Head-up display

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A head-up display, or HUD, is any transparent display that presents data without obstructing the user's view. Although they were initially developed for military aviation, HUDs are now used in commercial aircraft, automobiles, and other applications.

Contents 1 History 2 Types 3 Components 3.1 Combiner 3.2 Overhead Projection Unit 3.3 Computer 3.4 Factors 3.4.1 Field of Vision 3.4.2 Eyebox 3.4.3 Luminance/Contrast 3.4.4 Display Accuracy 4 Military aircraft applications 4.1 V/STOL approaches and landings 5 Civil aircraft applications 5.1 Symbology 5.2 Installation 5.3 Enhanced Flight Vision Systems, EFVS 5.4 Synthetic Vision Systems, SVS 6 Automotive applications 7 Experimental uses 8 Pop Culture 9 Further reading 10 Notes 11 External links 12 Commercially Available HUDs 13 Commercially Available EVS Cameras

[edit] History

Head-up displays were originally created as a modified gun-sight for military fighter aircraft. It took information such as speed and direction and showed the pilot where the bullets of their gun would land if fired. This greatly increased the accuracy pilots could achieve in air to air battles. In Great Britain, it was soon noticed that the pilots who had these new gun-sights were also becoming better at piloting their aircraft. It was at this point that the HUD expanded its use into a piloting tool. In the 1960s, french test-pilot Gilbert Klopfstein created the first modern HUD, and a centralized system of symbols for HUDs so that pilots would only have to learn one system and be able to transition between aircraft without re-learning the entire HUD. 1975 saw the development of the modern HUD to be used in instrument landings.^[1]

Head-up displays were pioneered by Klopfstein in fighter jets and military helicopters to minimize information overload by centralizing critical flight data within the pilot's field of vision. In the 1970s, this innovation was introduced to commercial aviation. These early HUDs featured data such as airspeed, altitude, and localizer readings, which had previously been accessible on head-down primary flight displays (PFD).^[2]

Currently, a typical HUD in a commercial aircraft will display airspeed, altitude, a horizon line, a compass, turn/bank and slip/skid indicators. These instruments are the minimum required by 14 CFR Part 91.

In 1988, the Oldsmobile Cutlass Supreme became the first production car with a head-up display.^[3]

Until a few years ago, the Embraer 190 and Boeing 737 New Generation Aircraft (737-600,700,800, and 900 series) were the only commercial passenger aircraft to come with an optional HUD. Now, however, the technology is becoming more common with aircraft such as the Canadair RJ, Airbus A318 and several Business Jets featuring the device. Furthermore, the Airbus A320, A330, A340 and A380 families are currently undergoing the certification process for a HUD.^[4]

[edit] Types

There are two types of HUD. Fixed HUDs require the user to look through a display element attached to the airframe or vehicle chassis. The system determines the image to be presented depending solely on the orientation of the vehicle. Commercial aircraft and automobiles usually incorporate a fixed HUD system. Helmet-mounted or head-mounted HUDs feature a securely-attached display element that moves with the orientation of the user's head. Such systems are often monocular, and are used in the AH-64 Apache and in some versions of the F-16 Fighting Falcon. The fixed HUD is actually being phased out in favor of the Helmet Mounted Display. The F-22 Raptor was the last military fighter to have the fixed HUD. The new F-35 fighters all have Helmet Mounted Displays.

[edit] Components

A typical HUD in civil aircraft contains three primary components: A Combiner, the Overhead Projector Unit (OPU), and the computer.

[edit] Combiner

The combiner is the part of the unit which is located directly in front of the pilot. It is the surface onto which all the information is projected so that the pilot can view and use it. The combiner is concave in shape and has a special coating that reflects the monochromatic light projected onto it from the OPU while allowing all other wavelengths of light to pass through. It is easily removable and also easily broken in case of a sudden stop by the aircraft that would cause the pilot's head to crash into it. This also allows for pilots who prefer not to use the unit to simply remove the combiner and fly the aircraft normally.^[5]

[edit] Overhead Projection Unit

The Overhead Projection Unit, or OPU, is located above the pilot's head in the cockpit. In tactical military aircraft, it is actually incorporated into the combiner in front of the pilot. The purpose of the OPU is to properly project the correct information onto the combiner for the pilot to view. In the early days of HUDs, this was done through refraction. Reflection is the current method that is used. The OPU uses a Cathode Ray Tube to project the image. In order to increase efficiency, the image is actually refreshed using a bi-direction scan. This causes the image to be updated twice as fast as normal. The OPU is also easily removed for repairs or replacements. This is possible because the brackets that hold both the OPU and combiner have been attached to the aircraft's structure using a process called boresighting. Since the HUD displays information regarding the orientation of the aircraft, it needs to be very precisely aligned with the aircraft's three axises. This is done during the very early part of the aircraft's building process. Once the brackets are permanently connected to the frame of the aircraft, the components of the HUD are line-replaceable.^[6] Recently, micro-display imaging technologies are being introduced. Currently, micro-display technologies that have been demonstrated include liquid crystal display (LCD), liquid crystal on silicon (LCoS), digital micro-mirrors (DMD), and organic light-emitting diode (OLED). HUD systems that project information directly onto the wearer’s retina with a low-powered laser (virtual retinal display) are also in experimentation.^[7][8]

[edit] Computer

The computer usually is located with the other avionics equipment and provides the interface between the HUD (i.e. the combiner and OPU) and the aircraft systems to be displayed. The computer is a dual independent redundant system. This means that is receives its input directly from the sensors aboard the aircraft and does its own computations rather than receive already computed data from the flight computers. The computer is very easily incorporated into an aircraft's system and allows connectivity onto several different data buses such as the ARINC 429, ARINC 629, and MIL STD 1553. The computer itself also has redundancy. It receives the data input into two separate partitions, and independently performs the calculations. It then compares the results within itself. If there is no error, the data is then converted into a form that can be used by the OPU to display. To check for errors, the HUD computer also sends its computed data to the main flight computers to be compared with their calculations. If there are errors, then the HUD unit can be shut down. Another error check that the computer performs is to check that the correct data is being displayed. This is accomplished because when the OPU projects the image onto the combiner, there is some reflection back to the OPU. The OPU then sends this data back to the HUD computer and compared to what should have been shown. This ensures that not only the data is correct, but that it is being shown correctly.^[9]

[edit] Factors

There are several factors that come into consideration when discussing the operation of a HUD. Most of these factors have to do with optics.

[edit] Field of Vision

The first concern has to do with field of vision. Since a person’s eyes are at two different points, they see two different images. To prevent a person’s eyes from having to change focus between the outside world and the display of the HUD, the display is focused at point infinity. In automobiles the display is generally focused around the distance to the bumper.

[edit] Eyebox

Another feature of the HUD is that the display is projected in such a way so that it can only be viewed while the pilot’s eyes are within a 3-dimensional area in the cockpit called the Head Motion Box or “Eyebox”. Modern HUD Eyeboxes are usually 5 by 3 by 6 inches. This allows the pilot freedom of head movement. It also allows the pilot the ability to view the entire display as long as one of his eyes is inside the Eyebox. This is achieved through the concavity of the Combiner.

[edit] Luminance/Contrast

Display luminance and contrast is also a concern for HUDs. The display can be brightened or dimmed to account for all foreseeable circumstances, whether it be a climb directly into the glare of a bright white cloud or a night approach to a desert airport where there is very little lighting.

[edit] Display Accuracy

HUDs are calibrated to have a display accuracy of +/- 7.0 milliradians. This allows the display to show the pilot exactly where the artificial horizon is, as well as the aircraft’s projected path with great accuracy.

[edit] Military aircraft applications

The use of HUDs was pioneered mainly for use in military aircraft platforms. The main advantage would be to allow the pilot to keep his attention on what was going on around him and not having to look down at his instruments for flight critical information very often.

[edit] V/STOL approaches and landings

During the 1980s, the military was testing the use of head-up displays for the use in aircraft that are capable of vertical take off and landings (VTOL) and short take off and landing (STOL). A head-up display (HUD) format was developed at NASA Ames Research Center to provide pilots of V/STOL aircraft with complete flight guidance and control information for Category-IIIC terminal-area flight operations. These flight operations cover a large spectrum, from STOL operations on land-based runways to VTOL operations on small ships in high seas. The principal features of this display format are the integration of the flightpath and pursuit guidance information into a narrow field of view, easily assimilated by the pilot with a single glance, and the superposition of vertical and horizontal situation information. The display is a derivative of a successful design developed for conventional transport aircraft.^[10]

[edit] Civil aircraft applications

The use of head-up displays allows commercial aircraft substantial flexibility in their operations. Systems have been approved which allow reduced-visibility takeoffs and landings, as well as full Category IIIc landings.^[11][12][13] Studies have shown that the use of a HUD during landings decreases the lateral deviation from centerline in all landing conditions although the touchdown point along the centerline is not changed. ^[14]

The image to the right, of a HUD in a NASA Gulfstream GV, shows several different HUD elements, including the combiner in front of the pilot. The green 'glare' in the lower right corner is a result of the backscatter of off-axis light from the OPU, as well as from reflection of the available light in the flight deck off the "only does green" coating on the combiner. Because the combiner has a pronounced vertical and horizontal curve to help focus the image, compensation is applied to the display symbols so they appear flat when projected onto the curved surface. When not in use, most combiners swing up and lock in a stowed position.

The Overhead Projector Unit in the Gulfstream GV image would be directly above the pilot's head. In smaller aircraft the design of the OPU can present interesting spacing and placement issues, as room has to be left for the pilot not only when normally seated but during turbulence and when getting in and out of the seat.

[edit] Symbology

In recent years, several new symbols have been added to the flight deck by HUD designers. A boresight symbol is fixed on the display and shows where the nose of the aircraft is actually pointing. A flight path vector (FPV) symbol shows where the aircraft is actually going, the sum of all energies acting on the aircraft.^[15] For example, if the aircraft is pitched up but is losing energy, then the FPV symbol will be below the horizon even though the boresight symbol is above the horizon. During approach and landing, a pilot can fly the approach by keeping the FPV symbol at the desired touchdown point on the runway.

An "acceleration indicator" symbol has also been added to HUD designs. This symbol, typically to the left of the FPV symbol, will be above the symbol if the aircraft is accelerating, and below the FPV symbol if decelerating.

Since being introduced on HUDs, both the FPV and acceleration symbols are becoming standard on head-down displays (HDD). The actual form of the FPV symbol on an HDD is not standardized but is usually a simple aircraft drawing, such as a circle with two short angled lines, (180 ± 30 degrees) and "wings" on the ends of the descending line. Aligning one of these angle lines on the horizon allows the pilot to easily fly a coordinated 30 degree turn while maintaining altitude.

For approach and landing guidance, the flight guidance system can provide head-up visual cues based on navigation aids such as an Instrument Landing System or augmented Global Positioning System such as the Wide Area Augmentation System. Typically this is a circle which fits inside the flight path vector symbol. By "flying to" the guidance cue, the pilot flies the aircraft along the correct flight path. Keeping the pilot "in the loop" in this way is an economical alternative to an autolanding system, where the flight guidance system and autopilot system are given the control of the aircraft and the pilot simply provides a monitoring function.

[edit] Installation

During the installation of a HUD in a commercial aircraft, the combiner is boresighted to the aircraft's centerline so that its displayed data corresponds to reality. For example, the line on the HUD used to depict the horizon must be conformal^[16] to the actual horizon (at lower altitudes, due the curvature of the earth the display horizon line will be above the visual horizon at higher altitudes.)

[edit] Enhanced Flight Vision Systems, EFVS

In more advanced systems, such as the FAA-labeled Enhanced Flight Vision System,^[17] a real-world visual image can be overlaid onto the combiner. Typically an infrared camera (either single or multi-band) is installed in the nose of the aircraft to display a conformed image to the pilot. In one EVS ^[18] installation, the camera is actually installed at the top of the vertical stabilizer rather than "as close as practical to the pilots eye position." When used with a HUD however, the camera must be mounted as close as possible to the pilots eye point as the image is expected to "overlay" the real world as the pilot looks through the combiner. "Registration" or the accurate overlay of the EVS image with the real world image is one feature closely examined by the authorities prior to approval of a HUD based EVS. When the pilot is coming in for a landing and "sees" the runway and runway lights through the EVS display, it is really a good thing when they come out under the clouds and the real world runway is right where the camera said it was as the pilot has a very short period of time to (a) take in the reality of "what is displayed is not what is real" (b) decide that action needs to be taken (c) take action and (d) allow the airplane some time to respond. During the design of such a system, the supplier would perform a safety analysis to determine the consequences of "EFVS Image not aligned with real world at or below decision height." Using regulatory guidance (FAA Advisory Circular 25.1309-1A [15]for example) this would be initially evaluated as a Major hazard ^[19] where if it does occur the design community anticipates that the flight crew will be able to take the appropriate action. The pilot may choose to initiate a missed approach (climb immediately and then figure out what to do because altitude and speed are your friend when trying to deal with "unexpected events") or perhaps to immediately blank the HUD/EVS display (typically there is a thumb switch on the control column for exactly this circumstance) and continue the landing using what can be seen through the window.

While the EVS display can greatly help, the FAA has only "relaxed" operating regulations ^[20] where an aircraft with EVS operating can perform a CATEGORY I [16]approach to CATEGORY II minimums. In all other cases the flight crew must comply with all "unaided" visual restrictions. (For example if the runway visibility is restricted because of fog, even though EVS may provide a clear visual image it is not appropriate (or actually legal) to maneuver the aircraft using only the EVS below 100' agl.)

[edit] Synthetic Vision Systems, SVS

HUD systems are also being designed to utilize a synthetic vision system (SVS), which use terrain databases to create a realistic and intuitive view of the outside world.^[21][22][23]

In SVS image to the right, immediately visible indicators include the airspeed tape on the left, altitude tape on the right, and turn/bank/slip/skid displays at the top center. The boresight symbol (-\/-) is in the center and directly below that is the Flight Path Vector symbol (the circle with short wings and a vertical stabilizer). The horizon line is visible going across the display with a break at the center, and directly to the left are the numbers at ±10 degrees with a short line at ±5 degrees (The +5 degree line is easier to see) which, along with the horizon line, show the pitch of the aircraft.

The aircraft in the image is wings level (i.e. the flight path vector symbol is relative to the horizon line and there is zero roll on the turn/bank indicator). Airspeed is 140 knots, altitude is 9450 feet, heading is 343 degrees (the number below the turn/bank indicator). Close inspection of image shows a small purple circle which is displaced from the Flight Path Vector slightly to the lower right. This is the guidance cue coming from the Flight Guidance System. When stabilized on the approach, this purple symbol should be centered within the FPV.

The terrain is entirely computer generated from a high resolution terrain database.

In some systems, the SVS will calculate the aircraft's current flight path, or possible flight path (based on an aircraft performance model, the aircraft's current energy, and surrounding terrain) and then turn any obstructions red to alert the flight crew. Such a system could have prevented the crash of American Airlines Flight 965 in 1995.

On the left side of the display is an SVS-unique symbol, which looks like a purple, dimishing sideways ladder, and which continues on the right of the display. The two together define a "tunnel in the sky." This symbol defines the desired trajectory of the aircraft in three dimensions. For example, if the pilot had selected an airport to the left, then this symbol would curve off to the left and down. The pilot keeps the flight path vector alongside the trajectory symbol and so will fly the optimum path. This path would be based on information stored in the Flight Management System's data base and would show the FAA-approved approach for that airport.

The Tunnel In The Sky can also greatly assist the pilot when more precise four dimensional flying is required, such as the decreased vertical or horizontal clearance requirements of RNP. Under such conditions the pilot is given a graphical depiction of where the aircraft should be and where it should be going rather than the pilot having to mentally integrate altitude, airspeed, heading, energy AND longitude and latitude to correctly fly the aircraft.^[24]

[edit] Automotive applications

Head-up displays are becoming increasingly available in production cars, and usually offer speedometer, tachometer, and navigation system displays. BMW, Citroën, GM, and Nissan currently offer some form of HUD system. Motorcycle helmet HUDs are also commercially available.^[25]

Add-on HUD systems also exist, projecting the display onto a glass combiner mounted on the windshield. These systems have been marketed to police agencies for use with in-vehicle computers.

[edit] Experimental uses

HUDs have been proposed or are being experimentally developed for a number of other applications. In the military, a HUD can be used to overlay tactical information such as the output of a laser rangefinder or squadmate locations to infantrymen. A surgical HUD could also display overlaid x-rays or other medical data or imagery onto the surgeon's view of a patient undergoing surgery, allowing the surgical team to "see" structures that are normally invisible. A prototype HUD has also been developed that displays information on the inside of a swimmer's goggles.^[26]

[edit] Pop Culture

The 2008 movie Cloverfield was filmed in the style of a handheld camcorder. The full name of the character doing the filming is Hudson Platt, nicknamed Hud, most likely a reference to the HUD seen on most cameras.

[edit] Further reading

• Head-mounted display
• Augmented reality
• EyeTap
• Scanned-beam display
• Wearable computer

[edit] Notes

[edit] External links

• BBC Article - 'Pacman comes to life virtually'
• 'Clinical evaluation of the ‘head-up’ display of anesthesia data'

[edit] Commercially Available HUDs

• El-Op
• Trivisio Monocular
• Microvision
• MicroOptical Consumer Electronics
• Olympus Prototypes
• Rockwell Collins Head Up Displays
• [17]

[edit] Commercially Available EVS Cameras

• CMC Electronics IR Camera and systems
• Kollsman Enhanced Vision System
• Max-Viz IR Camera and systems