ABSTRACT
The Honeywell Visual Target Acquisition Set utilizes a helmet mounted device to provide real-time data on a pilot's helmet position and thus line-of-sight. The practicability of the helmet sight concept has been demonstrated, but it has also been le:arned that a careful blending of VTAS requirements with those of the helmet as a protective device is necessary to achieve success.
This paper provides a basic description of how VTAS works, discusses helmet mounted unit design constraints, describes the experience gained to date with helmet mounted units, and shows how this was applied to develop the NADC VTAS III helmet. This lightweight, compact unit utilizes the latest materials and techniques available to integrate the helmet sight functions into the helmet itself.
Future efforts in this area including continued acquisition of flight test data and environmental testing are also described.

INTRODUCTION

What is VTAS?
The AN/AVG-8V Visual Target Acquisition Set is a head position referencing system. It is popularly referred to as a "helmet sight" system. It is used in the F-4B, J and N aircraft to determine the pilot's helmet position and thus his line of sight in aircraft coordinates. Once this information is known, a variety of tasks can be performed. In the case of the F-4, the aircraft radar or Sidewinder are selectively slaved to the pilot's line-of-sight for use in air-to-air combat. Wherever the pilot looks, the radar or Sidewinder "look". The pilot merely superimposes the collimated VTAS reticle image upon a target aircraft, actuates a trigger switch on the control stick and causes the radar or sidewinder to lock onto the target aircraft. He can then deal with the target using his conventional on-board fire control systems.

DISCUSSION

VTAS Helmet Development History
I considering a head position or line-of-sight reference system, one can imagine a variety of applications. Furthermore, it is reasonable to assume that some part of the reference system must be mounted upon the operator's head. The necessary elements include sensor assemblies to sense the orientation of the operator's head and some sort of optical sub-system that the operator can use as a reference to sight against, much as one would sight through a rifle scope or through a camera lens when taking a picture.
Helicopters were among the earliest applications planned for the head position referencing system. Here the designated operator, the pilot, was already encased in a protective helmet. The first approach taken was to fasten the reference system elements to the existing helmet. Figure I shows an early version of a "helmet mounted unit". Light sensitive silicon diodes were mounted in pairs on each side of the unit to act as sensors and the optical sub-system or sight piece mechanism consisted of a glass cube beam-splitter mounted in a plastic paddle. The additional weight, size and effect on centre of gravity are obvious.

In the case of VTAS, the designated operator was the pilot of a high performance aircraft and he was also already encased in a protective helmet (and oxygen mask). Figure 2 shows a later version of a helicopter helmet mounted unit that was used for some of the first flight tests of a Honeywell helmet sight system in the F-4. Although it is more compact than the unit of Figure 1, it was still too large for use in a confined area such as the F-4 cockpit with closed canopy. The protrusion of the "front porch" prevented the pilot from getting close to the canopy to look out or look back. Furthermore, in the high "g" manoeuvring environment of the F4, the added weight and forward shift of C.G. were totally unacceptable.

The first production version of the VTAS helmet mounted unit is shown in Figure 3. The sensor assemblies have been separated from the sight piece mechanism and are attached by screws to each side of the helmet. The sight piece, colloquially referred to as a "granny glass", stows inside the visor. It is deployed and stowed using a slide knob located on the helmet shell.
Although pilots are generally enthusiastic about VTAS - especially those with combat experience - the "granny glass" unit met with only mixed success. Any forward protrusion had been eliminated and weight and balance improved to the point of acceptability by lower and further aft mounting of the sensor assemblies. But during the flight test phase of the program, a considerable amount of helmet slippage had been found to exist using the conventional adhesive backed fitting pads. The Crew Systems Department of the Naval Air Systems Command specified the use of a form-fit liner in a special lightweight helmet shell to minimise the amount of slippage. Even with these improvements, some pilots complained of "losing the reticle". The exit pupil of the sight piece mechanism was too small. Even the slight shifting of the helmet under "g" loading caused the sight piece combiner glass to shift in front of the pilot's eye thus causing him to lose sight of the reticle image. In addition to this, some pilots were distracted by having something in front of one eye.
To eliminate these disadvantages, the Honeywell Corporation completed the development of the Visor Reticle Helmet Mounted Unit shown in Figure 4. In this unit, the sight piece mechanism is replaced by a parabolic visor and reticle generator assembly. There is no distracting combiner glass or support stem. The reticle image merely appears before the pilot's right eye when the system is energised. The sensor assemblies have been re-packaged and moved in closer to the helmet shell. All of the elements are contained in a one piece housing. This unit, in sizes medium and large, is presently being flown in the fleet.

How does VTAS work?
To describe the operation of VTAS, a discussion of the reticle image provided to the operator is necessary. The helmet mounted unit generates a collimated virtual reticle image. A collimated image is defined here as one that appears to be located an infinite distance away. The rays of light defining such an image are to all intents and purposes parallel. A collimated image has a definite "pointing direction." When a pilot superimposes the collimated reticle image on a target aircraft, he is very definitely looking exactly in that direction. In contrast to this, if the reticle image were focused at some discrete distance (such as 10 or 20 feet), it would not appear in sharp focus when superimposed on a distant target and would not have a definite pointing direction.
If the helmet were to shift slightly on the pilot's head due to manoeuvring for example, his eye would bear a different relationship to the uncollimated image and an error would be introduced into the system because of the lack of a definite pointing direction. A collimated image appears fixed against a distant target (as if it were fastened to the target). The uncollimated image moves against a distant target; the movement being determined by the viewer's eye relationship to the image generator.
In the case of the parabolic visor, the reticle image generator is placed at the focus of the paraboloid of revolution. All rays of light emitted at the focus will be parallel to one another after being reflected off the parabolic surface. Therefore, the rays of light reaching the pilot's eye create a virtual, collimated image of the reticle pattern.
All of the different versions of helmet mounted unit have sensor pairs mounted on each side of the helmet. These sensors represent two points that in turn define a straight line. The "pointing direction" of the reticle image is aligned parallel to the two straight lines defined by the sensor pairs on each side of the helmet. Therefore, whenever the reticle image is superimposed on a target, the sensors are aligned in that direction also. This information is converted into aircraft coordinates by individually surveying each sensor and determining its position in space. Fans of infrared light generated by rotating mirrors located in a device called a Sensor Surveying Unit pass over each sensor which causes the sensor to deliver a digital pulse to the general purpose digital computer (See Figure 5). The computer in turn resolves these pulses into azimuth and elevation angles in aircraft coordinates.
In the case of the F-4, there are two Sensor Surveying Units mounted at an angle on the canopy rail just behind the pilot's shoulders. When the pilot looks out the port side of the aircraft, the computer accepts data only from the port SSU and the sensor pair on the left side of the helmet. When the pilot looks to starboard, the reverse is true.
The requirements of the aircraft installation dictate the location of the sensor pairs on the helmet shell. To eliminate the interference with the pilot as he moves about in the cockpit and to stay out of his vision area, the SSU's must be mounted close to the canopy rail. If the sensors are mounted any lower on the helmet shell, the pilot's shoulder would block the fans of infrared light surveying the sensors. If the sensors were mounted higher on the helmet shell, they would be further from the pilot's tragion, would translate a greater distance as the pilot turns his head and thus decrease the angular coverage available in what is known as the "head motion box". The head motion box describes the limits through which the pilot may move his head and still transmit data on line-of-sight.

VTAS and the Fleet
The VTAS I HMU came on the scene at the same time that the U.S. Navy was uncovering a series of problems related to helmet weight and bulk. The added weight and bulk caused by the helmet mounted components tended to emphasise these problems. In some ways the helmet sight aggravated the existing helmet deficiencies. Some pilots had not realised they had a helmet slippage problem but the shifting reticle image provided visible evidence.
To combat these problems, the Navy revised its design philosophy regarding helmets and applications. The previous design philosophy had been to use the APH-6 series helmet, to which emphasis was placed on impact and penetration protection, as a standard helmet for all missions; fighter, attack, helicopter and ASW. The present program is to develop a "mission specific" design approach. The helmet will be designed around the mission rather than having the mission ride "piggyback" on an existing helmet (1).
As mentioned above, as an interim step, the Crew Systems Department specified the use of a form-fit liner in a special lightweight helmet shell for VTAS equipped helmets. This change, in conjunction with the VTAS II Visor Reticle Helmet Mounted Unit has produced an assembly that is acceptable for fleet use today. For the future however, a fighter mission helmet has been specifically designed for air combat manoeuvre with low weight and profile, maximum visibility, limited impact protection, a minimal weight and profile helmet integrated oxygen mask and integrated VTAS electronics. (See Figure 6.)

In response to the Naval Air Systems Command, the Naval Air Development Center, Warminster, Pennsylvania directed the Honeywell Corporation in the integration of the VTAS helmet mounted components into the NADC designed HGU-35/P helmet. The resulting NADC VTAS III helmet is based on fleet experience with VTAS and increased knowledge of helmet sight systems. These factors were combined with the results of parallel investigations of changing helmet requirements and a survey of the latest materials and techniques available to produce a lightweight, compact helmet that is fully in consonance with the Navy's current design philosophy of developing mission specific helmets.

Description of the VTAS III Helmet
The VTAS III helmet shell is constructed of Kevlar-49 cloth impregnated with epoxy resin. The crown of the helmet shell contains a layer of 1/8 inch Nomex honeycomb. The shells were fabricated, trimmed and painted by the JMR Corporation of Salem, New Hampshire. The design of the shell is based on the NADC designed HGU-35/P helmet developed and fabricated by the TME Corporation of Salem, New Hampshire. The profile of the TME shell is basically that of the APH-6 with modifications below the crown to provide much better visibility, much greater earcup adjusting range and (to some observers) a more pleasing contour without earcup-area bulges. The visibility improvement is achieved by cutting back the APH-6 front-edge contour by about ½ inch all around. The total height of the shell is identical to that of the APH-6; total width is 1/2 inch less (2). The major modifications to the basic design lie in the addition of "bumps" for the VTAS sensors and electronics, new tracks and glides for the parabolic visor and a new visor housing also fabricated of Kevlar-49 cloth.
The helmet communications were provided by the TME Corporation. They consist of a modified version of the NADC developed NAACH (Non-Acoustic Audio Coupling to the Head) earcup mounted skin contact microphone in conjunction with miniature Roanwell earphone units.
The integral oxygen system features a new lightweight low profile mask, an .internal oxygen duct through the helmet, and an external hose with communications wires connecting to the helmet at the rear. The NADC designed mask was developed and supplied by Carleton Controls Corporation (2).
The mould-in-place liner system was provided by the V-TEC Corporation of Hopewell, Virginia. It makes use of a one piece, leather covered hollow fibreglass shell which actually receives the foam while in a fixture on the pilot's head. It is then placed inside the helmet shell (3). It is the closest system yet to a custom fitted, foamed-in-place, walk-away-with-the-product liner.

CONCLUSIONS

Comparison of Statistics
Width - A measurement across the sensors is by far the best index to bulkiness of VTAS helmets for use in high performance aircraft. Extra width not only prevents the pilot from getting close to the canopy but also increases the moment of inertia of the entire assembly which shows up under high "g" loading, high roll rates and/or rapid head movements by the pilot.
Weight - The best way to visualise the overriding importance of helmet weight is to remember that an APH-6 helmet/mask combination that weighs over five pounds during straight and level flight weighs over forty pounds in an 8 "g" turn.
Table 1 shows the significant improvement in these two key factors in the design of the NADC VTAS III helmet. Table I shows no weight reduction in the transition from VTAS I to VTAS II. This is so because the slight reduction that resulted from the repackaging of the sensor electronics was balanced by a slight increase in the weight of the one piece housing and the heavier (than standard) visor tracks for the parabolic visor. However, a significant reduction in width was achieved.

Future Developments
The first VTAS III helmet is size medium. A large size version will be developed under NADC direction in the near future for use in flight tests where several pilots (and thus head sizes) are involved. Meanwhile the existing VTAS III helmets will continue to be subjected to a battery of environmental and actual flight tests in a current program directed by NADC.

REFERENCES

1. Snyder, R. Z., The Navy Fighter Pilot Helmet Oxygen Mask Improvement Program, 75-ENAs-19, Intersociety Conference on Environmental Systems, San Francisco, California, 1975.

2. Jagoe, W. M. and Radzelovage, W., Development of an Air Combat Maneuver Helmet System, SAFE JOURNAL, Spring, 1974.

3. Lamb, M. J. and DeSimone, D. N., U. S. Navy Development of a Mission Specific Fighter Helmet, PROCEEDINGS OF THE ELEVENTH ANNUAL SAFE SYMPOSIUM, 1973.

BIOGRAPHIES

Mr. Hedges is a Principal Development Engineer in the Government and Aeronautical Products Division of Honeywell Inc. He has produced many designs for helmet mounted sights and displays since his assignment to the Fire Control Section in 1968. Prior to this, he functioned in a series of administrative positions and aircraft instrument design assignments. Mr. Hedges received his degree of Bachelor of Mechanical Engineering from the University of Minnesota. He is the holder of four patents.