Two Ways to See

By Charlotte Adams | November 1, 2003
Send Feedback

Two major U.S. helicopter programs have adopted different helmet-mounted sight/display (HMS/D) technologies. The HMS/Ds are alike in allowing pilots to fly and fire weapons without constantly looking down at the cockpit instruments. Both use highly accurate head tracking units to point sensors and steer guns. But one HMS/D, part of a U.S. Marine Corps helicopter upgrade program, uses mature analog display technology, with night vision imagery optically relayed from helmet-mounted sensors. The other system, being developed for the U.S. Army’s new reconnaissance/attack helicopter, will use a more complex digital design, displaying forward-looking infrared (FLIR) and night vision video from externally mounted sensors.

The H-1 Upgrade

The U.S. Marine Corps H-1 Upgrades program, which is remanufacturing 180 AH-1W attack and 100 UH-1N utility helicopters into AH-1Z and UH-1Y configurations, has chosen the Thales Avionics TopOwl helmet-mounted sight and display. It already is in production for the Eurocopter Tiger, NH Industries NH-90 and Denel Aviation Rooivalk helicopters. With two helmets in each AH-1Z and UH-1Y aircraft, the Marine Corps acquisition includes 560 systems, plus about 40 spares.

The Thales helmet system was integrated into the UH-1Y aircraft in mid-September, and flight tests with the equipment were expected to begin soon thereafter, as a continuation of the aircraft’s engineering manufacturing development (EMD) phase. This phase is a joint effort between the Naval Air Systems Command (NavAir) program office and Bell Helicopter Textron. The U.S. Defense Acquisition Board (DAB) was to decide imminently on when the Marines can proceed to Lot 1 of low-rate initial production (LRIP) on the Yankee and Zulu platforms, including six UH-1Ys and three AH-1Zs. Initial operational capability (IOC) for both configurations is planned for FY2008.

TopOwl is equipped with built-in night vision (NV) sensors–image intensification tubes (IITs)–that amplify the visual and near-IR light available from stars, moon and terrestrial lighting. The NV imagery obtained from the IITs is projected onto a pair of circular reflective areas on the helmet visor. Flight and tactical symbology–produced using a pair of standard, miniature, analog cathode ray tubes (CRTs)–is overlaid on the IIT images through a series of lenses and mirrors.

The Comanche

The U.S. Army’s RAH-66 Comanche, developed by manufacturing partners Sikorsky Aircraft and Boeing Co., represents a long-running effort to develop a next-generation reconnaissance/attack helicopter. It will employ an HMS/D developed by Rockwell Collins (Kaiser Electronics). Sikorsky selected Collins as the system provider in 1991, and the electronics firm has transitioned in that time from an analog, CRT-based design to a digital, active-matrix liquid crystal display (LCD)-based HMS/D–the first for any aircraft.

As of September, Collins was delivering 35 of these helmet integrated display/sight subsystems (HIDSS) for flight test in Comanche’s ongoing EMD phase. According to the current schedule, EMD is to conclude in FY2007, LRIP to commence in FY2007, and IOC to occur in FY2009. The Comanche helmet system presents both image-intensified night vision and infrared video–the latter from the navigation FLIR– relayed from externally mounted sensors.

Both helicopter programs have arrived at their current helmet systems after an initial design was either superseded or discarded. The Comanche program’s duration has allowed time to adopt digital technology. And integration issues with the H-1 program’s initial helmet system dictated moving to an alternative technology. At the time the decision was made to find an alternative Marine Corps system, "we weren’t given any relief in the flight test schedule," recalls Sheldon Freegard, H-1 human systems integration lead with NavAir’s Human Systems Department, supporting the H-1 Upgrades program office. The TopOwl HMS/D appeared to present less technical and schedule risk.


TopOwl projects the night scene and associated symbology onto two circular reflective surfaces with a fully overlapped, 40-degree, binocular field of view. With training, the pilots also can look through the visor to read moving maps and other information on head-down displays. There are five declutter modes, allowing the pilots to gradually reduce the amount of data overlaid on the display. The system’s true binocular presentation and the spreading of the image intensification tubes at a distance wider than the spacing of the pilot’s eyes help to give a "3-D feel" to the 2-D night vision imagery.

TopOwl has the ability to project FLIR and NV imagery to the visor, so that pilots and copilots can select either view, explains Phil Beck, program manager for the USMC H-1 program with Thales Avionics. Although the UH-1Y and AH-1Z helicopters are equipped with FLIRs, the program office chose not to project the IR video in order to reduce the risk to the EMD schedule, according to Maj. Brad Schieferdecker, NavAir’s deputy for EMD in the H-1 Upgrades program office.

TopOwl will duplicate all flight symbology, such as attitude, heading and airspeed, and all tactical symbology, such as cross hairs and weapons cursors. Symbols can be read clearly in full sunlight, says Beck. The contrast ratio is greater than 1.4-to-1.0 against 10,000 foot-lamberts, the equivalent of a full-daylight environment, he says. (The minimum NavAir contrast specification is 1.2-to-1.0.) The magnetic head tracker is accurate to within 3.8 milliradians or about 0.22 degree, according to Thales.

Enabling the pilot to see the night scene and overlaid information on a large, curved surface without distortion is no easy task, however. The optical path is pretty complex, Freegard says, and "it took quite a few years of engineering for [Thales] to pull it off." Thales "pre-warps the image in the optical path, so that when it displays on the inside of the curved visor, it appears to be straight."

The maximum head-supported weight for Marine Corps pilots will be about 4.85 pounds (2.19 kg), but can be reduced to 4.4 pounds (2 kg) in daylight operations by removing the helmet-mounted image intensifier tubes, explains Jacky Cloué, Thales’ manager of export sales.


The utility UH-1Ys will be armed with rockets only, while AH-1Zs will carry guns, rockets and missiles. To use the 20-mm gun, the pilot can superimpose the helmet system’s cross hairs on the target by looking at it and then manually fire the weapon. TopOwl also will display the information necessary to fly the aircraft into the appropriate orientation to use the rockets and missiles effectively. Head tracker position information is updated at a rate of 60 Hz.

The Marine Corps HMS/D, however, will not be used as a primary flight reference system, according to the NavAir program office. To certify the equipment for that capability would require more testing than the service has funding for. "We’d prefer pilots to go heads-down for IFR [instrument flight rules] operations," Freegard says. The helmet sight and display system is mission-critical but not flight-critical.

Comfort is another TopOwl attraction, Thales stresses. At the heart of the system is the "personalized liner concept," which tailors the helmet exactly to the pilot’s head. Manufactured according to laser-based measurements of the head, the liner allows the pilot to use 100 percent of the available field of view, Beck asserts.

Integration Challenge

After moving to the lower-risk Thales design, the Marines still had to integrate TopOwl, whose external footprint is bigger than its predecessor’s was. Each helmet comes with a quick-release connector module to bring in the voltages driving the CRTs. Each one also has a head tracking module, a set of backup batteries, and an electronics unit (EU). The EU draws symbols based on mission computer inputs and passes the information to the CRTs. The EU also processes the head tracker information and holds the "cockpit map," a magnetic picture of the cockpit that allows the avionics to know where the pilot is looking from moment to moment. Because the mission computer no longer handles many of these helmet-related operations, the processor’s tasks have been cut in half.

Software integration has been more straightforward, although symbology developed for the original helmet design had to be adapted to the new system. The software driving the HMDs has been delivered to the program’s two software integration labs and has been tested successfully.

The biggest integration challenge was physically shoehorning all the TopOwl hardware into the aircraft. The AH-1Z cheek cowl, for example, had to be expanded to mount the two EU boxes–one on each side–giving the aircraft "chubby cheeks."

The 6-week-long developmental flight tests for the Yankee and Zulu configurations are intended to prove that the helmet systems are safe to fly. The tests will look at possible latency–or processing delay–issues that may come up. The exercises probably will start with some basic traffic pattern work, followed by night flights to get used to the effects of "hyperstereopsis," the phenomenon associated with the placement of the optical sensors at a separation wider than the pilot’s eyes. The TopOwl design places the sensors 11.25 inches (28.6 cm) apart–center to center. This can be an "enhancing feature, with training," Freegard says. (Imagery from IITs spaced greater than 18 inches [46 cm] apart, however, becomes difficult for the brain to process.)

During the test period pilots will probably check out the weapons symbology and accuracy, short of actually firing the weapons. There are no flight simulators yet, but test pilots can use the software integration labs to familiarize themselves with the helmet system prior to flight. Seven military and four Bell test pilots, as part of an integrated NavAir/Bell test team, will conduct the developmental flight tests.


Unlike the Marines’ Hueys and SuperCobras–which use helmet-mounted NV sensors–the all-digital Comanche system displays FLIR and night vision video to the pilot that is electronically transmitted from external sensors. This means the helmet is relieved of the weight of image intensification tubes. More importantly, data from co-located visual light and IR sensors can be fused, enhancing image fidelity and improving night and adverse weather pilotage. The Army also plans to enable Comanche RAH-66 pilots to fly IFR with the HIDSS in the ultimate production configuration.

Going digital means eliminating high-cost, low-reliability items like deflection amplifiers, video amplifiers, and high-voltage power supplies, says Curt Casey, manager of business development with Rockwell Collins in San Jose. The Gen II FLIR provides a resolution of 960 lines per viewable display, compared with typical FLIR resolution of 875 lines. Raster graphics symbology is overlaid on the night imagery.

Collins’ earlier analog, proof-of-concept HMD required approximately 13,000 volts per CRT to drive each of the two image sources. To establish a high-voltage, quick disconnect, moreover, required an expensive connector weighing several pounds. The digital interface eliminates this component. Going digital also reduced the head-borne weight from 5.5 pounds (2.5 kg) to 4.2 pounds (1.9 kg).

The HIDSS display is referred to as "biocular," in that the two display surfaces (each with a separate optical path) show the same image. The AC magnetic head tracker is accurate to within 6 to 8 milliradians, with a 60-Hz update rate. The total field of view is 52 degrees.

Slimmed-Down Design

Compared with its predecessor, the AH-64 Apache helmet system, the HIDSS reduces the electronics volume by 50 percent and the total system power by 58 percent. The Collins display can be fitted on standard U.S. Army aviator helmets, another cost-saver, Casey says. The optical module, called the "aircraft retained unit," stays in the helicopter as a piece of aircraft equipment. The HIDSS will present cueing information associated with the FLIR and the fire control radar to the pilots. They can use the helmet to steer the aircraft’s machine gun and employ the Hellfire air-to-ground and Stinger air-to-air missiles.

The HIDSS’ light-emitting diode (LED) light source can generate 24,000 foot-lamberts, or twice the light on a clear sunny day, which compensates for the reduction in brightness caused by passing the light through the inefficient image source and optics. Some 1,300 foot-lamberts are presented to the pilot’s eye, which "is more than enough to be able to use this display in the brightest daylight," Casey avers. The array of approximately 24 miniature LEDs fits into the area of a postage stamp. The devices can offer a service life of about 100,000 hours.

The two LCDs provide 1280-by-1024-pixel resolution for both the FLIR and the night vision video. The response time for the LCDs is 9 milliseconds, which is almost as good as a CRT, Casey says. Collins developed a proprietary "warp" chip which compensates for the image distortion resulting from the complex optics and correlates the data with the images, yielding a total system latency of less than 20 milliseconds.

Receive the latest avionics news right to your inbox