Helmet-mounted display


A helmet-mounted display is a head-worn device that uses displays and optics to project imagery and/or symbology to the wearer's eyes. It provides visual information to the user where head protection is required – most notably in military aircraft. The display-optics assembly can be attached to a helmet or integrated into the design of the helmet. An HMD provides the pilot with situation awareness, an enhanced image of the scene, and in military applications cue weapons systems, to the direction their head is pointing. Applications which allow cuing of weapon systems are referred to as helmet-mounted sight and display or helmet-mounted sights.

Requirement

Aviation HMD designs serve these purposes:
  • using the head angle as a pointer to direct air-to-air and air-to-ground weapons seekers or other sensors to a target by the pilot merely turning the helmet towards the target and operating a switch via HOTAS. In close combat, without HMDs, pilots have to align the aircraft to shoot at a target. HMDs allow pilots to simply point their heads at a target, designate a weapon and shoot.
  • displaying targeting and aircraft performance information to the pilot while "heads-up", eliminating the need to look inside the cockpit.
  • displaying sensor video for the purpose of:
  • * verification that the chosen sensor has been cued to the right target or location without requiring the pilot to look inside the cockpit
  • * viewing outside terrain using sensor video in degraded visual conditions.
HMD systems, combined with High Off-Boresight weapons, allow aircrew to attack and destroy nearly any target seen by the pilot. These systems allow targets to be designated with minimal aircraft maneuvering, minimizing the time spent in the threat environment, and allowing greater lethality, survivability, and pilot situational awareness.

History

In 1962, Hughes Aircraft Company revealed the Electrocular, a compact CRT, head-mounted monocular display that reflected a TV signal onto a transparent eyepiece.
One of the first aircraft with simple HMD devices appeared for experimental purpose in the mid-1960s to aid in targeting heat seeking missiles. The US Navy's Visual Target Acquisition System, made by Honeywell Corporation that was flown in early 1970s in F-4J and 1974–78 ACEVAL/AIMVAL on U.S. F-14 and F-15 fighters. VTAS received praise for its effectiveness in targeting off-boresight missiles, but the U.S. did not pursue fielding it except for integration into late-model Navy F-4 Phantoms equipped with the AIM-9 Sidewinder from 1969. HMDs were also introduced in helicopters during this time – examples include the Boeing AH-64 Apache with the Integrated Helmet and Display Sighting System demonstrated in 1985.
At the same time the Mirage 3CZ and Mirage F1AZ of the South African Air Force used a locally developed helmet-mounted sight integrated with the Armscor V3A heat-seeking missile. This enables the pilot to make off-bore attacks, without having to maneuver to the optimum firing position. After the South African system had been proven in combat, playing a role in downing Soviet aircraft over Angola, it is popularly claimed the Soviets embarked on a crash program to counter the technology. As a result, the MiG-29 was fielded in 1985 with an HMD and a high off-boresight weapon, giving them an advantage in close maneuvering engagements.
Several nations responded with programs to counter the MiG-29/HMD/R-73 combination once its effectiveness was known, principally through access to former East German MiG-29s that were operated by the unified German Air Force.
One successful HMD was the Israeli Air Force Elbit DASH series, fielded in conjunction with the Python 4, in the early 1990s. The U.S., UK, and Germany pursued a HMD combined with ASRAAM systems. Technical difficulties led to the U.S. abandoning ASRAAM, instead funding development of the AIM-9X and the Joint Helmet-Mounted Cueing System in 1990. American and European fighter HMDs became widely used in the late 1990s and early 2000s.
The first civilian use of HMD on aircraft was the Elbit SkyLens HMD on ATR 72/42 airplane.

Technology

While conceptually simple, implementation of aircraft HMDs is quite complex. There are many variables:
  • precision – the angular error between the line-of-sight and the derived cue. The position of the helmet is what is used to point the missile; it thus must be calibrated and fit securely on the pilot's head. The line between the pilot's eye and the reticle on the visor is known as the line of sight between the aircraft and the intended target. The user's eye must stay aligned with the sight; in other words, current HMDs cannot sense where the eye is looking, but can place a predicted impact point marker between the eye and the target.
  • latency or slew rate – how much lag there is between the helmet and the cue.
  • field of regard – the angular range over which the sight can still produce a suitably accurate measurement.
  • weight and balance – total helmet weight and its center of gravity, which are particularly important under high "g" maneuvers. Weight is the largest problem faced by fighter aircraft HMD designers. This is much less a concern for helicopter applications, making elaborate helicopter HMDs common.
  • safety and flightdeck compatibility, including ejection seat compatibility.
  • optical characteristics – calibration, sharpness, distant focus, monocular vs. binocular imagery, eye dominance, and binocular rivalry.
  • durability and ability to handle day-to-day wear and tear.
  • cost, including integration and training.
  • fit and interfacing the aviator's head to the aircraft – head anthropometry and facial anatomy make helmet-fitting a crucial factor in the aviator's ability to interface with the aircraft systems. Misalignment or helmet shift can cause an inaccurate picture.

    Head tracking

HMD designs must sense the orientation and in some cases the position of the pilot's head relative to the airframe with sufficient precision even under high "g", vibration, and during rapid head movement. This is known as 3D tracking. Five basic methods are used in current HMD technology – inertial, optical, electromagnetic, sonic, and hybrid. Hybrid trackers use a combination of sensors such as inertial and optical to improve tracking accuracy, update rate, and latency.

Hybrid inertial optical

Hybrid inertial tracking systems employ a sensitive Inertial Measurement Unit and an optical sensor to provide reference to the aircraft. MEMS based IMUs benefit from high update rates such as 1,000 Hz but suffer from precession and drift over time, so they cannot be used alone. In this class of tracker, the optical sensor is used to constrain IMU drift. As a result, hybrid inertial/optical trackers feature low latency and high accuracy. The Thales Scorpion HMCS and HMIT HMDs utilize a tracker made by InterSense called the Hybrid Optical-based Inertial Tracker.

Optical

systems employ infrared emitters on the helmet infrared detectors in the flightdeck, to measure the pilot's head position. The main limitations are restricted fields of regard and sensitivity to sunlight or other heat sources. The MiG-29/AA-11 Archer system uses this technology. The Cobra HMD as used on both the Eurofighter Typhoon and the JAS39 Gripen both employ the optical helmet tracker developed by Denel Optronics.

Electromagnetic

sensing designs use coils placed in an alternating field to produce alternating electrical voltages based on the movement of the helmet in multiple axes. This technique requires precise magnetic mapping of the flightdeck to account for ferrous and conductive materials in the seat, flightdeck sills and canopy to reduce angular errors in the measurement.

Sonic

sensing designs use ultrasonic sensors to monitor the pilot's head position while being updated by computer software in multiple axes. Typical operating frequencies are in the 50 to 100 kHz range and can be made to carry audio sound information directly to the pilot's ears via subcarrier modulation of the ultrasonic sensing signals.

Optics

Older HMDs typically employ a compact CRT embedded in the helmet, and suitable optics to display symbology on to the pilot's visor or reticle, focused at infinity. Modern HMDs have dispensed with the CRT in favor of micro-displays such as liquid crystal on silicon or liquid crystal display along with an LED illuminator to generate the displayed image. Advanced HMDs can also project FLIR or night vision imagery. A recent improvement is the capability to display color symbols and video.

Major systems

Systems are presented in rough chronological order of initial operating capability.

Integrated Helmet And Display Sight System (IHADSS)

In 1985, the U.S. Army fielded the AH-64 Apache and with it the Integrated Helmet and Display Sighting System, a new helmet concept in which the role of the helmet was expanded to provide a visually coupled interface between the aviator and the aircraft. The Honeywell M142 IHADSS is fitted with a 40°-by-30° field of view, video-with-symbology monocular display. IR emitters allow a slewable thermographic camera sensor, mounted on the nose of the aircraft, to be slaved to the aviator's head movements. The display also enables Nap-of-the-earth night navigation. IHADSS is also used on the Italian Agusta A129 Mangusta.

ZSh-5 / Shchel-3UM

The Russian designed Shchel-3UM HMD design from 1981, has been fitted to the ZSh-5 series helmet, and has been used on the MiG-29 and Su-27 in conjunction with the R-73 missile. The HMD/Archer combination gave the MiG-29 and Su-27 a significantly improved close combat capability.

Display and sight helmet (DASH)

The Elbit Systems DASH III was the first modern Western HMD to achieve operational service. Development of the DASH began during the mid-1980s, when the IAF issued a requirement for F-15 and F-16 aircraft. The first design entered production around 1986, and the current GEN III helmet entered production during the early to mid-1990s. The current production variant is deployed on IDF F-15, and F-16 aircraft. Additionally, it has been certified on the F/A-18 and F-5. The DASH III has been exported and integrated into various legacy aircraft, including the MiG-21. It also forms the baseline technology for the US JHMCS.
The DASH GEN III is a wholly embedded design, where the complete optical and position sensing coil package is built within the helmet using a spherical visor to provide a collimated image to the pilot. A quick-disconnect wire powers the display and carries video drive signals to the helmet's cathode-ray tube. DASH is closely integrated with the aircraft's weapon system, via a MIL-STD-1553B bus. Latest model DASH IV is currently integrated on India's HAL Tejas.