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ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 1
Synthetic Vision Systems – Terrain
Database, Symbology and Display
Requirements
Srikanth K P
Scientist, Flight Mechanics and Control Division National Aerospace Laboratories, Bengaluru, India
Dr Abhay A Pashilkar
Scientist, Flight Mechanics and Control Division National Aerospace Laboratories, Bengaluru, India
ABSTRACT
Synthetic Vision Systems (SVS) are designed to improve pilot’s situational awareness, thus lowering his workload. Synthetic Vision provides virtual out-of-window view of terrain and obstacles irrespective of weather conditions. SVS uses terrain databases and onboard sensors as inputs to render out-of-window cockpit view to the pilot. The dependability of synthetic vision is related to the accuracy of terrain elevation database and navigation data such as Differential Global Positioning System, Radar Altimeter etc. Sensors such as Radar Altimeter, Weather Radar can be used to monitor the integrity of the terrain databases. This paper provides an overview of SVS, sensors required to improve the reliability of such a system. A study of critical technologies such as synthetic database, flight symbology and display systems have been carried out. Tunnel in the sky symbology used in SVS displays have been studied. Accordingly, recommendations have been made regarding HUD FOV, accuracy and resolutions of synthetic database. A survey has been carried out regarding commercial SVS products that are available with state of art technology.
Keywords
Elevation Database, Head-Up-Display, Integrity Monitoring, Synthetic Vision System
Abbreviations
CFIT Controlled Flight Into Terrain
CRT Cathode Ray Tube
DEM Digital Elevation Model
DGPS Differential Global Positioning System
EVS Enhanced Vision System
FAA Federal Aviation Agency
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ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 2 GPS Global Positioning System
HDD Head Down Display
HFOV Horizontal Field of View
HSI Horizontal Situation Indicator
HUD Head Up Display
IESVS Integrated Enhanced and Synthetic Vision System
ILS Instrument Landing System
ISRO Indian Space Research Organisation
LCD Liquid Crystal Diode
LED Light Emitting Diode
LiDAR Light Detection and Ranging
NASA National Aeronautics and Space Agency, USA
NAV Navigation Mode
ND Navigation Display
NDB Non Directional Beacon
NRSA National Remote Sensing Agency, India
PFD Primary Flight Display
RADAR Radio Detection and Ranging
RTCA Radio Technical Commission for Aeronautics
SA Situational Awareness
SRTM Shuttle Radar Topography Mission
SVS Synthetic Vision System
TAWS Terrain Awareness Warning System
VFOV Vertical Field of View
VOR Very High Frequency Omni-Directional Radio Range
1. INTRODUCTION
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ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 3 Enhanced Vision Systems (EVS) and Synthetic Vision Systems (SVS) have the capability to permit pilots to take advantage of different image sources available. Data from imaging sensors are fused digitally in an EV system. This provides a clear view of external world even in impaired visibility conditions within the cone of visibility of the imaging sensors. SVS renders the image using a priori database depending on the current pilot view. This displays terrain and flight path information to the pilot, which is not possible with EVS. With the technology advancement, it is possible to obtain more accurate terrain and obstacle data for most part of world. The availability of low cost 3-D graphics cards facilitates simulation of external world as on a clear day. The ability of pilot to see in all directions, even in bad weather conditions provides substantial operational usefulness and safety benefits [2].
A detailed description of synthetic vision system is presented in Section 2. The synthetic vision systems that are commercially available are discussed in Section 3. The requirements for the development of a typical SVS are presented in Section 4. The recommendations of a typical SVS are provided in Section 5.
2. SYNTHETIC VISION SYSTEM
According to the definition of SVS by Federal Aviation Administration (FAA) [3], ―Synthetic Vision (SV) is a computer-generated image of the external scene topography from the perspective of the flight deck, derived from aircraft attitude, high-precision navigation solution, and database of terrain, obstacles, and relevant cultural features‖. NASA added more information to the SVS by augmenting it with flight display symbologies, data links and navigation systems [4]. These systems represent the visual cues as seen by a pilot in broad daylight.
Information from SVS and weather penetrating sensors or actual imagery from enhanced vision sensors are fused together to form Integrated Enhanced and Synthetic Vision System (IESVS) [5].
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ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 4 (TAWS) to be installed on most aircraft. SVS is developed for applications from being advisory to flight critical systems but TAWS are purely advisory in nature. The SVS with terrain data integrity monitoring would aid the pilot in avoiding CFIT which could be flight critical. As SVS is used to support decision making depending on terrain depiction, it is essential that terrain database is certified to flight critical levels.
Figure 1. Baseline Head-Down Display Figure 2. SVS Head-Down Display
Synthetic Vision can be classified as either advisory, strategic or as tactical applications [6]. Based on the application, SVS can be categorized as non-essential, essential and critical systems. The integrity levels are calculated as acceptance of probability of a failure which is not detected. Systems with undetected failure rate greater than 10-3 (per flight or per flight hour) are classified as advisory system applications. Systems with probability between 10-4 and 10-7 are termed as strategic essential applications. For flight-critical applications, the integrity levels are between 10-6 and 10-9. The benefits of SVS are discussed in the following section.
2.1 Benefits of Synthetic Vision System
The safety and operational benefits of a SVS are briefly explained in this section.
2.1.1 Safety Benefits
A SV system provides a visual representation of outside world resembling visual flight conditions [7]. This has the potential to warn about loss of attitude pathway and terrain awareness, traffic and altitude awareness, run way incursions, spatial disorientation etc, thus reducing loss due accidents. These benefits are visible during emergency conditions when pilot mental workload is high. SVS can enhance situation awareness of pilots, thus reducing his workload.
2.1.2 Operational Benefits
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ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 5 and proved that airlines can make huge savings on operational costs [7]. These savings are possible, provided other aiding technologies are developed and the units are to be certified for operational efficiencies offered by these technologies are to be analyzed further. SVS can provide operational benefits such as enhanced surface operations, decreased runway occupancy time during low visibility, reduce inter-arrival separations, improved path guidance and alerting mechanisms, enhanced flight management etc. A cost-benefit study conducted by NASA for 10 major US airports has predicted savings of about $2.25 billion for years 2006 to 2015 for the airliners.
SVS consists of Enhanced intuitive view, Hazard detection and display, Integrity Check and Alerting and Precision navigation guidance display. As explained earlier, SVS displays relevant and critical environment features of out of window visuals using computer generated terrain images even when weather conditions are inferior. As the pilot will see the display as he sees in clear day light environment, the display is termed as intuitive. Symbology can be added to the display to increase pilot’s awareness.
To maintain pilot’s SA and provide terrain and hazard separation, terrain, traffic, obstacles and other hazards are pictorially displayed. SVS provides pilot detection, identification, geometry awareness and overall SA which is not possible by standard avionics displays.
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ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 6 Figure 3. Block Diagram of a Downward Looking Integrity Monitoring System
SV elements such as surface guidance, tunnels/highways-in-the-sky, velocity vector etc allow pilot to correlate aircraft position to outside environment. These elements facilitate pilot to check navigation precision to meet Required Navigation Performance criteria without depending on land-based navigation aids. The components which form a SV system are discussed in next section.
2.2 Synthetic Vision System Components The components of a SV system are [7]:
- Synthetic Vision Database/Sensors - Synthetic Vision Displays
- Computers/Embedded Computational Functions - Equipment
- Associated Aircraft Systems
Database used can be generated statically and carried on-board or can be generated using Light Detection and Ranging (LIDAR). Sensors such as Weather Radar, Radar Altimeter, Global Positioning System (GPS) and other forward looking sensors such as Millimeter wave radar or Infra-Red Sensors can also be used.
The virtual out-the-window (OTW) view can be generated using accurate, ortho-rectified satellite imagery, airport details, elevation database and cultural features. Jeppesen provides the Aerodrome Mapping Database (AMDB) for most important airports in the world. High resolution terrain database has become a pre requisite for Low Level Flights, Terrain
-
p (ti)
hDEM(ti)
lonDGPS(ti)
Radar Altimeter
DGPS
DEM Database
T Algorithm
Test Statistic Algorithm
latDGPS(ti)
hDGPS(ti)
-hRA(ti)
+
-
hSYNT(ti)IJCSBI.ORG
ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 7 Following and Terrain Avoidance under Instrument Meteorological Conditions. Highly accurate SRTM terrain database is suitable for aviation use. The SRTM terrain database with 3-arc sec resolution is available in public domain. National Remote Sensing Agency (NRSA) has released CartoDEM data with accuracy of 3-arc second for Indian sub-continent region in public domain [9]. LiDAR system can be used to generate terrain databases. A LiDAR system has a scanning laser ranger, an Inertial Measurement Unit and a GPS receiver. The principle of LiDAR is similar to that of RADAR. Information from above sensors can be used to create synthesized terrain profiles. The precision of measurements can be as high as up to 20 centimeters. [10].
RTCA / DO-272 defines the accuracy and resolution specifications of a airport for a SV system [11]. The data requirements for an airport and obstacle data is indicated in Table 1. The accuracy is categorized as Fine, Medium or Coarse. The data accuracy of aerodrome shall meet the confidence level of 95% for Fine and 90% for Medium or Coarse quality categories.
Table 1. Accuracy and Resolution requirements of Obstacle Data as per RTCA
Region Area 1
The World
Area 2
Terminal Airspace
Area 3 –
Cat II/III Operation
Area
Horizontal Accuracy 50 m 5.0 m 2.5 m
Vertical Accuracy 30 m 3.0 m 1.0 m
Vertical Resolution 1.0 m 0.1 m 0.1 m
Data Integrity 10-3 10-5 10-5
Confidence Level 90% 90% 90%
Terrain database Post Spacing
3 arc second (~90 m)
1.0 arc second (~30 m)
0.3 arc second (~10 m)
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ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 8 2.3 Synthetic Vision Displays
The different types of Synthetic Vision Displays [7] are, - Head-Up / Helmet Mounted Display
- Primary Flight Display / Head Down Display - Navigation Display
Table 2. Accuracy and Resolution requirements of Terrain Data Region Area 1
The World Area 2 Terminal Airspace Area 3 CAT II/III Operations Airport Surface Horizontal Accuracy 50 m (90%) 5 m (90%)
2.5 m 0.5 m (95%) Vertical Accuracy 30 m (90%) 3 m (90%)
1 m 0.5 m
(95%) Post Spacing Integrity 3 arc-sec 10-3 1 arc-sec 10-5 1 arc-sec 10-5 20 m 10-5
2.3.1 Head-Up-Display / Helmet Mounted Displays
The Head-Up-Display (HUD) is used to improve position awareness and guidance during flight [12]. This is a see-through display projected onto the wind shield of the aircraft. Many important aircraft parameters such as air speed, velocity vector, rate of climb, aircraft attitude and position are projected on the HUD for quick reference to the pilot.
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ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 9 Figure 4. Areas of Coverage
Development of HUD technology can be categorized into four generations [12]. In the first generation, CRT technology was used to generate display image. This used phosphor screen which used to degrade over time. In the second generation HUDs, solid state light source such as LEDs are used to display the symbology. The advantage of this technology is that the symbols do not fade over time. This technology is adapted in commercial airliners. Optical wave guides are used to produce images in the combiner in third generation series. Scanning laser technology is used to display images in fourth generation HUDs.
Various factors to be considered during design of a HUD are field of view, eye box, luminance/contrast ratio, display accuracy and ease of integration to existing aircraft systems. FOV can be defined as Total FOV, Instantaneous FOV, Binocular FOV or Monocular FOV [13]. In this paper,
FOV means Total FOV defined by Horizontal and Vertical FOV. Figure 5 shows a generic HUD symbology set [14].
FAA has recommended HUD symbologies for Enhanced Flight Vision System in FAA Part No 91.175. The recommended symbols are Airspeed, Altitude, Horizon bar, Heading, Bank and Side Slip markers, Flight Path Markers or Velocity Vector, Glideslope and Localiser raw data path deviation indicators and Guidance information such as horizontal and vertical ball guidance cues.
Area 1
(World) Area 2
(Terminal Airspace)
Area 3
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ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 10 A generic SVS HUD symbology is shown in Figure 6. The HUD should have haloing effect so that the symbology is highlighted with scene imagery as background. The physical characteristics of the required dynamic range and the grey level resolution required must be considered during design of a HUD system [15]. Field trials were conducted by Advanced 3D Primary Flight Display System on Honeywell Citation V aircraft to identify the field of view (FOV) required for the HUD. For terminal operations, the preferred HFOV is about 45 degrees and 60 degree HFOV for en-route operations for generic HUD [16].
For SVS operations, NASA has recommended Head-Up Display FOV of 32 x 24 degrees in raster format is preferred [15]. As mentioned earlier, to highlight the symbology against scene imagery, ―haloing‖ effect is necessary. Overall HUD brightness and controls are to be provided for the pilot. The pilot can be provided with a de-clutter control switch.
Figure 5. Generic Head-Up-Display Symbology set
Pilot trials were conducted by NASA to find out appropriate display sizes and FOV for HUD and HDD SVS displays [17]. It was found that the path
performance of pilots does not vary with display size of HDD. A experimental size of 22 degrees VFOV by 28 degrees HFOV was set to
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ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 11 significant differences among SVS concepts. The study recommended a variable FOV depending on the phase of flight [4].
Figure 6.Generic SVS Head Up Display
Another study was conducted by NASA on retro-fitting HUD displays to non-glass cockpit aircraft. A SVS retro-fit was recommended along with existing head-down PFD. A SVS Navigational Display has been suggested to replace the existing HSI display [4]. The most important symbol integrated into SVS PFD and SVS HUD is the velocity vector. Along with this, the pathway or the highway symbology, explained below provides the pilot an awareness of current and future spatial situation.
2.3.2 Head Up Display Hardware
In Aircraft, HUD symbologies are realized by specialized hardware. ARINC 764 is the technical standard for HUD avionics [18]. This standard describes the physical form factors, dimensions, interface definitions and functionality of HUD. Two areas of focus are integration of Enhanced Vision System (EVS) and Synthetic Vision System functionality to HUD and develop alternatives to CRT image projection system for use in smaller aircraft. Initial displays were built with CRT projection to the combiner glass. Later this was replaced with LCD image source which provides a wider field of view. Airbus (A340-600), Boeing (B787) and Embraer (ERJ 190) aircraft have used LCD images source displays for HUD implementation.
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2.3.3 Additional SVS Head Down Display Symbologies
To aid the pilot, more symbologies have been evolved by research. Some of the symbologies researched were Flight Path Marker, Flight Director Guidance Markers, Pathway/Highway/Tunnel Marker and Pitch Ladder. The Flight Director Guidance and Tunnel markers are discussed in this section.
The guidance markers explored were the integrated cue circle (―ball‖) used in several HUDs, a ―follow me‖ aircraft concept (―ghost‖), and a ―tadpole‖ guidance symbol [4]. The tadpole symbology has been implemented in F-16 military aircraft HUD. It was found there were no statistically detectable differences between the symbols, although pilots favored tadpole symbol and the ghost airplane symbol over the ball guidance symbol. The ghost airplane symbol was preferred over the tadpole symbol due to the anticipatory information provided by the symbol. Each of these symbols is shown in Figure 7.
Figure 7. Guidance symbols: Integrated cue “Ball” (left), “Tadpole” (center) and Ghost aircraft
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ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 13 Pilots have evaluated four tunnel concepts – Box, Minimal, Dynamic Crow’s foot and Dynamic Pathway along with no tunnel display [23]. Box tunnel, as defined before, consists of a series of boxes with the corners connected, which forms the boundary path likely to be flown by the pilot. The tunnel displayed can be of length of 10nm with five segments per nautical mile. Five segments of tunnel per nautical mile are displayed for a distance of three nautical miles. The symbology fades away to invisibility gradually. Pilots get a feedback about the aircraft position with respect to the tunnel. The tunnel walls grow on increase of path error which helps pilot gauge the deviation. In this concept, when the aircraft flies outside the tunnel, the tunnel opens up on the side where the aircraft leaves the tunnel indicating the pilot to fly into the tunnel. The pilots were found to prefer dynamic crow’s feet over other symbologies [24]. This symbology is found to reduce workload of pilot during landing tasks and complex maneuvers in helicopters too [25].
Figure 8. Conventional tunnel in the sky concept
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2.3.4 Primary Flight Displays
On the flight deck, Attitude Direction Indicator is displayed on a Primary Flight Display (PFD) [23]. The PFD also contains other critical information such as calibrated airspeed, altitude, heading, attitude and vertical speed. If a Horizontal Situational Indicator (HSI) is displayed on the PFD, then this display is known as Navigation Display. The PFD integrates important analog instruments which can improve pilot’s SA during flight. This display can also alert then pilot during harmful situations such as low airspeed, high rate of descent etc by generating audio signals.
To improve situational awareness of pilots, more functionality is added to the normal PFD display and can be denoted as SVS display. In addition to the existing symbology, this can display synthetic terrain, command guidance indicating the possible path to fly, horizontal and vertical path deviations, velocity vector etc. [27].
2.3.5 Navigation Display
A Navigation Display (ND) provides lateral position information of the aircraft. Depending different modes such as ILS, VOR or NAV, it displays a compass rose or an arc of the compass rose. Lateral flight plan as well as additional points like NDBs, VORs, airports are displayed on the ND. The next section explains some of the commercial SVS products available in the industry today.
3. COMMERCIAL OFF THE SHELF SVS PRODUCTS
This section describes the SVS products developed by various aviation majors. It is found that, Honeywell Inc and Universal Avionics, two major companies from USA are supplying SVS related products. Thales, Garmin, Chelton Systems and Elbit Systems have also come up with certified SVS displays.
Some of the certified SVS Systems along with a brief explanation of capabilities of the product is explained below:
- SmartView Synthetic Vision System from Honeywell System
SmartView is a revolutionary product to increase safety and situational awareness. SmartView is currently available on Gulfstream aircraft equipped with the PlaneView™ cockpit, and Dassault aircraft featuring the EASy flight deck. [28]
- Universal Avionics Systems Corporation’s Vision-1+ SVS (FAR Part 23 and 25 Supplemental Type Certificates)
Terrain and runways are displayed using 15-arc second data resolution worldwide and 6-arc second data for airport area [30]
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ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 15 Garmin’s SVT displays 3D terrain, obstacles and traffic on G1000 PFD simulating pilot’s outside the cockpit view during clear weather conditions. This unit has implemented Highway-In-The-Sky guidance symbology also [31]
- Thales’ Helicopter Flight Vision System (H-SVS)
This system provides enhanced situation awareness to approach, with confidence, unfamiliar airports in all weather conditions. The display is intuitive, real-time image is provided during all phases of flight, higher mission reliability and lower rate of missed approaches. This system meets the latest FAR 91.175 requirements and has been implemented On Bombardier Global XRS AND Global 5000 Business Aircraft [32] - Genesis Aerosystems’ (Previously known as Chelton Flight System) 3D
Synthetic Vision System
Synthetic vision with three-dimensional Highway-In-The-Sky navigation (FAR Part 23 Supplemental Type Certificate)
- Rockwell Collins’ Helisure Helicopter Synthetic Vision System (H-SVS)
This displays high-resolution terrain and identified obstacle database with a resolution of 3-arc second [33]
- Elbit Systems’ Mission Safety Equipment Package (MSEP) for the C-130 and ORIA Integrated Display System [34]
- Aspen Avionics’ Evolution Synthetic System which renders 3D computer generated terrain with obstacles and traffic as viewed from cockpit by pilot [35]
4. REQUIREMENT SPECIFICATIONS FOR THE
IMPLEMENTATION OF SVS
i. Elevation Database Resolution: SVS terrain depiction can match the actual terrain environment if high DEM resolution data used. Elevation reference (average elevation, maximum elevation or elevation of the geometric center of the area) which is related to DEM resolution has to be chosen to determine the manner of assigning elevation values to the cells in the DEM for terrain depiction. The data base resolution requirements [23] depend on the flight phase. Experiments conducted by NASA indicate that although one and three arc second resolutions are preferred, adequate situation awareness can be obtained by pilot from a 30-arc second database also
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ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 16 way. The challenge in rendering arises as representation of realistic terrain requires high resolution terrain databases while real-time rendering requires optimized terrain models. Methods for adaptive terrain meshing and depiction have been explored [28].
iii. Storage: Avionics quality storage devices are readily available for SVS database applications. Typical storage requirements for a 1º by 1º cell (approximately 60 square miles at the equator) [4] are of the order of 5 MB for DTED Level 1 data, 54 MB for DTED Level 2 data, 6.3 GB for DTED Level 4 data. [4]
iv. Texture / Color / Shading: To convey terrain information to pilot, Terrain coloring and shading techniques have been found to be very effective. NASA research has demonstrated that colored terrain portrayal techniques convey more information than constant color terrain displays. Two of the commonly used texturing methods are: (i) elevation-based color-coding with generic texturing of the DEM (ii) ortho-rectified photographic imagery overlays on the DEM ("photo-realistic"). Other enhancements that could be tried include: (a) coloring bands with each band representing a 100 foot change in
elevation to show the height of the terrain
(b) Shading, texturing and shadowing to avoid the obscuration of important terrain features by shadows due to the light source positioning
(c) Hybrid textured format, created by false-color coding monochromatic imagery (aerial photographs) of the flight test areas of interest
While creating synthetic vision photo-realistic terrain database, color balancing of imagery and time of year are important aspects to be addressed.
v. Data base creation: A typical database model which can be used for studies can be defined as below:
a. To generate an airport database, the airport surface needs to be surveyed, especially the runway markings, lightings, buildings on the runway vicinity. The markings can be accurate up to one foot to carry out low visibility operations
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ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 17 c. Varying multi-resolution imagery with appropriate color balancing
can be overlaid on DEM to depict SV photo realistic terrain database d. Cultural and obstacle features needs to be included for more realistic
rendering
vi. HUD considerations:
a. Use of a grid pattern for a SV-HUD terrain representation alone or in addition to generic-textured, photo-realistic textured or hybrid-textured databases in the SV-HUD
b. Available HUD luminance and resultant contrast ratios for imagery content characteristics
vii. Other Graphics Issues:
a. Use of a compressed version of DEM to avoid ―Terrain popping‖ which can be due to rounding off errors
b. Eliminate coordinate transformation issues by storing databases by latitude and longitude position coordinates, instead of storing in other traditional coordinate systems
c. Rendering differences while using a ―flat-earth‖ or ―spherical earth‖ approximations
5. RECOMMENDATIONS
Head-Up-Display (HUD) symbologies improves the pilot’s situation awareness especially during landing tasks under bad weather conditions. The typical FOV is 32 x 24 degrees.
A typical Head-up-Display is shown in Figure 10. The reticles in this display have been realized using FAA Part 91.175 as reference. This can be used as HUD or can be superimposed on the Synthetic Vision displayed on a head-down display. A super-imposed Synthetic Vision with Head-Up-Display symbology is shown in Figure 11. This can be used as part of pilot evaluation of HUD / HDD symbology using simulation platforms.
The synthetic database specifications of the simulator at NASA Langley Research Center which is used for ESVS studies are:
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ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 18 3D models of airport area such as runway and terminal buildings are derived from aerial photographs of about one feet accuracy.
Figure 10. Recommended HUD symbology as per FAA Part 91.175 requirements
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ISSN: 1694-2108 | Vol. 15, No. 5. SEPTEMBER 2015 19 Figure 11. Synthetic Terrain with recommended HUD symbology for SVS operations
6. CONCLUSION
A literature survey for Synthetic Vision Systems has been carried out. The importance of SVS, its safety and operational benefits and various components were studied. Different types of SV displays were deliberated. A baseline SVS HUD and HDD display symbology has been developed using FAA recommendations. Field tests of different field of views of HUD have been elaborated. A study of tunnel-in-the-sky symbology and its different implementations has been carried out. Certified SV Systems available with state of art technology have been identified and listed. Finally, the requirement specifications for implementation of a typical synthetic system such as database resolution, rendering, storage space etc. are elaborated.
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