In aviation, a helmet-mounted display (HMD) is a head-worn device that uses digital 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 (HMSD) or helmet-mounted sights (HMS).
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 (e.g., radar, FLIR) 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 (such as airspeed, altitude, target range, weapon seeker status, "g", etc.) 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.
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 (VTAS), 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 (IHADSiSy) demonstrated in 1985. At the same time (1975) the Mirage 3CZ and Mirage F1AZ of the South African Air Force (SAAF) 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 (R-73), giving them an advantage in close maneuvering engagements. Several nations responded with programs to counter the MiG-29/HMD/R-73 (and later Su-27) 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 (LOS) 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 (or collimation, a technique used to present the images at a distant focus, which improves the readability of images), 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 (elevation, azimuth and roll) and in some cases the position (x, y, and z) 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.
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