The Human Interface Technology Laboratory at the University of Washington is developing a new display device, the Virtual Reality Display (VRD), in which a coherent light source is used to scan an image directly on the retina of the viewer's eye. Development work is funded by Micro Vision, Inc., Seattle, which holds an exclusive license to manufacture and distribute the VRD. Using the VRD technology it is possible to build a high resolution, wide field-of- view, full color personal display device that is light weight and will operate in a high brightness environment. Current work is aimed at developing the technologies that will make the VRD a commercially viable product from both a performance and cost standpoint. Prototypes produced to date include a full color, VGA resolution device based on a unique mechanical resonant scanner as the horizontal scanning element. This paper will briefly explain the VRD concept and discuss potential applications of the technology. It will also describe the current research and development efforts which are aimed at creating a high performance yet low cost display system.

The use of cathode ray tubes in airborne helmet mounted displays (HMD) imposes a design requirement to safely discharge high voltage potentials (up to 13 kV), upon disconnection of the helmet from the cockpit electronics. For ejection safety, the discharge of energy stored in the HMD must be both rapid and complete, requirements difficult to reliably meet using traditional vacuum spark gaps. Experiments demonstrating the problem are presented in this paper, together with an alternate circuit approach which solves the problem by using all solid state components. The device is packaged as a miniature encapsulated module to meet airborne requirements, and has completed more than 1,500 -55 degree(s)C/+85 degree(s)C thermal cycles of failure-free stress testing. A Standard Electronic Module format high voltage power supply incorporating this device was designed for the Comanche RAH-66 HMD system.

In 1991, the Advanced Research Projects Agency (ARPA) established a head-mounted display (HMD) program as part of their High Definition Systems Program. The goals were to investigate, then develop new display technologies that would overcome the technical challenges of cathode-ray tubes, and satisfy DoD needs for improved HMDs. A Joint Services Working was formed to identify and define display specifications through common program goals. The technologies, Active Matrix Electroluminescent and Active Matrix Liquid Crystal Display were selected as the candidate display technologies. The Combat Vehicle Crew-HMD program resulted as the testbed for integrating the new display technologies. Many military systems leveraging the ARPA-developed technologies have emerged as a result of the ARPA HMD program. The dual-use applications of these technologies comply with ARPA and user goals to have HMD systems that will have wide acceptance meeting the requirements of the military, medical, commercial, and consumer markets.

A laser diode measurement concept is described that can be effectively used in a helmet mounted sight system; it offers a factor of 5 resolution improvement over conventional optical sensing techniques. It also has no moving parts and eliminates the compensation problems associated with helmet mounted metal in magnetic approaches. In the laser helmet mounted sight approach, the feedback from a helmet mounted reflector interacts with an airframe mounted diode to form a 3 mirror laser cavity; this semi-conductor laser cavity technique is used to provide 20 micrometers range resolution for the triangulation of fixed reflection points on the helmet. Geometric and radiometric models have been developed that show the feasibility of this approach. Geometric models include a discussion of resolution requirements. Radiometric models include a discussion of the laser cavity conceptual approach and a performance analysis of resolution and detectability.

The Combat Vehicle Crew (CVC) head-mounted display (HMD) program has built the first high-resolution (1280 X 1024) flat panel head mounted display. The CVC HMD is designed for use by the tank commander of an M1 A2 main battle tank and will show both tactical IVIS information and thermal imagery from the commander's independent thermal viewer. The CVC HMD uses 1280 X 1024 active matrix electroluminescent image sources with 24 micrometers pixels and integrated digital drivers. The use of flat panels in the HMD design has allowed new optical and head integration approaches and has required new approaches to the HMD drive electronics. The integration of the first AMEL image sources has been completed and preliminary photometric and subjective image quality evaluations performed. The preliminary findings from these evaluations will be discussed and conclusions regarding the application of flat-panel HMDs presented.

The U.S. Army is currently investigating the pilotage benefits of fusing imagery from infrared and image intensified sensors for tactical night terrain display missions. This program just completed 7 months of night helicopter flight tests and analysis of the performance of a proprietary real-time image fusion system. This paper provides an overview of the program, flight test, and results.

Military aviators have a need for lightweight Helmet Mounted Display (HMD) systems capable of displaying Night Vision (NV) imagery (which has been collected at the O.665m-O.93Optm portion of the spectrum), Forward Looking Infrared (FLIR) imagery (which has been collected at the 3-5im and 8-l2im waveband portions of the spectrum) and/or Heads-Up Display (HUD) information. Present HMD systems have excessive head—supported weights and poor centers of gravity (CG), inducing fatigue and creating an unsafe ejection condition. These problems are created mostly by the weight of the optics and optics support structure attached to the side and front of the aviator's helmet. The optics are necessary to redirect the imagery output of a phosphor screen to the aviator's eyes with as much fidelity as possible. An alternative approach to present HMD systems involves the use of a microlaser based image output array. The micro—lasers would be an array of Vertical Cavity Surface Emitting Laser (VCSEL) diodes. VCSELs have circular output beams, as opposed to edge emitting laser diodes which have astigmatic output beams. The VCSEL circular output beam geometry lends itself to the use of micro-lenses. The micro-lenses allow for f/number modulation with a significantly reduced number of optical elements in the optical path during the redirection of the output image to the user's eyes. Reduced optical elements equate to reduced weight and better CG. In addition, this approach may allow for significantly greater fields-of-view (FOV), possibly in excess of 100 degrees. This paper addresses the attributes and drawbacks of current HMD systems (especially NV systems) and the attributes, drawbacks, and technical challenges associated with realization of a HMD utilizing VCSELs as a display illumination source.

A cockpit revolution is in the making. Many of the much ballyhooed, much promised, but little delivered technologies of the 70's and 80's will finally come of age in the 90's just in time to complement the data explosion coming from sensor and processing advances. Technologies such as helmet systems, large flat panel displays, speech recognition, color graphics, decision aiding and stereopsis, are simultaneously reaching technology maturities that promise big payoffs for the third generation cockpit and beyond. The first generation cockpit used round dials to help the pilot keep the airplane flying right side up. The second generation cockpits used Multifunction Displays and the HUD to interface the pilot with sensors and weapons. What might the third generation cockpit look like? How might it integrated many of these technologies to simplify the pilots life and most of all: what is the payoff? This paper will examine tactical cockpit problems, the technologies needed to solve them and recommend three generations of solutions.

New aircraft are being developed incorporating head—up displays (HUD5 ) to serve as primary flight references (PFRs) in all phases of flight. HUDs have been used in military airplanes as weapon displays and as primary flight references. They have been approved in some civil aircraft as part—time displays for approach and landing or as supplementary displays. The HUD offers several advantages over conventional instruments: Reduced pilot workload; Increased flight precision; Direct visualization of trajectory; and Less eyes—inside—the—cockpit. At the same time, these advan— tages can sometimes create problems (I. e • , There is no free lunch) . Two instances are clutter and rapid display motion In spite of these difficulties, the use of a HUD can be an overall unproveinent in flight instrumentation, provided that attention is paid to the problem areas. A design guide is available)HUDs have been certificated on civil aircraft since the early 1970s. They have usually been restricted to specific flight phases (approach and landing). All have supplemented head down instruments, not replaced them. A Flight Dynamics HUD is being developed for the Lockheed C-130J for use as the primary flight reference (a stand-alone PFR). There is interest in the airline community for enhanced vision systems (EVS) or synthetic vision systems (SVS). As envisioned by proponents, EVS/SVS would use a raster-capable HUD which would show electronic imagery from some kind of sensor. Both millimeter wave radar and FLIR have been proposed. The operational scenario would use the electronic image in place of direct view of the runway environment. The pilot would base descent below instrument minimums on the view of the sensor image of the runway. A certification project is underway for an enhanced vision system to be installed on B—747 airplanes. Helicopter operators would like to use night vision technology, based on image intensifier night-vision goggles or on infrared head—mounted displays (HMD5). Such displays have been suggested to aid emergency medical service flights to accident sites. Several helicopter ambulance operators have requested operational approval for night—vision goggles.

The DRA's research effort into Visually Coupled Systems is intended to assist the operational pilot fly aggressively at night and day, by giving him an informative view of the near-earth environment. A number of simulator trials have been conducted to explore some of the issues identified in Helmet-Mounted Display symbol set development. The development of successful symbol sets will be iterative. HMD symbol set evaluation techniques are the subject of considerably interest, not only within research establishments. A careful study of the operational requirement should produce a series of tests which will demonstrate compliance; however, we must be cautious, when we are testing the ability of the visionic system to feed cues to a pilot, in how we set out the cues to define the task. International cooperation on VCS research has been formalized through a Memorandum of Understanding with the United States, which defines a 48 months program; considerably effort has been put into achieving an efficient flow of information between its partners. The paper outlines past research, describes the planned future activities and raises some human factors issues which have been identified.

The German Government selected Eurocopter Deutschland (ECD) to coordinate the development of a new 40 degree(s) Integrated Helmet System (IHS) for the PAH2 (Panzer Abwehr Hubschrauber 2. Gen) helicopter. ECD were tasked with integrating the IHS into the BK117/AVT (Ausrustungs-Versuchs-Trager) for flight trials. A contract was placed in 1992 with GEC-Marconi Avionics of the UK for the IHS. This IHS consists of a new light weight helmet shell equipped with an individual form fit liner, intercom and a removable Optical Module which includes two Cathode Ray Tubes (CRT), two Image Intensifier Tubes (I²T), two see-through combiners and a Helmet Mounted Sight (HMS) Receiver. An A- model helmet with a 35 degree(s) Field of View (FOV) was tested on the BK117-AVT, with flight symbology during Nap of the Earth flights in early 1994. A similar helmet was assessed during 1990/91 on a PAH1 in Celle. In this version, no HMS was installed in the HC. An A/B-model helmet (a B-model helmet with an A-model electronics) with 40 degree(s) FOV was received at the end of 1994 and tested in the ECD Visionics laboratory. The flight trials started on a PAH1 in Manching January 1995 and in the new moon phase of February/March 1995 in Buckeburg. The wearing comfort of the helmet and the night channel were evaluated. The new IHS has been specially designed for PAH2. In C-model configuration the IHS shall have a total helmet weight of 2.2 kg, an optimized Centre of Gravity, exit pupils with > 15 mm, IITs with > 50 Lp/mm and gain in excess of 2000, CRTS with high resolution and high brightness for daytime application. There is a requirement for the HMS to have an accuracy of 0.2 degree(s) in a large Head Motion Box. This HMS steers a nose mounted FLIR platform and has the task of aiming precise weapon systems on targets. The paper presents the results of IHS assessment in the laboratory and flight trial assessments with the IHS on a PAH1.

The magnitude of requirements, specifications, testing, and liability associated with producing an item for military use has, no doubt, caused many potential helmet vendors to stay away from the military customer. This is a dilemma for the military as well as the vendors. This paper is an attempt to educate people on the requirements for one aspect of advanced helmet development. That aspect is the Safety-of-Flight requirement. Safety-of-Flight encompasses every aspect of flight to include preflight and post flight. Emergency egress, high-voltage containment, helmet fit, donning and doffing, windblast protection and other aspects of protection during ejection, comfort, aural protection, and compatibility with life support equipment are a few of the items that must be evaluated prior to flying in a multi-million dollar fighter. This paper will highlight the types of testing required with a short explanation of why this testing is necessary as well as what must be accomplished in order to pass the stated requirement.

The Head-Up Display (HUD) and Helmet Mounted Display (HMD) potentially offer the pilot several critical abilities: maintenance of head-out posture; enhanced situational awareness; real world target location and engagement; and provision of enhanced vision displays (e.g. FLIR). Experience with the HUD in current fast-jet cockpits has led to user acceptance and to the display of a wide range of information. Conversely, there is currently minimal experience of flight with HMDs and hence little is known of this technology on mission performance. Although the HMD has the benefit of cueing pilots as they move their head, it is unclear what the appropriate selection of stabilization cues should be, e.g. attitude information. This review considers a number of technological and psychological factors of which designers of HMDs should be aware.

The Aeroflightdynamics Directorate of the US Army's Aviation Research, Development and Engineering Center initiated a study to determine the effects of limiting a rotorcraft pilot's field-of-view (FOV) on performance and workload. Pilot FOV was restricted to simulate current and next generation helmet mounted display FOVs used in night vision systems. A helmet visor was constructed for this test to restrict the horizontal peripheral limits of the square shaped FOV to values of 100, 80, 60, 40, and 20 degrees. The vertical limits and overlap were held constant to 40 degrees (except for the 20 degree FOV). Six pilots executed a series of prescribed low altitude maneuvers with an instrumented NAH-IS (Cobra) rotorcraft at the Crows Landing Airfield located in California. The aircraft flight path was measured with a laser tracker. On board data was recorded, as well as pilot handling quality ratings and visual cue ratings. This test is not yet complete, but some preliminary observations and results are provided. It was observed that reducing FOV increased the difficulty of controlling the aircraft and altered head movement. Reducing FOV also diminished the pilots' situational awareness. The reduction in situational awareness affected the ability of the pilots to provide an accurate report of their own flying performance, and the ability to observe warning indicators inside the cockpit.

Virtual environment technologies, such as helmet-mounted displays (HMDs), are challenged by problems involving time delay--the time between an input to a system, and its corresponding output. An experiment was conducted to evaluate two methods of time delay compensation--algorithmic prediction and perceptual adaptation--during a time-delayed, head- slaved tracking task using an HMD. Predictive algorithms attempt to compensate for time delays by predicting future head position in order to update images effectively in the HMD. Perceptual adaptation refers to the ability of humans to adapt to the time delay by modifying their tracking strategies. Subjects were assigned to either a perceptual adaptation or algorithmic prediction condition, and participated in four experimental sessions during which they attempted to center a reticle over a moving circular target using a HMD. Tracking performance was evaluated in terms of RMS error, and the adequacy of the adaptation and prediction solutions was evaluated by several comparisons of tracking efficiency within and between sessions. Results indicated that the algorithmic prediction solution was superior to the perceptual adaptation solution for compensating for the effects of time delay in a head-slaved tracking task.

Helmet-mounted displays (HMDs) present flight, navigation, and weapon information in the pilot's line of sight. The HMD was developed to allow the pilot to retain aircraft and weapon information while looking off boresight. Symbol stabilization is a key issue for HMDs. In current equipment, the lack of compensation for pilot head motion creates excessive workload during hovering and nap-of-the-earth flight. This high workload translates into excessive training requirements. At the same time, misleading symbology makes interpretation of the height of obstructions impossible. A set of standardized coordinate transformations are necessary for the development of HMD symbology and the control laws. Part of the problem is there is no agreed upon set of definitions or descriptions for how HMD symbols are driven to compensate for pilot head motion. A candidate set of coordinate definitions is proposed to address this issue.

Head-Up Displays have been used worldwide for almost 30 years in over 10,000 aircraft. Helmet-Mounted Displays exist in less than 500 aircraft, most of which are helicopters. This paper examines some of the tradeoffs involved in the selection of each type of display and their related performance comparisons.

Eight veteran USAF fighter pilots, experienced with AGM-65 Maverick air-to-ground missiles, flew a night, low-level ground attack mission in a flight simulator equipped with a helmet-mounted display (HMD). The mission was performed by delivering five Maverick missiles against ground vehicles using either an aircraft-fixed forward-looking infrared (FLIR) sensor image on a head-up display (HUD) or a head-steered FLIR as the missile aiming device. Additionally, the pilots employed their weapons by two methods: fixing and launching missiles singly or in varying numbers (multiple method). The purpose of the experiment was to determine what, if any, advantage there is to employing the AGM-65 using the HMD FLIR image to slew the missile seeker onto the target versus the conventional method of using the FLIR image displayed on the HUD. With a head-steered sensor (and fixing and launching weapons singly) subjects released their weapons quicker (14.6 second interval between launches vs. 17.1 sec.), at a higher altitude (1739 feet vs. 1603 ft.), and slightly farther from the target (3.42 nautical miles vs. 3.37 nm). Furthermore, data indicated the pilots looked farther off-boresight when searching for and locking the weapon onto a target, thereby more effectively using the full field-of-regard of the missile seeker. The participants also contributed their opinions of the advantages and disadvantages of the two mechanizations.

The old saying, `Safety is paramount.' was never more true than it is in the area of ejection safety for high-speed fighter aircraft. The fighter aircraft of today has been designed to endure tremendous structural loading during dogfight or evasive maneuvers. It can fly faster, turn quicker, stay in the air longer (with in-flight refuel) and carry more bombs than its predecessor. Because of human physiological limits, the human has become the weak link in today's fighter aircraft. The fighter pilot must endure and function with peak performance in conditions that are much worse than anything the majority of us will ever encounter. When these conditions reach a point that human endurance is exceeded, devices such as anti-g suits and positive pressure breathing apparatus help the fighter pilot squeeze out that extra percentage of strength necessary to outperform the opponent. As fighter aircraft become more sophisticated, helmet trackers, helmet displays and noise cancellation devices are being added to the helmet. Yet the fighter pilot's helmet must remain lightweight and be aesthetically appealing, while still offering ballistic protection. It must function with existing life support equipment such as the Combined Advanced Technology Enhanced Design g-Ensemble (COMBAT-EDGE). It must not impede the pilot's ability to perform any action necessary to accomplish the planned mission. The helmet must protect the pilot during the harsh environment of ejection. When the pilot's only resort is to pull the handle and initiate the ejection sequence, the helmet becomes his salvation or instant death. This paper discusses the safety concerns relative to the catapult phase of ejecting from a high-speed fighter while wearing an advanced fighter helmet.

Flight instrument interpretability has been a key piloting issue because it is directly related to operator performance and inversely related to operator error. To improve interpretability we have developed the Sky Arc, a new symbology initially developed for attitude control, particularly for a helmet-mounted display. It consists of an integrated set of graphic symbols which vary in a continuous, analog fashion with changing flight parameters. The Sky Arc currently integrates, pitch, roll, heading, air speed, and terrain avoidance. The display can be integrated into a head down display, a head up display, or a helmet mounted display. In this preliminary study the usability of the Sky Arc as an attitude indicator was compared to a conventional head-up display pitch ladder symbology. The test involved six test subject pilots and a medium-fidelity simulator. The pilots were asked to fully recover from a series of unusual attitude conditions that were presented on the simulator. The time taken to recover and the correctness of the recovery procedure served as the objective evaluation measures. A Likert-type rating scale and open-ended subject matter expert opinions served as the subjective measures of evaluation. To examine whether there was a relationship between usability of the attitude indicator and difficulty of the unusual attitude, the workload levels involved in performing the unusual attitude recoveries were grouped into three levels, low, medium, and high. At each workload level there were four conditions, for a total of 12 different conditions. Each pilot was asked to recovery twice from each condition, for a total of 24 unusual attitude recovery trials. The test trials were counterbalanced and displayed in a prearranged order. No differences due to difficulty of the unusual attitude were detected. Overall, the study revealed that the Sky Arc led to generally faster recoveries than did the standard display, as well as higher subjective preference ratings. Although the pilots had thousands of hours of familiarity with pitch ladder symbologies, they recovered more quickly using the Sky Arc symbology with which they had had no previous experience. This evidence suggests that further research is warranted to examine the utility of the Sky Arc symbology.

This study was conducted to investigate the effects of different candidates transparent head- coupled helmet-mounted display (HMD) target location symbology orientations on search and intercept performance during an air-to-air task. Orientation, as it refers to HMD symbology, is the method by which the information is referenced and related to the observer. HMD symbology can be coded so that it responds to ownship maneuvering (within the world coordinate system) as well as observer head movement (within the ownship coordinate system). Three symbology orientations were compared to the traditional head-up-display target location symbology: One of the candidate symbologies was referenced to ownship maneuvering, a second was referenced to head movement, and the last was referenced both ownship and head movement. The objective and subjective findings suggest that HMD target location information is advantageous and that ownship referenced information presented via a HMD may be more useful than previously believed.

The design and evaluation process for novel displays in military fast jet cockpits is far from optimal. In this paper, the design of symbology is discussed in terms of those involved in the design process and the sources of information available to provide a sound basis for the design of novel Helmet-Mounted Display (HMD) symbology. Further, the evaluation process is discussed in terms of the available methods (subjective and objective), experimental media (flight and simulations), experimental tasks (full mission and part tasks) and the measurement of the key design drivers (workload and situational awareness). Finally, it is argued that the cycle should be more structured and that greater feedback between the operational use of displays and their design is required. These points are summarized in the form of ten guidelines for the design and evaluation of HMD symbology.

The US Army Aviation RDEC's Aeroflightdynamics Directorate (AFDD) in cooperation with the Department of Defense Flight Symbology Working Group, the United Kingdom's Defense Research Agency (DRA), and The Technology Cooperative Program Helicopter Technical Panel 6 (HTP6), conducted a Helmet Mounted Display (HMD) symbology investigation using AFDD's Crew Station Research and Development Facility helicopter simulator located at the Ames Research Center, Moffett Field, California. The objectives of the experiment were to examine HMD symbology stabilization, pitch ladders, flight path presentations, and tasks and measures that capture objective and subjective performance differences. Symbology presentation techniques closely modeled specific presentations found in the US Army's AH- 64D Apache helicopter and proposed symbology techniques for the RAH-Comanche and Longbow Apache rotorcraft. Eight helicopter pilots from DOD and DRA participated in the study flying simulated low-altitude rotorcraft maneuvers. This paper describes the simulation flight tests, test results, implications of test findings and recommendations for future HMD investigations.

Over the past 18 months the Army has been developing a terrain following capability in it's next generation special operations aircraft (SOA), the MH-60K and the MH-47E. As two experimental test pilots assigned to the Army's Airworthiness Qualification Test Directorate of the US Army Aviation Technical Test Center, we would like to convey the role that human factors has played in the development of the MMR for terrain following operations in the SOA. In the MH-60K, the pilot remains the interface between the aircraft, via the flight controls and the processed radar data, and the flight director cues. The presentation of the processed radar data to the pilot significantly affects the overall system performance, and is directly driven by the way humans see, process, and react to stimuli. Our development has been centered around the optimization of this man-machine interface.

In an effort to address some of the fundamental issues that must be considered in any long- range progression from `targeting' to `flight instrument' for any helmet-mounted display (HMD), this study investigated the impact of incorporating an HMD with one of the objective flight performance tasks used in head-up display (HUD) evaluation. The task, an unusual attitude recovery, was adapted to require head movement by having the pilot acquire a target either on- or off-axis prior to initiating a recovery. Experiment One did not include any HMD orientation symbology and established the increase in reaction time 0.82 seconds for the on- axis condition to 1.35 seconds for the extreme off-axis condition (+/- 90 degrees off-axis with a +30 degree head tilt). Experiment Two included minimal HMD orientation information in half of the trials. In trials where no HMD symbology was presented, initial reaction times significantly increased (i.e., from 1.06 for on-axis to 1.56 seconds for +/- 80 degrees). In trials where pilots were provided minimal HMD orientation information off-axis, initial reaction times did not significantly increase (i.e., 1.06 on-axis to 1.12 seconds for +/- 80 degrees).

Daimler-Benz Aerospace's Military Aircraft Division has investigated a HMD system in combination with a moveable FLIR integrated into a TORNADO fighter aircraft. That study was performed under a contract of the German MoD Research and Technology Program. The aim was to assess the systems suitability for enhancing situational awareness at night and for ground target acquisition and to deduct a specification for in series applications. For this purpose a low cost simulation environment was set up at Dasa and successfully used for system test, new symbol format development and evaluation, ergonomic assessment and initial pilot familiarization. Flight tests were conducted first in the rear cockpit of the TORNADO combat aircraft and later on in the front cockpit by test pilots from the German Official Test Center and from German Air Force and Navy tactical evaluation groups. Findings from simulation and flight testing in terms of ergonomic, flight medical, physiological and operation aspects will be reported.

For the X-31 Inflight Weapon System Simulation for tactical utility evaluation and the Agile Warrior follow-on program a virtual adversary HMD symbology and a digital adversary maneuver pilot have been developed by Daimler-Benz-Aerospace (Dasa) and evaluated in manned realtime simulations. The simulations proved the utility of the virtual target symbology as a viable and cost saving alternative to any real airborne target for air combat training and air combat maneuvering exercises. The symbology is compatible with existing HMD hardware, however, some limitations were found inherent in the current available display technology.

In parallel with the development of Helmet Mounted Displays, significant development work has been focused on systems to track and accurately measure the orientation and position of aircrew helmets within the cockpit of military aircraft (fixed and rotary wing). Such systems have wide application in both the commercial and military environments. Two different types of Helmet Tracker Systems are discussed both of which have been selected for major European aircraft programs, one for fast jet and one for rotary wing. The fast jet system utilizes optical techniques and the rotary wing system is electromagnetic. The development of one system is traced from initial concept, through prototype system development and flight test, and how the lessons learned have now been applied to the production system configuration. The paper also provides a review of the basic design requirements that are peculiar to each system.