Head-up display

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For other meanings of Hud, see HUD.

A head-up display, also known as a heads-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 Display 3.1 Focus 4 Civil Aircraft Applications 4.1 Symbology 4.2 Installation 4.3 Enhanced Flight Vision Systems 4.4 Synthetic Vision Systems 5 Automotive applications 6 Experimental uses 7 Further reading 8 Notes 9 External links 10 Commercially Available HUDs

[edit] History

Head-up displays were first pioneered 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).^[1]

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.^[2]

[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.

[edit] Display

A typical HUD in civil aircraft contains three primary components: A computer, Overhead Projector Unit (OPU), and a combiner. 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. Flight data is received from the inertial reference system, flight management system, and other flight guidance systems, and processed into a form compatible with the Overhead Projector Unit^[3]. The OPU takes this data and projects it onto the combiner. The combiner is usually made of glass with a special coating that reflects the monochromatic light from the OPU while allowing all other wavelengths of light to pass through, creating a superimposed image.

Tactical military aircraft usually rely on a projection unit incorporated into the combiner.

Traditionally, the source for the projected image has been a Cathode Ray Tube (CRT), but micro-display imaging technologies are now 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. ^{[4] [5]}

[edit] Focus

To avoid refocusing of the user’s eyes while reading a HUD, the display is focused at infinity in aircraft, while in automobiles the display is generally focused around the distance to the bumper.

[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. ^{[6] [7] [8]}

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 pilots 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.^[9] For example, if the aircraft is pitched up but is loosing 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 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 a HDD is not standarized 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 ^[10] to the actual horizon (at lower altitudes. Because of the curvature of the earth at higher altitudes the display horizon line will be above the visual horizon.)

[edit] Enhanced Flight Vision Systems

In more advanced systems, such as the FAA-labeled Enhanced Flight Vision System, ^[11] a real-world visual image can be overlaid onto the combiner. Typically a multiband infrared camera is installed in the nose of the aircraft to display a conformed image to the pilot.

[edit] Synthetic Vision Systems

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.^{[12] [13] [14]}

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 - a fact not lost on those developing Synthetic Vision Systems.

On the left side of the display is an SVS-unique symbol, what looks like a purple, dimishing sideways ladder, 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 slected an airport to their 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 dimentional flying is required, such as the decreased vertical or horizontal clearance requirements of RNP. Under such conditions the pilot is given an graphical depiction of where the aircraft should be and where it should be going rather then the pilot having to mentally integrate altitude, airspeed, heading, energy AND longitude and latitude to correctly fly the aircraft. ^[15]

[edit] Automotive applications

Head-up displays are becoming increasingly available in production cars, and usually offer speedometer, tachometer, and navigation system displays. BMW, Nissan, Lexus, Citroën and GM currently offer some form of HUD system. Motorcycle helmet HUDs are also commercially available.^[16]

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 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.^[17]

[edit] Further reading

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

[edit] Notes

1. ^ Pope, Stephen. "The Future of Head-Up Display Technology." Aviation International News. Jan. 2006. 12 Feb. 2007 [1]
2. ^ "Oldsmobiles Pace "the Race"" Oldsmobile Club of America. 2006. 12 Feb. 2007 [2]
3. ^ This is different between manufactures and also can be dependent on the technology used in the OPU.
4. ^ "Virtual Retinal Display (VRD) Technology." Virtual Retinal Display Technology. Naval Postgraduate School. 13 Feb. 2007 [3].
5. ^ Lake, Matt. "How It Works: Retinal Displays Add a Second Data Layer." New York Times 26 Apr. 2001. 13 Feb. 2006 [4].(registration required)
6. ^ ORDER: 8700.1 APPENDIX: 3 BULLETIN TYPE: Flight Standards Handbook Bulletin for General Aviation (HBGA) BULLETIN NUMBER: HBGA 99-16 BULLETIN TITLE: Category III Authorization for Parts 91 and 125 Operators with Head-Up Guidance Systems (HGS); LOA and Operations EFFECTIVE DATE: 8-31-99[5] (Document)
7. ^ Falcon 2000 Becomes First Business Jet Certified Category IIIA by JAA and FAA; Aviation Weeks Show News Online September 7, 1998
8. ^ Design Guidance for a HUD System is contained in Draft Advisory Circular AC 25.1329-1X, "Approval of Flight Guidance Systems" dated 10/12/2004 [6]
9. ^ "Forces in a Climb" NASA Glenn Research Center [7]
10. ^ Note that in this case the word "conformal" has been taked to mean "when an object is projected on the combiner and the actual object is visible, they will be aligned." The displayed horizon line and the actual horizon for example. When Enhanced Vision is used, the display of runway lights must be alighted with the actual runway lights when the real lights become visible.
11. ^ DOT Docket FAA-2003-14449-45 [8] Enhanced Flight Vision Systems.
12. ^ Part 23 Synthetic Vision Approval Approach; FAA Synthetic Vision Workshop, Lowell Foster, Feb 14 2006. [9]
13. ^ For additional information see Evaluation of Alternate Concepts for Synthetic Vision Flight Displays with Weather-Penetrating Sensor Image Inserts During Simulated Landing Approaches, NASA/TP-2003-212643 [10]
14. ^ No More Flying Blind, NASA [11]
15. ^ "A Review of Pathway-In-The Sky Displays, FAA Presentation to 2003 Digital Avionics System Conference Synthetic Vision Workshop, Dick Newman, 15 February 2006[12]
16. ^ Mike, Werner. "Test Driving the SportVue Motorcycle HUD." Motorcycles in the Fast Lane. 8 Nov. 2005. 14 Feb. 2007 [13]
17. ^ Clothier, Julie. "Smart Goggles Easy on the Eyes." CNN.Com. 27 June 2005. CNN. 22 Feb. 2007 [14]

[edit] External links

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

[edit] Commercially Available HUDs

• Trivisio Monocular
• Microvision
• MicroOptical Consumer Electronics
• Olympus Prototypes
• Rockwell Collins Head Up Displays