.
Further, the North Atlantic Treaty Organisation (NATO) program aimed at enabling helicopter operations in a Degraded Visual Environment (DVE) has been in the planning since 2012 to address serious problems caused by this. DVE is reduced visibility of potentially varying degrees, wherein situational awareness (SA) and aircraft control cannot be maintained as comprehensively as they are in normal visual meteorological conditions (VMC) and can potentially be lost.
Some initial attempts at wire detection employed radio waves in the millimetre wave band or light from carbon dioxide (Co2) lasers at 10.6 micrometres or neodymium-doped yttrium aluminium garnet (Nd: Yag) lasers at 1.06 micrometres, and various laser diodes at 0.85 micrometres. But the requirement was to have a small, compact, lightweight, cost-effective obstacle-avoidance system. Such a system should also feature eye-safe operation for overflown personnel and aircraft ground crews, provide an effective pilot interface, and install conveniently in existing aircraft. Furthermore, it should operate during the day or night and under inclement weather conditions, not impairing aircraft survivability.
System Mechanism
The system for sensing objects in the flight path of an aircraft and alerting the pilot to their presence includes a laser radar subsystem for emitting a beam of laser energy, receiving returns from objects, and processing the returns to produce data related to the range of the objects from the aircraft. A scanning subsystem scans the beam and produces directional information related to the instantaneous direction of the beam relative to the aircraft. Processor circuitry controls operation processes the range data and directional information with instrumentation data from the avionics system. It then produces video information related to the range, direction, and type of the objects and interfaces the video information to the video display system.
The processor circuitry may be programmed to overlay video information on existing aircraft video display systems and provide acoustical warnings on an aircraft intercom. The system analyses return by subdividing the field of regard into a series of analysis windows, performing statistical analysis of the returns related to each of the analysis windows, and identifying returns that fall into a common range interval. It then transforms the coordinates of objects measured relative to the aircraft to a horizon-stabilised coordinate system independent of the aircraft’s attitude. The system corrects for movements of the aircraft. Based on speed, and obstacle distance, the system also suggests the direction and quantum of avoidance manoeuvres that the pilot may execute.
Initial NASA Research
As early as 1996, the National Aeronautics and Space Administration (NASA) began research on helicopter collision avoidance systems. The initial systems at NASA’s Ames Research Center, along with Honeywell Military Avionics, had looked at 35 GHz Millimeter-Wave (MMW) Radar Forward Sensor designed to detect obstacles in the aircraft’s flight path and provided a cockpit display to help avoid them. Although developed for helicopters, the new radar system had potential uses for all aircraft. The collision avoidance requirement is much more demanding for helicopters because of the nature of their operations.
Military helicopter pilots often fly below tree level and face unique guidance and control tasks, such as aircraft concealment, obstacle avoidance and real-time mission planning. These tasks require a high degree of pilot concentration, which is intensified during bad weather and during stressful tactical situations. Automation of some of these tasks can reduce pilot workload while enhancing safety. These features are also required nowadays for civil helicopter missions such as emergency medical service helicopters, search and rescue missions and also by oil rig operators.
NASA carried out initial flight tests of the collision avoidance system on a UH-60 research helicopter. The test aircraft was equipped with satellite navigation systems, digital data recorders, panel-mounted displays in the cockpit and crew area and a Silicon Graphics imaging system to generate high-fidelity graphics. The new radar system was mounted on the nose of the aircraft, and a colour video camera was mounted adjacent to the radar to document the test conditions.
Existing 4.3 GHz radar altimeter components were used for the sensor transmit and receive functions to make it affordable. The radar altimeter signal is converted to 35 GHz and transmitted as a scanning, three-dimensional pencil beam through a small twist-reflector-type antenna measuring only nine inches in diameter. Initial flight tests were conducted over flat and moderately rugged mountainous terrain. While airborne, the new radar system constructed a database of the terrain and obstacles it detected in the aircraft’s flight path and produced a three-dimensional synthetic perspective grid panel display for the pilot. The new radar also operated well in bad weather conditions, such as fog or rain, when visibility was poor.
The UN Definition Of HTAWS Requirement
A 2013 United Nations definition of Helicopter Terrain Awareness and Warning Systems (HTAWS) operational requirements provides for Ground Proximity Warning System (GPWS), Forward Looking Terrain Avoidance (FLTA), and a display capability, which places an aircraft position symbol on a terrain and obstacle map and applies terrain display algorithms. Terrain and obstacle mapping information may be provided on a Weather Radar display, Electronic Flight Instrument System (EFIS) display, or other compatible display screens. The display of HTAWS information on a Multi-Function Display (MFD) that also serves other purposes should be displayed in a heading up or track up orientation, not in a North up orientation to avoid confusion.
The HTAWS should integrate a terrain and obstacle database with Global Positioning System/GLONASS vertical and horizontal positions to enhance terrain and obstacle awareness. The enhanced awareness shall incorporate a look-ahead function that provides cautions, warnings, and terrain and obstacle display. The HTAWS should provide automatically, as a minimum, warnings under the when excessive descent rate; excessive terrain closure rate; excessive altitude loss after take-off or go-around; unsafe terrain clearance while not in landing configuration; excessive downward deviation below the instrument glide path; excessive banking angle; excessive pitch angle; and vortex ring danger.
Initial Systems Development
The United States Special Operations Command’s (USSOCOM) Joint Operational Requirements Document (JORDS) lays down the need for a Cable Warning Obstacle Avoidance System for United States Military helicopters. The Helicopter Laser Radar System HELLAS was developed in Germany by EADS-Dornier. The Laser Obstacle Detection System (LODS) Foreign Comparative Test (FCT) program began in March 2002 to evaluate the HELLAS on a US Army helicopter. The managing agency of the LODS-FCT was the US Army Night Vision and Electronic Sensors Directorate (NVESD).
The Aviation Applied Technology Directorate (AATD) was contracted to mount the HELLAS sensor on the nose of a UH-60L Blackhawk helicopter and to conduct flight tests to evaluate the HELLAS obstacle detection sensor. The UH-6OL-mounted HELLAS system was to detect wires along the flight path and act as an aide to avoid the wires. Sensor installation, aircraft checkout, and flight testing were conducted to achieve this objective. Concurrent with this testing, laser raw data angle range, including INU/GPS data to support studies, was collected to determine the ability of the HELLAS system to detect general obstacles, poles, towers, etc. and to provide precision landing information to assist rotary wing aircraft and UAVs during the critical landing phases of shipboard operations.
Modern Helicopter Obstacle Avoidance Systems
Dornier Luftfahrt GmbH has developed a Laser Radar Sensor which has good capabilities and overcomes the shortcomings of other radar systems. The radar/laser sensor needs to be integrated onboard along with display and warning functions. Turkey-based Meteksan has developed the HETS, which uses a 1550 nm wavelength fibre laser that detects obstacles such as wires, poles, and trees. It detects vertical and horizontal obstacles and alerts the pilot promptly to avert an accident. The warning is through both aural and visual means. The system is operational with a 30-40 degrees Field-of-View and scans in 2 Hz for detecting vertical and horizontal obstacles up to 5 mm in diameter. The total instrumental range is 1500 m with the LIDAR technology, accurate signal processing and user-friendly visualisation characteristics.
Airbus Helicopters has developed an experimental onboard image processing management system aimed at performing automatic approaches and landing in challenging conditions, as well as paving the way for future sense and avoid applications on autonomous vertical take-off and landing (VTOL) systems. Codenamed Eagle, for Eye for Autonomous Guidance and Landing Extension, this system federates the entire helicopter’s image processing functions and feeds them into the avionics system, thus improving the crew’s situation awareness and reducing the pilot’s workload by automating and securing approaches, take-off and landing in the most demanding environments. Ground and flight tests of Eagle have been ongoing since 2017. While existing missions such as search and rescue and offshore transportation will benefit from Eagle’s capabilities, the system will also help address future requirements for operations in urban environments. Eagle will also contribute to improving the safety, autonomy and performance of future unmanned vehicles.
The system, which could be embedded in a variety of existing and future Airbus VTOL vehicles, relies on a gyro-stabilised optronics package, which includes three high-resolution cameras and state-of-the-art processing units, as well as onboard video analytics providing advanced functionalities such as object detection and tracking, digital noise reduction as well as deep learning. Newer versions of the Eagle system are integrating a laser, which, combined with the high processing capability, would open the door to other applications such as a new generation of search lights, obstacles detection and 3D terrain reconstruction.
The Thales HTAWS is a smart collision avoidance system for helicopters. By 2017, Thales had developed the most advanced HTAWS, which brought the next level of flight safety. More than 20 patents are incorporated into this innovative software solution. HTAWS aims to reduce the rate of accidents drastically by timely providing cautions and warnings to the pilot. It perfectly fits with the different mission levels requirements. It uses three databases to obtain essential information relative to airports, obstacles and terrain.
The terrain database is built upon multiple database sources, including state-of-the-art Shuttle Radar Topography Mission (SRTM) mapping data and other Thales proprietary high-accuracy data, all of which are fully World Geodetic System (WGS) 84-based. Thales has a long track record in supplying Terrain Awareness and Warning Systems (TAWS). In the fixed-wing aircraft domain, Thales TAWS are in service on a wide range of civil, regional and business aircraft from companies including Boeing, ATR, Sukhoi and Bombardier.
The Way Ahead
Hindustan Aeronautics Limited (HAL) is known to be interacting with global experts to develop Obstacle Avoidance Systems (OAS) for its extensive product line of Dhruv-based helicopters. The equipment is very critical for all the variants. HENSOLDT (erstwhile part of EADS) of Germany is known to be in discussion with HAL to offer such a product with a substantial transfer of technology (ToT). It is not known if there are other players in the competition. An early decision must be taken.
An October 2022 Federal Aviation Administration (FAA) report to the US House and Senate Committees on Appropriations details the net passenger and public safety benefits of installing HTAWS on all commercial helicopters, as well as any challenges with implementing such a mandate. Future HTAWS will be enhanced with the introduction of the updated artificial obstacle database and the remote natural locations database. New active sensors, such as wire detection, and eventually merging the best of active sensors and passive database technologies.
Air Mshl. Anil Chopra (r) is the Director General of Centre for Air Power Studies
