Human Factors in the Design of Tactical Display Systems for the Individual Soldier (1995)

This chapter provides a context for our initial discussion of the human factors considerations related to the communications subsystem of 21 CLW. It begins with an overview of the military environment in which he proposed new system and subsystems are expected to be used. Emphasis is given to the idea that the conditions under which military missions are or are likely to be conducted are increasingly varied with respect to both the physical conditions and the task conditions. Task conditions are described for war, anti-terrorist operations, catastrophe relief, and peace-keeping. Each of these variations could require different equipment configurations to achieve optimum soldier performance. Finally, we examine the sources of environmental stress and their implications for helmet-mounted displays.

The combat measures of performance for military units, including infantry squads and platoons, are the abilities of the soldiers and the unit to shoot, move, communicate, and survive. Advanced weapons systems and new technologies offer the possibility of increased individual and unit performance. Advanced weapons technology linked with rapidly unfolding information technology holds a promise of increased speed, accuracy, and lethality for the Land Warrior of “Force XXI” (see Army Focus, September 1994). These technological advances could make soldiers and their units significantly more effective than they are today. However, realizing this potential can be achieved only if the new technology helps reduce a soldier's fear of going into combat, reduces the uncertainty of success in combat, and makes it physically less demanding for soldiers and their units to achieve mission success. The land warriors of tomorrow, equipped with systems with sophisticated capabilities, will be more formidable fighters only if the new systems build their individual confidence and aid the confidence and cohesion of their units.

The mission of the infantry soldier in a war-fighting environment in the twenty-first century will not be dramatically different than that of today. In a war-fighting sense,

tomorrow's land warrior will still be required “to close with and destroy the enemy by means of fire and maneuver to defeat or capture him, or to repel his assault by fire, close combat, and counterattack” (U.S. Department of the Army, 1992). However, the role of tomorrow's land warrior will be changed in significant ways. Infantry soldiers have recently been committed to a series of less than combat employments in Somalia and Haiti. The infantry soldiers of tomorrow could be required to react to an even broader array of noncombat or not-active combat requirements, such as peace enforcement, stabilization and civil support missions, counterinsurgency and counter terrorism, humanitarian relief in a potentially hostile environment, and political interventions. Changing international power balances may increase the frequency of low-intensity conflicts, bring about increased counter-terrorism actions, and expand the range of peacekeeping tasks.

In these settings, rapidly changing danger situations, avoidance of civilian casualties, restrictive but variable rules of engagement, and widely dispersed small units place increased demands on effective communications, accurate position reporting, and detailed intelligence gathering. In general, such roles place soldiers and small units in an environment where the enemy threat is often unclear, while unit dispersions are potentially much greater, and the appropriate response and battle actions are more uncertain. Such changes add to the task of battle hardening soldiers and their units to deal with the uncertainty and fear of combat.

Since the end of World War II, the infantry, like the entire Army, has been required to be prepared to respond to a broad range of levels of combat. Infantry soldiers have been trained to meet the challenge of high- to mid- intensity conflict in Europe, the Middle East, and Korea and low-intensity conflict in third-world countries around the world. For the past 40 years, the Army has been guided by the presence of a clear threat that led to prescribed doctrine and to weapon systems and organizations driven by threat capabilities.

The end of the cold war has brought fundamental changes to the nature of the threats to U.S. security and to the approach to doctrine and organizations. Based on threat changes, advances in technology, new weapons, and digital information systems, the infantry soldier of the future may see significant changes in basic unit organization at squad, platoon, and higher levels. The requirement for new, flexible Army doctrine has been clearly presented (see U.S. Department of the Army Force XXI Operations, 1994). This first look into the future describes the new battle dynamics of the Force XXI with new dimensions in battle command, battle space, depth and simultaneous attack, and early entry. These dynamics project decreases in the number of soldiers committed to a mission, decreases in mission duration, increases in individual and unit dispersion, extended engagement ranges, increased speed of maneuver, and more complex combat maneuvers. Increased distances and faster tempos add emphasis to the need to prepare individual soldiers and small units to overcome the fear and uncertainty caused by isolation and rapidly occurring battlefield events.

All of these potential changes to the nature of the infantry soldiers ' battlefield reinforce the requirements for increased soldier performance and for the appropriate use of new technologies. These requirements include accurate and almost instantaneous information about the position of friendly and enemy elements; more effective communications, down to and within the infantry squad; more accurate and longer range day and night engagement systems; and increased soldier protection. All of this must be achieved without significantly separating a soldier from the physical environment. It must also be achieved without placing on soldiers unmanageable cognitive demands that shorten the duration times allowed for individual performance, retard individual response time, or are physically debilitating.

Other changes will also affect the nature of combat for tomorrow's infantry. The end of forward stationing of U.S forces on overseas bases adds greater importance to deployability and sustainability. Lower budgets reduce the level of infantry forces; they also reduce training dollars and increase the importance of added soldier effectiveness. These same constraints add emphasis to requirements for joint and combined operations and the need to have U.S. infantry soldiers trained, equipped, and able to interface flexibly with a wide range of organizations and systems. These requirements add to the number of infantry tasks for which soldiers need to be trained to maintain a level of readiness and as well, to the range of environmental conditions under which missions are performed. The world of the infantry soldier and the future land warrior will continue to be a muddy, dry, wet, icy cold, desert--and always unfriendly--environment.

In addition to all of these challenges, it is most likely that a greater number of future infantry soldiers will be part of the National Guard or Army Reserve force structure. This sharply reduces the time available for training, since Guard and Reserve soldiers have significantly less time available to train and maintain readiness. All of these changes add new dimensions to the problems of maintaining individual and small unit combat readiness: demands for developing increased physical and psychological readiness, increased numbers of training tasks while faced with reduced training time, and increases in the number of technically complex weapons systems and equipment.

The complex technology and pervasive impact of the proposed Land Warrior System on infantry soldiers raises a number of basic doctrine questions about the autonomy of an individual soldier. Is it a design objective to give tomorrow's land warriors information that could allow them to make more independent decisions about their individual tactical actions? At the system design level, this issue translates into questions about control prerogatives. For example, should an individual soldier decide when a display will be used or moved out of view? If so, should an individual soldier be able to choose the kind of information being presented? For example, should soldiers be able to call up the latest map overlay whenever they want to see it? The concept of a helmet-mounted display with a flexible set of modes or

displays that allow soldiers to view different levels of information presentations has significant potential for cognitive distraction. (The negative and positive effects that multimodal displays can have on immediate individual situation awareness is addressed below.)

Other questions concern the nature of two-way communications in the vertical dimension. Information acquired by an individual land warrior from advanced sensor systems (i.e., image intensifier, infrared, video camera, lasers, etc.) is intended for use at battalion and higher levels. Should such information be transmitted directly to higher levels or through each level of the chain of command? Is there an operational tradeoff between delays in transmission and the possibility of misunderstanding the message at higher levels of misunderstanding the operational situation if intermediate levels do not process the information? Historically, in demanding combat situations, low-level leaders could and would reduce the flow of higher level instructions and orders by ignoring or not answering transmissions. Will the land warrior communications system cause even greater potential for selective avoidance of communications? How will communications security be addressed? What happens if a Land Warrior System falls into enemy hands? How will intelligence updates be processed down to the soldier level? Other questions concern the amount or level of interaction among squad members to correct or update visual data and set priorities for information. While procedures can be developed to address these questions, the most significant issue is whether the Land Warrior System will create a situation of task and information overload for individual soldiers and lower level leaders. The issue is information management. Is there a team-based approach to information management and if so how is the information distributed among squad members?

The essential human performance measures for tomorrow's land warrior are not different from those of today's infantry soldier--speed and accuracy. The time it takes a soldier to execute, with precision, a critical combat task is the measure of battlefield success and survival. In combat situations, soldiers ' survival depends on their ability to rapidly detect, identify, and successfully engage a hostile enemy before the enemy can successfully engage them or their units. Detection and identification rely on the ability to perceive obstacles in the environment, the layout of the terrain, and the location of the present site within the more general situation. For small unit combat actions, this infantry capability is supported by at least three other essential capabilities: accuracy by unit leaders in knowing and reporting their unit's location; the accuracy and timeliness with which unit leaders know the location of friendly units and elements within the range of the weapon systems they control; and the ability to report enemy locations accurately, rapidly, and stealthily.

The infantry rifle platoons and squads of today, and probably of tomorrow, must be prepared to conduct three basic tactical operations: movement, offense, and defense (U.S. Department of the Army Field Manual 7-8, 1992). All of these operations are conducted day and night, in conditions from desert heat to winter snows, and in terrain that varies from flatlands to swamps to mountains. Under wartime conditions, all infantry missions are

planned for execution on a 24-hour-a-day basis. This places great stress on the need for endurance and physical conditioning.

Battle success requires the successful execution of a series of critical individual, collective, and leader tasks. Many of these critical infantry tasks overlap and recur many times across all squad and platoon missions. Table 1-1 highlights the key battle tasks for an individual infantry soldier, an infantry team or squad leader, and an infantry platoon leader. The relative importance of the tasks will differ, depending on the mission and the level of intensity of combat. For example, the time available for planning varies on the basis of the mission and whether it is a mid- or high-intensity operation or a low-intensity operation or operation other than war. In planning for a move in a mid-intensity environment, there may be days available, but in a hasty attack in the same environment there may be only minutes. In an operation other than war, a squad leader with a mission to defend a key area and control elements of the civilian population would be very focused on the rule of engagement, identification of potential threats, and ensuring appropriate use of force; in contrast, in a mid-intensity defend mission, the leader would be concerned with planning fires, controlling direct fires, and rapid response with all and any firepower available.

To understand the human factors effects of the proposed Land Warrior System with its helmet-mounted displays, it is necessary to correlate the critical tasks that infantry soldiers and leaders must accomplish and the functional capabilities of the proposed system. Table 1-2 is a matrix of Land Warrior System components associated with their expected battlefield enhancements (lethality, tempo, survivability, and mobility) and with key tasks performed within a rifle platoon by infantry soldiers, squad leaders, and platoon leaders. The table does not indicate whether the component aids a soldier or leader in accomplishing a task but only whether the component potentially affects the task.

The tasks of the infantry soldiers and their units may appear routine and simple when viewed in Table 1-2, but they are difficult because of the need for precise execution by a soldier (with varying cognitive, physical, and general skill levels) and for swift coordinated execution by an infantry unit under battlefield conditions. For example, during a small unit tactical movement, if contact with the enemy is made, accurate and responsive reporting of all information is essential in order to rapidly generate combat power at the point of enemy contact.

The first few seconds after enemy contact determines the fate of the maneuvering fire team. On enemy contact, soldiers might immediately drop to the ground, roll for cover, and return rapid fire in an attempt to suppress the enemy fire, or they may be required to rapidly move out of the kill zone and seek cover. In an instant, soldiers wearing a helmet-mounted display would have to execute a broad range of cognitive processing and physical activity: detect and analyze the threat, its location, and effect; drop and roll and return fire or run at a sprint; or drop to cover and call for fires on the enemy position and, perhaps, return direct fire on the location. If at that moment the Land Warrior System display interferes with the soldiers' local situational awareness or hinders their physical movements, all the other Land Warrior System capabilities may not matter.

The first minutes after an initial enemy contact determine whether the squad and platoon will be successful in maintaining contact by rapidly increasing rates of fire and executing squad maneuvers to fix the enemy. Knowing friendly positions and the location of the enemy accurately is essential if requests for fire support are to be timely and effective. In the future, because of the potential for increased dispersion and more rapid movement, it will be more imperative and time sensitive for higher level command and control nodes to know the exact location of all friendly forces in order to provide responsive coordinated support and at the same time avoid friendly fire casualties.

During the conduct of an attack, the physical and neurological responses of a soldier could be critical to his survival. Soldiers already have an almost overburdening load of equipment. If the added weight of Land Warrior System components slow them or distract their attention from the local surround, even unintentionally, the outcome could be negative. In an attack, knowing the precise location of friendly forces and the identification, size, and location of enemy forces is critical, but as in the movement example, the immediate initial actions of soldiers may hold the key to their survival and a unit 's success.

The design of the helmet-mounted display must be compatible with the physical responses a soldier has to make and, to be effective, must account for the psychological and neurological differences and capabilities of different soldiers. A flexible display capability with a selection of modes of informational display and levels of detail may not be fully compatible with the capabilities of a given soldier or with the actions required in battle action. An illustration of a negative impact is the disorientation that a soldier would experience if he were moving at night wearing a helmet-mounted display in a night vision mode with a field of vision of 60 degrees when suddenly his unit is engaged. He drops to the ground and rolls to cover, switches to night thermal sight with a field of vision of 9 to 15 degrees, and attempts to determine the direction of enemy fire of which he is totally unsure. At the same time, the impact of a bright flare shuts down his night vision devices, and the soldier may be disoriented to the point of not being able to make a response.

At the squad platoon level in an attack, the situation may be different. The critical tasks that the helmet-mounted display supports include controlling direct fires, maneuver, and coordinating indirect fire support. A successful squad attack requires a squad leader to rapidly adjust the direct fires of his unit; accurately report the situation, including the

location of his unit; make timely calls of fire support; and control the maneuver of his squad. All of these actions must occur under enemy fire and amidst confusion and uncertainty.

Defensive operations may have a different set of effects on the design of the Land Warrior System. A defense can be characterized by periods of intense preparation, followed by irregular periods of boredom and observation waiting for enemy activity, and then the overwhelming impact of an enemy closing in on the defensive position. Preparation for defense is an intense day-night physically demanding, individual and team effort. After intense preparation, leaders and soldiers are often physically and psychologically stretched. The impact of the helmet-mounted display can be a combat multiplier if the design does not cause a false sense of capability or a lack of local situational awareness that leads to ineffective soldier responses. Using a helmet-mounted display that has a selection of modes means that the leaders and soldiers must have a way of determining what information is needed and that everyone has the correct situational awareness at the same critical time. In defense, it also means designing a system that allows for the individual capabilities and differences of the soldiers using it. In defense, leaders fight by ranging targets, setting priorities for targets--what and when to fire--controlling fires to destroy the nearest or most dangerous, and shifting direct, indirect, and indirect supporting fires. These actions require constant communications and rapid information flow with accurate reports when soldiers and leaders may not be at peak physical and psychological levels. The design of the Land Warrior System and the helmet-mounted display have to meet these challenges to achieve their expected potential.

Any given land warrior task may not be particularly demanding in a benign environment. However, the collective squad or platoon tasks associated with shooting, moving, and communicating are performed under conditions of great stress. The sources of stress include the nature of the mission, resource availability, perceived risk, information uncertainty, physical demands, fatigue, time, environmental conditions associated with weather and terrain, and surprise. As a result of these stresses, a soldier's state of arousal varies from prolonged boredom to short periods of stark terror. Much of the cognitive workload results not only from the tasks to be performed, but also from the standards to be achieved and conditions under which the tasks have to be performed. The engineering psychology design challenge is to avoid unanticipated negative consequences that will impair confidence, cohesion, commitment, and communication. Losing or degrading the quality of information a soldier is currently receiving under daylight conditions and which the soldier perceives to be critical could affect how and under what conditions the soldier will use the system.

Success in combat requires that infantry soldiers control their fear and behave in a predictable manner, no matter how tired they are or how uncertain the situation is. In order

to elicit this behavior, commanders attempt to build high-performing, cohesive, and confident units by clearly defining roles and jobs, keeping soldiers informed, conducting realistic and demanding individual and group training, and providing strong leadership and competent supervision. The introduction of new technologies and capabilities adds, deletes, or modifies tasks; affects how tasks are performed; changes job requirements; enhances or degrades confidence and cohesion; changes training requirements; and affects the acquisition and retention of personnel. The Land Warrior System is not an exception: it will affect how leaders define soldiers' roles and jobs and how they communicate with and train and lead their units.

Military commanders have known for centuries that fear is easier to cope with in groups than in isolation. Building confidence at the individual and unit level is essential for overcoming fear. As a result, “buddy” systems have existed for years. Extending the buddy system concept through the fire team, squad, platoon, and company levels builds unit cohesion and instills trust and interdependence in a larger group. Technology can facilitate or hinder unit cohesion, depending on how it is designed and used. Communication technology allows soldiers to feel a part of a unit despite geographical separation. In contrast, a display that reduces vision in daylight, induces motion sickness, increases workload and reading time, causes discomfort, or requires constant adjustment will affect individual confidence and unit cohesion.

Individual and collective training is conducted to improve confidence, overcome fear, and enhance performance reliability. With sufficient training, soldiers know that they can execute assigned tasks under all conditions and achieve or exceed the expected standards. Training helps the soldiers gain confidence in their roles within the unit for any specified mission. The military structure assists in establishing, maintaining, and reinforcing clearly defined roles. Role clarity fosters interdependence and trust as well as confidence. The reporting procedures associated with new technologies can undermine this structure and erode soldier confidence in unit leaders if it is used to bypass the structure.

Radios have been used reliably at the platoon and higher level and for years. Although platoon radio nets and squad radios have existed, these have not always been reliable, and squad members have not had radios. As a result, communication below the platoon level has historically relied on visual or auditory contact with another member of the unit. But hedge rows, dense jungles, mountainous terrain, and urban environments make it difficult to maintain visual and auditory contact. The effects of poor communication on maintaining a cohesive fighting force, avoiding fratricide, and shortening battle execution time are obvious.

Communications in the military usually include orders and instructions flowing from the headquarters to soldiers, and information about the battle situation flowing from small unit leaders to the headquarters. As communication technology has improved, so has the demand for information at the headquarters level. When these demands from headquarters are not synchronized with the work ongoing in units that are engaged in battle, the communication process itself can reduce battle effectiveness by diverting leader attention and

by adding stress to an already overstressed situation. Personnel who are overworked will shed tasks until they achieve a level they can manage. Reporting takes time and can be a major workload factor; technology should be used to reduce this workload, not to increase it.

The natural human tendency is to want more information, not less. Each information requirement should be questioned as to who needs it, when they need it, under what conditions they need it, and how they will use it. More information frequently leads to more complex screen designs, navigation menus, function keys, and other control mechanisms. Ultimately this can further contribute to reduced situational awareness, with the unintended consequence of decreased survivability.

General Bruce C. Clarke (1959) stated: “The truth is that the most expensive weapon that technology can produce is worth not an iota more than the skill and will of the man who uses it.” The engineering design challenge is to use technology to improve a soldier's will and skill. History is replete with examples of technological advances that were introduced into the military before they were introduced into society in general (Guilmartin and Jacobowitz, 1984).

Both physical and psychological conditions influence the way in which a helmet-mounted display is used and the efficacy of such use in an operational environment. As noted above, infantry missions are conducted day and night, in all weather conditions and in all terrains and under threat conditions ranging from civil disobedience to high intensity war. These factors place great stress on the individual soldier. In this section we look at various forms of stress as external conditions that could amplify or diminish the utility of a helmet-mounted display. These factors include threat, heat, cold, noise, vibration, and fatigue.

The threat stress of military engagements can be extreme. When under intense stress, the basic individual response is a narrowing of the range of cue utilization. It appears that the stressed person limits his or her acceptance of information to only a few categories of content and only a few symbol sets or formats perceived to be central to task performance (Easterbrook, 1959). Such narrowing is an attentional phenomenon. Given this effect, the design of displays must vary according to contexts. This variation is achieved by software manipulation (Wickens et al., 1993). The same can be said for what can be called inactivity or underload stress--the boredom or inattention that is the inverse of threat stress. Loss of vigilance is the consequence of this form of stress as illustrated by the events in the case of the U.S.S. Stark, when the operator simply overlooked the clear indication of an inbound missile.

Hancock and Warm (1989) present an approach to the prediction of stress effects by means of a model that establishes ranges of adaptability. Violation of these ranges of adaptability causes progressive failure in performance. As the model uses both a physiological and a behavioral basis, it can deal with the question of combined physical and mental demands; it should be used in association with the testing of advanced technologies associated with 21 CLW.

Heat is one of the more ubiquitous and disruptive forms of environmental stress that is encountered. Recent operations of the U.S. forces in Grenada, Haiti, and the Arabian Gulf have had to face the problem of heat stress. However, heat stress is not only a function of the geographical location of operations. In any encounter in which chemical or biological weapons are suspected to be in use, military personnel wear protective clothing and equipment that generate considerable heat on the wearer. On occasion, such heat load has been high enough to cause missions to be canceled or mission goals to be changed.

For researchers, it is natural to focus directly on the complex effects of heat on the functioning of the central nervous system. This orientation is understandable given that heat stress investigations are often parts of a more general search for stress effects upon human performance (Vercruyssen et al., 1988). However, in the military context, there are certain physical disruptions from heat that have to be considered first. People under heat stress sweat, and sweat is a problem for the use of any visual display since profuse sweating has a significant impact on visual capabilities including visual acuity. In addition, the presence of sweat means that soldiers brushing sweat from their eyes may miss information solely from this mechanical problem and not from some more complex effect on cognitive activity.

For a soldier wearing a head-mounted display, sweat is particularly problematic. A large percentage of heat load from the human body is lost from the head and particularly from the front of the face. Covering this area with an exclusion display system may generate heat-related problems. Heat loss also comes from vapor exchange in breathing. Displays that cover the mouth and nose can lead to extensive fogging problems, as experienced, for example, in space suit operations. Leaving half the face uncovered might seem a solution, but it ignores mandates about full protective masks for toxicological warfare. Sweat is also going to present a problem with respect to fit of equipment. Injecting a fluid medium into the interface between the display unit and a soldier's face can have significant short- and long-term problems--slippage and the maintenance of complex electrical equipment that is exposed to water and secreted salts on a regular basis. In addition to the concerns of geographical heat stress and garment heat stress, prolonged wearing of a system over the eyes may cause thermal balance problems. Of necessity, any proposed displays need extensive field testing, including system integration with existing equipment ensembles. Solving the problem of localized and general heat stress is thus one important step in realizing the goals of 21 CLW.

Human tolerance to heat stress where nontrivial physical load is expected has been included in regulations issued by the National Institute of Occupational Safety and Health and is further described in Vercruyssen et al. (1988). The latter also provides information with respect to body temperature and performance and on procedures that can be used to evaluate the combined effects of physical and mental demands.

While heat presents a number of unique problems, cold also provides considerable contextual challenge for advanced systems. Paradoxically, many heat-related effects, such as sweating and fogging, also apply in the cold, since soldiers are covered in heavy clothing to resist cold effects; they are usually “overclothed” to provide protection against extended exposure, on an emergency basis, should it occur. The physical form of disturbance in cold exposure is shivering. Shivering is a process designed to spend stored energy solely to heat the body: the oscillating motion experienced is not directed to behavioral goals and, in fact, directly interferes with them. Hence, the central problem of shivering is mechanical interference. Traditionally, the research literature has looked at how this interference debilitates psychomotor performance, such as pursuit tracking; however, there are obvious effects on perception as well. Although fewer studies have examined this facet of performance, it is clear that some degradation is to be expected (Hancock, 1984; Boff and Lincoln, 1988).

Unfortunately, by the time the individual is shivering there is a direct indication of incipient physiological problems. Although individuals can still perform during approaching physiological failure, they are not as invulnerable as the old concept of “physiological adequacy ” implies, and by the time shivering becomes a mechanical problem it has already begun to affect central functions such as speed of response and decision-making (Hancock, 1989). Consequently, cold effects can become as much of a contextual challenge as heat effects, although they are somewhat less likely to be encountered.

Noise is also a form of stress. Because of the lack of a direct link to a single physiological paradigm, however, the concurrent and aftereffects are less well understood than those of temperature, for example. Noise effects are much more diverse and difficult to localize than other stresses. Unless noise drops below approximately 30Hz, the effects are mainly aural: that is, hearing is partially masked. However, even relatively low-intensity sound at very low frequencies can cause problems analogous to mechanical vibration.

One obvious concern in physical ergonomics is the provision of auditory messages against a noisy background. In the true sense, this is a signal-to-noise problem. At the risk of visual overload, it might be reasonable to suggest the provision of multimodal forms of information transmission when the context dictates that soldiers are going to be exposed to battle noise (Wickens, 1987).

In any battle, noise stress will be present. Furthermore, noise has persistent aftereffects, which have to be considered when trying to integrate developing technologies. It might well be that head-mounted systems are specifically designed to insulate a soldier from excessive ambient noise and associated shock effects. However, low-level vital auditory cues for situation awareness might be present, which the soldier needs to assimilate to survive. Again, a design of flexibility and choice of configuration, that is, an adaptive system, seems desirable.

While noise is primarily airborne vibration (it can also travel through liquids and solids), one typically thinks of vibration as motion transmitted by some solid substance. Vibration effects are very similar to shivering, with one critical exception: the frequency of vibration can vary over a considerable range. In the 3-6 Hz band, the resonance frequency of the major organs of the body, vibration is exceptionally disruptive, to the point of nausea and beyond. Vibration might come from vehicle operations such as helicopter or armored personnel vehicles. These effects, like shivering, are liable to affect both perception and action. Individuals also vibrate as a function of locomotion. Usually, this vibration is masked by internal compensatory mechanisms, so that people do not see the world oscillate as they walk. With head-mounted displays, however, there are questions about suppression of this intrinsic form of vibration, and much empirical research needs to be done on working while walking (see Sampson, 1993). Such research is critical since it lies at the heart of the connection between the physical characteristics of the battlefield and the design of the information systems. Quantitative limits in vibration for visual capability, are presented by Boff and Lincoln (1988) who discuss a wide coverage of such effects across differing sources of stress.

Fatigue is often agreed to be a critical facet of performance that can vitally effect mission success particularly in prolonged operations, yet major research efforts by the armed services and others have not led to real understanding. Defined recently as the “subjectively experienced disinclination to continue performing the task at hand, ” attempts to define fatigue have each had to fight hard against tautology. In large part, these attempts have not been successful. In a classic work, Muscio (1921) suggested the term fatigue be abandoned; his

suggestion was not adopted. This is probably because almost everyone has experienced fatigue and recognizes it as it begins and progresses. Also, one cannot abandon a study of such an important operational topic.

Currently, fatigue is viewed as a subset of general stress effects and as a combination of a variety of physiological and psychological factors. Among the precursors are extensive hours of work; among the physiological factors are components like circadian rhythms; and among the behavioral factors is vigilance or sustained attention. Fatigue is experienced as a region in a multidimensional space of stress in which the interaction of factors push an individual across a threshold. It may be with the identification of the appropriate contributory variables and the mapping of the interactive space of these variables that objective measures can be provided to the boundaries of the conditions to which we attribute to the subjective experience of fatigue.

The definitions suggested by researchers again tend to relate most strongly to central nervous system effects. That is, they refer to the impact on cognitive functioning. However, one major facet of fatigue is physical fatigue. In the context of this report, perhaps the most important element is the effect of prolonged physical activity on such factors as visual fatigue. Like any other component of the body, the eye is moved by muscle, and these muscles are vulnerable to fatigue. Repetition of use usually leads to fatigue, although there is much evidence that even this putative muscular fatigue is actually mediated by central control. That is, a muscle that an individual says is fatigued will respond virtually without degradation. Since the plans for helmet-mounted displays assume that much of the information will be provided by visual displays, the question of visual fatigue during operations deserves empirical study. Physical fatigue is liable to be a considerable problem for soldiers (see Chapter 3). This fatigue is not directly a result from operating the proposed systems but is intrinsic to efforts involved in battle operations, such as moving from point to point. Therefore, design of portable devices must specifically consider facilitation of cognitive operations under this form of stress.

The operating environment of today's infantry soldier is varied, complex, and demanding. The environment of tomorrow's land warrior may well be more varied, more complex, and more demanding. The design issues for the helmet-mounted display of Land Warrior System that are raised by discussions of the future infantry battlefield can be summarized in four key themes. First, what critical doctrinal and employment priorities must or should the Land Warrior Systems and the head-mounted display meet? Second, what are the limitations of cognitive, psychological, and physical capabilities of the infantry soldiers in the active force and the National Guard and Army Reserve who will use the Land Warrior System? Third, what are the significant Land Warrior System program test and evaluation implications? Fourth, what are the critical issues in selection, retention, and training of future infantry soldiers?

Force XXI projections identify changes in the scope of future infantry tasks and present a view of an expanded range of future infantry missions. These projections for tomorrow's land warrior highlight the need for a more clearly defined Land Warrior System doctrine. An integrated battlefield information and weapon system incorporating the latest electronic technology cannot be effectively achieved without clear employment concepts. Developing the design of the helmet-mounted display requires technical tradeoffs not only in achievable technology, but also in human performance. No one set of tradeoffs will optimize human performance under all conditions. The most advanced technical capabilities will not improve human performance unless the system is reliable and builds confidence.

In order to achieve the expected enhancements in human performance, the Land Warrior System and its head-mounted display must be designed to support the mental, psychological, and physical characteristics of tomorrow's infantry soldier. Complex electronic displays and battle sights will not achieve the Army's goal if the displays provide overwhelming levels of information that a soldier may not need and, in fact, that he may not be able to process effectively under critical battlefield conditions.

Physical ergonomics also play a critical role in achieving the success of 21 CLW. There can be no more disastrous scenario for advanced battlefield technology than the situation in which a soldier is so engrossed in making the equipment work that he or she becomes an easy target for the enemy. In contrast, a well-designed, effective information system would not only improve efficiency, but mitigate stress effects, which have been a traditional barrier to mission effectiveness in conflict situations. There should be an enhanced focus on understanding the mechanisms of stress and how they interact with proposed technology support systems. One particularly important area for research is the question of how to suppress the vibration in a head-mounted display that is caused by walking.

The desire to rapidly develop and achieve a Land Warrior System that applies significant technological advantages to the infantry battlefield requires the development of a comprehensive and thoughtful test and evaluation strategy. The rapid advances made in computer and electronic systems can sometimes lead to expectations about capabilities that are not achievable, do not work, and are not supportable under any realistic battlefield or employment conditions. The Land Warrior System and head-mounted system must be systematically tested in a range of realistic battle conditions, not only to aid the development and refinement of employment doctrine, but also to validate technical capabilities, system performance, and development priorities. Given the lack of information about human performance with head-mounted displays in which the body is in motion, a comprehensive test program is needed that considers helmet stability, helmet display capabilities, perceptual understanding and comprehension, and variance associated with individual differences.

Finally, the area of training requires focused attention. The design of the Land Warrior System and its head-mounted display must not only be logistically supportable from a reliability, availability, and maintainability point of view, but must also result in a system that soldiers can be taught to use, effectively, efficiently, and confidently. If the

complexities of the display and the system require extraordinary personnel selection or training demands that will not allow the development and maintenance of combat proficiency of infantry soldiers, their combat potential could be lost.