February 2012 

NASA/TM–2012-217340 

Conceptual Design Standards for eXternal 

Visibility System (XVS) Sensor and Display 

Resolution

Randall E. Bailey, Susan J. Wilz, and Jarvis (Trey) J. Arthur III 

Langley Research Center, Hampton, Virginia 
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February 2012 

NASA/TM–2012-217340 

Conceptual Design Standards for eXternal 

Visibility System (XVS) Sensor and Display 

Resolution

Randall E. Bailey, Susan J. Wilz, and Jarvis (Trey) J. Arthur III 

Langley Research Center, Hampton, Virginia 
Available from: 

NASA Center for AeroSpace Information 

7115 Standard Drive 

Hanover, MD 21076-1320 

443-757-5802 

Acknowledgments 

This work was conducted under the Fundamental Aeronautics Program, Supersonics 

project. The support of the Principal Investigator, Mr. Peter Coen, is gratefully 

appreciated.

This report documents the conceptual design standards required for the development of 

an eXternal Visibility System to support a low-boom/no-boom supersonic aircraft. This 

work builds on a significant body of work that was conducted previously under NASA’s 

High Speed Research (HSR) program and extends it based on technology and 

programmatic efforts completed since that time. References are cited where appropriate, 

although much of the HSR work was not be fully disseminated due to limited release 

restrictions in place at that time. The many individuals who contributed toward the HSR 

eXternal Visibility System element are too numerous to cite. However, the contributions 

of Russ Parrish, Mike Lewis, Lynda Kramer, R. Michael Norman, Steven Harrah, Steve 

Williams, Lou Glaab, George Boucek, Rich Edwards, Mike Johnson, and Jerry Rousch 

merit specific acknowledgement.

The use of trademarks or names of manufacturers in this report is for accurate reporting and does not constitute an 

official endorsement, either expressed or implied, of such products or manufacturers by the National Aeronautics 

and Space Administration.
 i 

Table of Contents

Abstract .............................................................................................................................. 1

1. Introduction ............................................................................................................... 2

2. Background ................................................................................................................ 4

3. Design Standards Basis ............................................................................................. 7

4. Human Vision Equivalence ...................................................................................... 9

5. Visual Acuity for “See-and-Avoid” ........................................................................ 15

5.1 Resolution Requirements Based on See-and-Avoid and Johnson’s Criteria ..... 18

5.2 Adequacy of See-and-Avoid Based on FAA AC90-48 ..................................... 21

6. Visual Acuity for “See-to-Follow”.......................................................................... 26

7. Technology Trends and Associated Research Needs............................................ 28

7.1 XVS Proof-of-Concept Evaluation .................................................................... 32

8. Concluding Remarks ............................................................................................... 35

9. References................................................................................................................. 36

Appendix A ...................................................................................................................... 39
 ii 

List of Tables

Table I: Snellen Acuity Related to Display Resolution ................................................... 11

Table II: Johnson's Criteria - Resolution Requirements .................................................. 17

Table III: Recognition and Reaction Times – AC90-48C ............................................... 18

Table IV: See-and-Avoid-Based XVS Resolution Requirements .................................... 20

Table V: Time and Distance to Achieve 500 ft Separation in Head-On Scenario ........... 23

Table VI: See-to-Follow-Based XVS Resolution Requirements ...................................... 27
 iii 

List of Figures 

Figure 1: Conceptual Low-Boom Supersonic Aircraft Configuration .............................. 2

Figure 2: TU-144 Drooped Nose Deployed ....................................................................... 4

Figure 3: Clear Area of Vision as per AC25-773 .............................................................. 5

Figure 4: Snellen Acuity Related to Display Resolution ................................................. 10

Figure 5: Visual Acuity and FOV vs. Display Size, Using NTSC Resolution ................ 12

Figure 6: Visual Acuity and FOV vs. Display Size, using HDTV Resolutions .............. 13

Figure 7: Probability of Detection and Recognition vs. System Resolution (from 

Ratches, 1997)................................................................................................................... 17

Figure 8: God’s Eye View of Worse-Case Head-On See-and-Avoid Scenario ............... 19

Figure 9: Dimensions of Aircraft in See-and-Avoid Analysis ......................................... 20

Figure 10: God's Eye View of Final Encounter Position ................................................. 21

Figure 11: God's Eye View of Encounter Geometry ....................................................... 23

Figure 12: TIFS Flight Test HDTV XVS Display ........................................................... 29

Figure 13: FL5 Flight Test XVS Prototype Display ........................................................ 29

Figure 14: IBM T221 Quad-HD Resolution Display ...................................................... 31

Figure 15: Quad-HD Camera ........................................................................................... 32

Figure 16: Quad-HD XVS Proof-of-Concept System ..................................................... 32

Figure 17: XVS Proof-of-Concept Quad-HD End-to-End Latency Test ......................... 34

Figure A-1: Visual Target Detection - Expansion With Horizontal Trajectories ............ 40

Figure A-2: Visual Target Detection - Departure Level-Off ........................................... 42

Figure A-3: See-to-Follow Equivalent Visual Operation ................................................ 44

Figure A-4: See-and-Avoid / See-to-Follow Performance – Parallel Runway Ops ........ 46
 iv 

Nomenclature 

AC Advisory Circular 

AFL Above Field Level 

ANSP Air Navigation Service Provider 

C Contrast 

CFR Code of Federal Regulations 

DERP Design Eye Reference Point 

FAA Federal Aviation Administration 

EFVS Enhanced Flight Vision System 

EV Enhanced Vision 

EVO Equivalent Visual Operations 

EVS Enhanced Vision System 

FAR Federal Aviation Regulation 

FOV Field of View 

H Horizontal 

HD High Definition 

HDTV High Definition Television 

HSCT High Speed Civil Transport 

HSR High Speed Research 

IFR Instrument Flight Rules 

IMC Instrument Meteorological Conditions 

IOD Image Object Detection 

KIAS Knots, Indicated Air Speed 

L Luminance 

LCD Liquid Crystal Display 

MSL Mean Sea Level 

MTF Modulation Transfer Function 

NTSC National Television Standards Committee 

NASA National Aeronautics and Space Administration 

ppd pixels per degree 

SV Synthetic Vision 

SVS Synthetic Vision System 

TIFS Total In-Flight Simulator 

UAV Uninhabited Air Vehicle 

V Vertical 

VFR Visual Flight Rules 

VMC Visual Meteorological Conditions 

XVS eXternal Visibility System 

(·)min Minimum Value 

(·)max Maximum Value 
1

Abstract

NASA is investigating eXternal Visibility Systems (XVS) concepts which are a 

combination of sensor and display technologies designed to achieve an equivalent level 

of safety and performance to that provided by forward-facing windows in today’s 

subsonic aircraft. This report provides the background for conceptual XVS design 

standards for display and sensor resolution.

XVS resolution requirements were derived from the basis of equivalent performance. 

Three measures were investigated: a) human vision performance ; b) see-and-avoid 

performance and safety; and c) see-to-follow performance. From these three factors, a 

minimum but perhaps not sufficient resolution requirement of 60 pixels per degree was 

shown for human vision equivalence. However, see-and-avoid and see-to-follow 

performance requirements are nearly double. 

Two areas are identified for additional investigation: a) the need to verify and validate 

these analytically-derived values by in-flight testing; and, b) the development of XVS 

display and sensor resolution requirements using modulation transfer function-based 

criteria as a measure of optical performance. 

This report also reviewed historical XVS testing. The trends in commercial video 

systems are very favorable such that emerging technology has the potential to meet the 

needs of an XVS system. Proof-of-concept testing has identified potential limitations. 
2

1. Introduction

A successful low boom supersonic aircraft design drives the shaping and configuration of 

the vehicle. One such conceptual configuration is shown in Figure 1. As evident in this 

figure, the forward visibility for the flight crew is severely compromised as a result of the 

vehicle shaping.

The National Aeronautics and Space Administration (NASA) is performing fundamental 

research, development, test and evaluation of flight deck and related technologies which 

may provide the required pilot visibility for these low-boom, supersonic configurations 

by use of an eXternal Visibility System (XVS). XVS is a combination of sensor and 

display technologies designed to achieve an equivalent level of safety and performance to 

that provided by forward-facing windows in today’s aircraft. In this memorandum, 

conceptual standards for “equivalent performance and safety” are offered as a basis for 

the design of an XVS. Further, testing results for sensor and display resolution are 

briefly reviewed. 

Figure 1: Conceptual Low-Boom Supersonic Aircraft Configuration 

The XVS conceptual design standards are only notional and imply nothing more, since a 

formal basis for certification of an XVS has not yet been established by the Federal 

Aviation Administration (FAA) or other certification authority. However, these notional 

standards are offered using current FAA regulations and advisory material with support 

from previous NASA research and other related research areas. Using these notional 

standards, further research needs are identified by which to verify and validate these 

requirements. 

This report provides the background and summarization of notional XVS design 

standards for display and sensor resolution. Citations are used for textbooks and 

analytical and simulation models where more detailed factors associated with these 

requirements (e.g., visual acuity, sensor acuity and other factors) are necessary. This 
3

report is intended to summarize the pacing requirements and technologies while 

identifying the research and technology development required to produce validated and 

verified XVS acuity requirements to support low-boom Supersonic Aircraft emerging 

within the 2015-2025 time frame. 

Two significant requirements which interact with the sensor and display resolution in an 

XVS design are not addressed in this work – Field-Of-View (FOV) and display 

collimation. These issues are discussed but standards are not provided. These should be 

addressed in forthcoming XVS research.
4

2. Background

The Concorde and TU-144 supersonic aircraft used a mechanism which drooped the 

aircraft nose to enable pilot visibility from forward-facing windows for terminal 

operations including takeoff, landing, departure, arrival, and surface operations. For an 

aircraft of the class anticipated for the High Speed Civil Transport (HSCT) aircraft, the 

maximum takeoff weight of a fixed-nose configuration design using an XVS was 3.2% 

lower than that of a drooped-nose configuration. This substantial weight difference 

created a significant cost-benefit for the development of an XVS (Andrastek, 1999). 

Figure 2: TU-144 Drooped Nose Deployed 

Without an XVS or a drooped nose, the pilot of a low-boom supersonic aircraft cannot 

see in front of the aircraft by using their natural vision, since the fuselage obstructs 

extensive portions of the view where transparent windows would normally be located. 

Such a design would violate Chapter 14 of the Code of Federal Regulations (14 CFR) 

Part 25.773 which stipulates that “the flight deck windshield must provide sufficient 

external vision to the pilot to safely perform any maneuvers within the operating limits of 

the aircraft.” FAA Advisory Circular (AC) 25-773 further instantiates this requirement 

as stipulating a clear area of vision, as shown in Figure 3.
5

Figure 3: Clear Area of Vision as per AC25-773 

In addition, the FAA requires under 14 CFR Part 91.113, across all classes of airspace,

that “when weather conditions permit, regardless of whether an operation is conducted 

under instrument flight rules or visual flight rules, vigilance shall be maintained by each 

person operating an aircraft so as to see and avoid other aircraft.” The lack of outside 

visibility due to the fuselage obstructing significant areas required to be “clear” would be 

further grounds to deny certification.

Even if certification of this configuration were to be awarded, the lack of forward 

visibility by the pilot would – at best – severely restrict aircraft operations and airspace 

usage. For instance, it might be possible to receive special handling and stipulate that all 

flights and flight operations are flown under Instrument Flight Rules (IFR). The rationale 

would be that an aircraft without sufficient pilot visibility is essentially the same as one 

flying during Instrument Metrological Conditions (IMC).

From an operational standpoint, the supersonic aircraft without an XVS or drooped noise 

would be incapable of flying under Visual Flight Rules (VFR) and would require “special 

handling.” Both of these conditions are operationally problematic, for example: 

The lack of a VFR capability would prohibit operations to certain airspace 

classes and airports (e.g., see Drumm et al, 2004) where operational 

limitations around some airports and facilities cannot provide continual 

IFR services, such as those located in mountainous regions where radar 

coverage is not available.

The lack of a VFR capability can create inefficient flights during actual 

VFR conditions since pilots can no longer “cancel IFR” and go direct to 

their destinations or accept a visual approach clearance. IFR aircraft 

spacing and routing is dictated by Air Navigation Service Provider 

(ANSP) regulations and procedures.
6

“Special handling” by the ANSP necessitates that the ANSP is integral to 

the planning and execution of all flights. The Concorde required “special 

handling” in the United States to meet certification requirements. While 

this was viable for the fourteen Concorde aircraft that operated worldwide, this scenario is not economically viable for any reasonable volume 

of supersonic aircraft.

Finally, emerging Next Generation Air Transportation System (NextGen) 

concepts run counter to the concept of “special handling.” These concepts 

are exploring ways to optimize traffic flow without direct ANSP control 

over flight operations and shifting some traditional ANSP roles and 

responsibilities for aircraft separation to the flight deck.

Significant research was conducted under NASA’s High Speed Research (HSR) program 

during the 1990s on the design and development issues associated with an XVS for a 

conceptual HSCT aircraft. What emerged from this work – which still holds true today – 

is that the key challenge for an XVS design exists during VFR operations and when it is 

assumed that flight crew have natural visibility (even though they may be operating on an 

IFR flight plan).

The driving XVS design standards emerge from the three tenets of VFR operations: 

1. “see-and-avoid” 

2. “see-to-follow” 

3. “self-navigation”

These VFR operational requirements apply to all aircraft, not just to low-boom 

supersonic aircraft. The issues and requirements for these other vehicles are discussed in 

the following sections as their experiences are applicable to the design of an XVS. 

As one example, an emerging challenge - being brought forth in the evolution of 

NextGen - is the concept of Equivalent Visual Operations (EVO). The EVO concept is 

notionally the ability to achieve or even improve on the safety of current-day VFR 

operations, maintain the operational tempos of VFR, and even perhaps, retain VFR 

procedures - all independent of the actual weather and visibility conditions. While EVO 

capability is not required for a successful supersonic aircraft, the technologies and 

operational requirements to develop EVO are in many ways equivalent to XVS. As such, 

current research in the use of synthetic and enhanced vision systems and other interface 

modalities as enabling technologies to meet the EVO operational concept are pertinent to 

this work.

EVO technologies strive to create VFR operations for today’s aircraft when flying in 

actual IMC whereas XVS technologies try to enable an IMC cockpit (i.e., low-boom 

supersonic aircraft) to operate under VFR.
7

3. Design Standards Basis 

Design standards are drafted in the following on the basis that the XVS must provide 

equivalent levels-of-performance and safety to that of a pilot’s natural vision through a 

forward-facing window.

First, equivalent levels of performance and safety emerge from “human vision 

equivalence.” Secondly, equivalent levels of performance and safety can be derived from 

the three tenets of VFR operations – “See-to-follow,” “See-and-Avoid,” and “SelfNavigation.” These VFR tenets are engrained in the FAA regulations and advisory 

materials. As such, these materials and associated analyses are used as a basis from 

which to define design standards for XVS display and sensor resolution.

While three tenets to VFR operations must be met - based on past HSR and other 

research - the driving XVS requirements for sensor and display resolution are those 

necessary to enable “see-to-follow” and “see-and-avoid” operations. “Self-navigation” 

has not been found to be a driving factor and is not included in these standards 

development (e.g., see Summers, 1998). Self-navigation, in this context, includes the 

ability to identify and fly with respect to visual flight references (such as navigation with 

respect to cultural objects - roads, rivers, large man-made structures, etc) and the ability 

to conduct visual approaches, landings, and take-off operations. For example, flight 

testing has shown that pilots can safely conduct approach and landing operations, (i.e., 

“self-navigate”) even with severely restricted forward fields-of-view and minimal 

resolution (e.g., Perry, Dana, and Bacon (1967), Layton and Dana (1966), and Gaidsick, 

Dana, and McCracken (1969)) These works show that the tenet of “self-navigation” does 

not drive the resolution requirements of an XVS. 

The XVS design standards are derived using the following principles: 

1. XVS design standards are first derived by requiring equivalent performance to 

that provided by the human visual system. These requirements are briefly 

reviewed in Section 4.

2. Additional XVS design standards are established by assuming “equivalent” visual 

acuity from advisory material associated with “the pilot’s role in collision 

avoidance” – that is, “see-and-avoid.” Regulatory guidance is employed in 

defining XVS requirements for display and sensor resolution based on “see-andavoid” in Section 5.

3. Visual acuity requirements based on the capability to “see-to-follow” are 

presented in Section 6. “See-to-follow” is a capability - not a regulatory 

requirement. Therefore, advisory material does not exist, but one might consider 

that this requirement is derived from 14 CFR Part 25.1301 – satisfying the 

intended function of an XVS as being “functionality equivalent to that of a 

forward-facing window” and thus, enabling VFR operational capability. 

Background information for functional requirements for an XVS can also be found in 

Summers (2008). These functional requirements were derived during NASA’s HSR 
8

program, using similar methodology. The HSR functional requirements are more 

comprehensive than those presented in the following; however, these HSR-XVS 

requirements are colored by operational assumptions invoked at that time and they are 

also predicated by the technologies and operational concepts prevailing and foreseen at 

that time (circa 1999). The requirements given in the following reflect the present 

regulatory and operational environment and should be considered an update to these 

HSR-XVS requirements. 
9

4. Human Vision Equivalence 

Various factors influence the perceived image quality of an electro-optical sensor and 

display system including (Holst, 2008):

Sensor Performance 

- Resolution 

- Sensitivity 

- Noise

- Output/Input Transformation 

Display / Monitor 

- Resolution 

- Luminance 

- Contrast

- Distance from Observer 

Scene Content 

- Target Characteristics 

- Background Characteristics 

- Lighting 

- Motion 

- Clutter 

Atmospheric Transmittance 

- Haze

- Fog

- Rain

- Dust

- Clouds

- Etc.

All of these factors are, of course, critical in the assessment and evaluation of an XVS 

image. However, only the effects of resolution for the sensor and display systems are 

considered directly in the following as they pertain to the ability of an XVS to provide 

equivalent performance and safety to that of forward-facing windows. 

Human vision “equivalent performance” is impossible to summarize in one or two 

concise requirements. Human vision performance is described in numerous textbooks 

(e.g., Westheimer, 1972); the details of which are not repeated here. Nevertheless, some 

basic standards associated with human visual performance are investigated as XVS 

design standards for display and sensor resolution.

It should be noted that the effect of the aircraft windows are neglected in this analysis.

While some optical distortion and vision modifications due to the transparencies are 

always present, these effects must be minimal during aircraft certification (see Chapter 14 

Federal Aviation Regulations Part 25.773) and are therefore, not considered a significant 

factor. In addition, testing during the HSR program showed that the influence of the 
10 

transparencies on human vision performance was in fact negligible (Quinn, Larson, 

Roush, and Johnson, 1996).

The most commonly used, but not necessarily sufficient, measure of human visual acuity 

is provided by Snellen-type acuity tests. This standard is most often invoked as an XVS 

design standard. The rationale is simple: a) it is easily understood; and b) it is one for 

which virtually everyone has been tested; so familiarity is ensured. 

Display system performance is evaluated by viewing a Snellen acuity target (and others 

analogous to it, e.g., Landolt C) through the XVS sensor and display. Equivalent human 

visual acuity is said to occur if the user is able to demonstrate 20/20 vision (or 6/6 for 

metric assessments) – see Figure 4. 

By this measure, normal human visual acuity equates to the ability to resolve one arc 

minute. As a display image is created by pixels, it is often convenient to express visual 

acuity in terms of the number of pixels to define a degree (analogous to dots per inch 

(dpi) in printing). Thus, a display system resolution of less than 60 pixels per degree 

(ppd) would mean that the display is the “limiting” resolution – i.e., it cannot provide 

resolution at least equal to that of human vision. To relate Snellen acuity to display and 

sensor resolution, display resolutions which are “equivalent” to Snellen acuity values are 

listed in Table I. 

Figure 4: Snellen Acuity Related to Display Resolution 

To illustrate how this acuity test relates to XVS design standards, US broadcast television 

display standards are compared. (In these examples, the camera and display are assumed 

to be “loss-less” and they show their native resolutions without a magnification or 

minification - i.e., a so-called “unity” or “conformal” field-of-view - whereby the camera 

field-of-view subtends the same visual angle as the display when viewed from the 

observer eye point. The observer is assumed to sit 25 inches from the display.) 

In Figure 5, the effect of changing the field-of-view with a constant resolution 640x480 

video system is shown. The field-of-view is changed because a different monitor size is 

used as the display device while conformality is maintained. The abscissa shows the 

1 arc min

2 arc min

20/20 Letter 2 arc min 

per cycle

60 arc min

per degree

60 pixels

per degree

2 pixels 

per cycle

1 arc min

2 arc min

20/20 Letter 2 arc min 

per cycle

60 arc min

per degree

60 pixels

per degree

2 pixels 

per cycle
11 

diagonal dimension of the display. The data also shows the horizontal and vertical FOV 

displayed. (Note that this example does not use the 525 line resolution National 

Television Standards Committee (NTSC) video standard. Instead, Video Graphics Array 

(VGA) resolution of 640 horizontal (H) x 480 vertical (V) video is assumed to be 

equivalent for convenience.)

Table I: Snellen Acuity Related to Display Resolution 

Snellen Acuity Min. Angle of 

Resolution (arc minute) 

Pixels per Degree 

Resolution 

20 / 200 10 6.0 

20 / 160 8 7.5 

20 / 125 6.3 9.5 

20 / 100 5 12.0 

20 / 80 4 15.0 

20 / 60 3.2 18.8 

20 / 50 2.5 24.0 

20 / 40 2 30.0 

20 / 30 1.6 37.5 

20 / 25 1.25 48.0 

20 / 20 1 60.0 

20 / 16 0.8 75.0 

20 / 12.5 0.63 95.2 

20 / 10 0.5 120.0 

The display resolution varies between 30 and 15 ppd for the 640x480 display system. 

This display system provides the Snellen-equivalent resolution of between 20/40 (12 inch 

diagonal) and 20/80 (25 inch diagonal monitor). The higher acuity is provided by the 

smaller display size because of the higher pixel density, providing only 20 degrees fieldof-view. As the display surface increases in size, the same number of pixels (640x480) is 

now spread over a larger field-of-view. Thus, when this video is displayed across almost 

40 degrees field-of-view, the pixel density drops to 15 ppd; hence, the poorer visual 

acuity.
12 

Figure 5: Visual Acuity and FOV vs. Display Size, Using NTSC Resolution 

High Definition Television (HDTV) video examples are shown in Figure 6. In these 

examples, HDTV is portrayed using 1920 H by 1080 V pixel resolution. The increased 

display resolution from HDTV provides between 81 and 40 ppd for monitors sized 

between 12 inch and 25 inch diagonal. These displays provide a Snellen-equivalent 

acuity better than 20/15 (15 inch diagonal monitor) and better than 20/30 (25 inch 

monitor). This does not imply that a person with 20/20 vision will be able to have 20/15 

vision by using this system. Rather, the system has enough resolution to accurately 

display a scene with 20/15 acuity. This resolution improvement over VGA clearly 

highlights the attraction for the HDTV revolution in the US. The data shows that 20/20 

Snellen acuity is provided by a “loss-less” HDTV system if the display is 16 inch 

diagonal (or smaller), subtending ~20o

 V x 30o H FOV. 

NTSC (4:3) Resolution

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

0 5 10 15 20 25 30 35

Diagonal (inch)

H (FOV) deg

V (FOV) deg

Resolution (pixel/deg)
13 

Figure 6: Visual Acuity and FOV vs. Display Size, using HDTV Resolutions 

Snellen acuity must be considered a necessary, but perhaps not sufficient, prerequisite for 

the XVS display and sensor design standard. As mentioned before, Snellen acuity is just 

one measure of human vision performance. For instance, the test employs “full” contrast 

targets; that is, the test uses a fully lit black “target” with a white background. The 

Snellen acuity represents the spatial frequency at which the observer’s eye can no longer 

discriminate differences in the light and dark transitions in the image. Human visual 

acuity and discrimination associated with color and variations of contrast are not 

considered at this time. 

XVS acuity should optimally be evaluated using Modulation Transfer Function (MTF) 

testing. Display contrast (C) is defined as:

 

  max min

max min

L L

L L C



 

where Lmax is the maximum luminance and Lmin is the minimum luminance of the 

displayed imagery. The MTF measures the displayed image contrast compared to the 

input image contrast as a function of spatial frequency (Holst, 2008). Because the eye is 

normally considered to require an image contrast of approximately 3% in order to 

distinguish the light-to-dark transitions in the bars, the limiting resolution using a 

Snellen-type acuity test can be considered as the spatial frequency at which the MTF has 

fallen to a value of three percent. Thus, the Snellen acuity test represents just one point on 

a MTF. The MTF would capture the contrast sensitivity performance of the XVS display 

and sensor system. 

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

0 5 10 15 20 25 30 35

Diagonal (inch)

HDTV (16:9) Resolution

H (FOV) deg

V (FOV) deg

Resolution (pixel/deg)
14 

Ideally, the MTF should be used as a basis for XVS display system performance 

evaluation. At this time, however, MTF criteria for XVS applications have not been 

established.

Even if 20/20 Snellen acuity is measured, other human vision capabilities suggest much 

higher pixel densities than the 60 ppd Snellen-equivalent are necessary (Hooper, 2000). 

Glint, vernier line comparisons, etc. suggest pixel densities as high at 1500 ppd might be 

required. Thus, XVS resolution and system performance to achieve 20/20 Snellen acuity 

or better (60 ppd) must be considered a minimum but perhaps not sufficient XVS 

prerequisite. Other test parameters should also be considered (e.g., see Sweet and 

Giovannetti, 2008 and Winterbottom et al, 2004). 

Even if Snellen-equivalent performance is demonstrated, it may be difficult to prove

human vision equivalence, especially since some human vision performance capabilities 

far exceed that required for Snellen vision equivalence (Hooper, 2000). Functional 

performance requirements, such as those in Sections 5 and 6, may be necessary to 

demonstrate equivalent performance. 
15 

5. Visual Acuity for “See-and-Avoid”

The flight rules prescribed in Part 91 of the Federal Aviation Regulations (FARs) set 

forth the concept of “see-and-avoid.” See-and-avoid serves three primary functions 

(Australian Transport Safety Board, 1991): 

1. Self-separation of aircraft outside controlled airspace 

2. As a separation procedure for VFR aircraft in control zones, where the pilot is 

instructed to see and avoid another aircraft. This procedure only operates when 

the pilot can see the traffic and is therefore significantly different to other types of 

see-and-avoid which may involve unalerted searches for traffic. 

3. Last resort separation if other methods fail to prevent a conflict, regardless of the 

nature of the airspace. 

The merit of see-and-avoid operations and the associated human visual, human attention, 

and operational issues have been studied extensively (e.g., see Australian Transport 

Safety Board, 1991; Graham, 1989). Whether see-and-avoid actually is effective or not is

immaterial. The FAA requires under 14 CFR Part 91.113 across all classes of airspace

that “when weather conditions permit, regardless of whether an operation is conducted 

under instrument flight rules or visual flight rules, vigilance shall be maintained by each 

person operating an aircraft so as to see and avoid other aircraft.”

For certification of an XVS, a formal safety analysis would be performed to quantify the 

required equivalent levels of safety for see-and-avoid. This type of safety analysis to the 

see-and-avoid problem is currently being performed by the Uninhabited Aerial Vehicle 

(UAV) community (e.g., see Kuchar, 2004). Their work is of great interest to an XVS 

design problem, but to apply this analysis directly to an XVS design, is difficult at best.

The difference is that the UAV community is trying to develop “sense-and-avoid” 

whereby “see-and-avoid” is provided by a combination of sensors and computer 

processing, not a human pilot. Thus, the difficulty arises primarily from trying to 

sufficiently quantify and validate XVS performance as it impacts the “visual acquisition” 

component of this analysis. In sense-and-avoid, the process is deterministic (i.e., the 

sensor either can or cannot acquire and track other traffic to avoid). In see-and-avoid, the 

unquantifiable capabilities (and limitations) of the human are involved. 

The visual acquisition task in see-and-avoid is, in essence, a “target detection” problem 

of which a wealth of military data and research are available. Target detection research 

shows that the probability that a target will be recognized (by a human observer) is a 

function of:

the probability that the observer is searching an area that is known to contain a 

target, looks with his/her foveal vision for a specified glimpse time (l/3 s) in the 

direction of the target; and, 

the probability that the displayed target image is viewed foveally for one glimpse 

period with sufficient contrast and size to be detected;
16 

In the development of these XVS design standards, the issues associated with how and 

how efficiently a pilot “visually scans” a display is ignored even though it is an important 

design issue (e.g., see Boff and Lincoln, 1998). For instance, the amount of time to 

visually scan a larger field-of-regard to locate a target is theoretically longer than 

scanning a smaller field-of-regard. 

Also, these analyses do not consider the role of surveillance alerting that can alter and/or 

improve visual search importance, vigilance, and localization (e.g., where surveillance 

alerting might be generated from sources such as Air Traffic Control surveillance radar or 

Traffic Collision Alerting Systems). Data suggest that an alerted search is eight to nine 

times more successful in visual acquisition than an un-alerted search (Andrews, 1991 and 

Boff and Lincoln, 1988). 

The analyzes shown here only consider if a visually displayed target image is of 

sufficient contrast and size for visual detection. This component of human visual target 

acquisition is shown to be directly proportional to the target angular size and the apparent 

target-background luminance ratio (Boff and Lincoln, 1988). Thus, the resolved target 

contrast and size are the only determinants used for establishing the required XVS 

display and sensor resolution.

A series of experiments (Ratches et al, 1997) attempted to quantify the target detection 

probability as a function of the resolution of the sensor/display system (i.e., the target 

angular size). This work showed that, whether using an unaided eye or other sensors, 

such as night-vision goggles or thermal imaging systems, four tasks were involved in the 

target acquisition process: 

1) Detection: correctly discriminating an object in the image from background 

and system noise. 

2) Orientation: correctly determining the detected target aspect or direction of 

movement. 

3) Recognition: correctly determining the class membership of the target. For 

example, is it a truck? A tank? etc. 

4) Identification: correctly determining the exact identity of the target, e.g., for 

automobiles, is it a Ford or Chevrolet? 

These same and similar processes in detection, recognition, and identification for air-toair target acquisition tasks have also been found (e.g., see Rohrer, 1996).

Experimental data led to the development of Johnson’s criteria for target acquisition 

(Ratches et al, 1997). Johnson’s work presumed that the ability of the observer to detect, 

recognize and identify a target were a function of how well a critical dimension of the 

target could be resolved by the sensor-display system. An example of the data is shown in 

Figure 7, indicating the probability of detection and recognition (of a man or a vehicle) is 

a function of the number of sensor-display cycles (sensor/display line pairs) depicting the 

object.
17 

Figure 7: Probability of Detection and Recognition vs. System Resolution 

(from Ratches, 1997). 

Johnson’s work evolved into straight-forward criteria for display and sensor requirements 

for visual acquisition as shown in Table II. The “critical target dimension” was based on 

intuition and was usually chosen to be the minimum dimension. 

Table II: Johnson's Criteria - Resolution Requirements

Task Line Resolution per Minimum 

Dimension

Detection 1.0 ± 0.25 line pairs 

Orientation 1.4 ± 0.35 line pairs 

Recognition 4.0 ± 0.8 line pairs 

Identification 6.4 ± 1.5 line pairs 

These criteria only pertain to the image size of the visual target detection task; that is, the 

target angular size and the associated resolution of the display and sensor to generate the 

image for human visual acquisition. The other component of the visual acquisition task – 

the apparent target-background contrast - is dependent upon many other factors such as 

atmospheric conditions (clouds, fog, haze, moisture content, etc.), target color/material, 

background illumination and color, sun angle, background (sky/ground), etc, and 

sensor/display contrast performance. Even though these are critical issues to the task, 

these are not influenced by the display resolution directly and therefore, are not

considered herein.

Johnson’s criteria has been shown to be a rough, first-order approximation as numerous 

analyses have shown it to be flawed in several respects (McDonald and Vorst, 2002). 

Other, more elaborate analyses may more accurately predict human detection 
18 

performance and should be referenced (e.g., see Aviram and Rotman, 2000 and 2001, 

Vollmerhausen, Jacobs, and Driggers, 2004); however, for the comparative analyses in 

the following, Johnson’s criteria should be sufficient as it provides an intuitive measure 

for display and sensor resolution effects on visual target acquisition.

5.1 Resolution Requirements Based on See-and-Avoid and Johnson’s Criteria 

Johnson’s criteria are used with FAA advisory circular material to derive XVS resolution 

requirements to meet see-and-avoid precepts. The objective is to provide a rough order 

of magnitude assessment of XVS resolution requirements for equivalent safety and 

performance levels in the absence of a formal safety analysis. 

Under FAA AC90-48C - Pilot’s Role in Collision Avoidance – pilot recognition and 

reaction times are published for detection, decision, and evasive action in a see-andavoid, mid-air collision scenario. These times are shown in Table III. 

Table III: Recognition and Reaction Times – AC90-48C 

Recognition and Reaction Tasks Times 

See Object 0.1 sec 

Recognize Aircraft 1.0 sec 

Be Aware of Collision Course 5.0 sec 

Decision to Turn Left or Right 4.0 sec 

Muscular Reaction 0.4 sec 

Aircraft Lag Time 2.0 sec 

Total: 12.5 sec 

The recognition and reaction times stipulated in Table III are applied along with 

Johnson’s criteria (Table II) to develop see-and-avoid XVS display resolution 

requirements using the following guidelines and assumptions: 

1. From Table II, the number of pixels required for detection, orientation, 

recognition, and identification are equal to 3, 4, 9, and 14, respectively (i.e., 

approximately twice the number of line pairs plus one).

2. From Table III, a minimum of 12.5 seconds is needed between when detection of 

an aircraft is made and a mid-air collision can be averted. This function 

corresponds to the detection task in accordance with Johnson’s criteria (Table II).

This time defines the distance between two aircraft on a collision course which, of 

course, depends on their closure speeds and geometry. The XVS resolution is 

determined by this distance and the fact that 3 pixels of the XVS sensor/display 

system must resolve the target aircraft. 

3. From Table III, the pilot has a minimum of 6.4 seconds before the collision to 

recognize that the two aircraft are on a collision course and maneuvering is 

required to avert the collision. This function corresponds to the orientation task in 

accordance with Johnson’s criteria (Table II). Depending upon the closure rate 

between the aircraft, this time defines the distance between the two aircraft. The 
19 

XVS resolution is determined by this distance and the fact that 4 pixels of the 

XVS sensor/display system must resolve the target aircraft. 

A worst case, head-on mid-air collision scenario is assumed (Figure 8). 

Figure 8: God’s Eye View of Worst-Case Head-On See-and-Avoid Scenario

The most demanding mid-air scenarios requiring see-and-avoid occur in uncontrolled 

airspace below 10,000 ft Mean Sea Level (MSL) (e.g., see Drumm et al, 2004).

Assuming worst-case, ownship and the “target aircraft” are flying at 250 knots indicated 

airspeed (KIAS) in opposite directions at 10,000 ft altitude generating a closure rate of 

578 knots.

Three general classes of aircraft “size” were arbitrarily assumed for this analysis. These 

are shown in Figure 9. These aircraft were picked to demonstrate the influence of target 

size on the “see-and-avoid” problem. They scale by approximately a factor of 2. Note 

that the critical dimension under Johnson’s criteria is the minimum distance (aircraft 

height for the head-on collision scenario), but in this work, both vertical and horizontal 

(wing span) extent of the target aircraft are used. Other analyses use the target crosssectional area (e.g., Andrews, 1991).

Under these assumptions, the XVS resolution requirements are shown in Table IV. 

Detection

(in 12.5 s)

Orientation

(in 6.4 s)
20 

Figure 9: Dimensions of Aircraft in See-and-Avoid Analysis 

With a head-on closure in this analysis, detection of the target aircraft has to occur 

slightly inside of 2 nmi separation to meet the guidance of AC90-48C. Recognition has 

to occur slightly inside of 1 nmi separation. These separation distances are not 

unreasonable. They generally meet the expectations of subject matter experts (i.e., 

experienced intercept pilots, Rohrer, 1996).

Table IV: See-and-Avoid-Based XVS Resolution Requirements 

Detection Orientation 

Vehicle Range 12210 ft 6252 ft 

Size Resolution Reqr’d 3 pixels 4 pixels 

Big Height 14.4 ppd 9.8 ppd 

Span 5.1 ppd 3.5 ppd 

Medium Height 28.8 ppd 20.0 ppd 

Span 9.7 ppd 6.7 ppd 

Small Height 71.8 ppd 50.0 ppd 

 Span 17.7 ppd 12.1 ppd 

The XVS resolution requirements, based on Johnson’s criteria, show that the detection 

task is the most demanding. 

The XVS resolution requirements also show that the required resolution in pixels per 

degree (ppd) is highest for the smallest vehicle. This result is intuitively obvious. The 

data show that a C-172-sized aircraft on a head-on collision course would require more 

“Big” (B-757-200)

“Medium” (ERJ-145)

“Small” (C-172)

“Small” 36.1 ft 8.9 ft 27.2 ft

“Medium” 65.8 ft 22.2 ft 98.0 ft

“Big” 124.8 ft 44.5 ft 178.6 ft

Aircraft Span Height Length

“Class”

“Small” 36.1 ft 8.9 ft 27.2 ft

“Medium” 65.8 ft 22.2 ft 98.0 ft

“Big” 124.8 ft 44.5 ft 178.6 ft

Aircraft Span Height Length

“Class”

Span

Height
21 

than 70 ppd resolution for the pilot to correctly detect the target on the XVS in time to 

avert a mid-air. This result would initially seem peculiar based on a C-172 flying at 250 

KIAS. However, Very Light Jets, such as the Eclipse, are not radically different in size 

from a C-172 (or military jets for that matter). So the results are relevant. 

5.2 Adequacy of See-and-Avoid Based on FAA AC90-48 

An alternative analysis is performed to assess the FAA’s published timeline for evasive 

action in the see-and-avoid/collision avoidance task.

Under emerging standards for “sense-and-avoid” systems (ASTM, 2007), sensing must

be sufficient that “detection of the collision threat shall be at a range to allow a resolution 

maneuver that results in a required miss distance of 500 ft or greater.”

In this analysis, the same “worst-case” head-on collision scenario is assumed but a 500 ft 

collision resolution requirement is imposed. XVS resolution requirements are again 

derived and the implications of this requirement can be judged against FAA AC90-48C. 

Two aircraft – ownship and an intruder – are flying in opposite directions at co-altitudes.

Both ownship and the target aircraft are flying at 250 KIAS at 10K ft (578 knot closure 

rate). The vehicle sizes are varied as per Figure 9. Ownship maneuvers to avoid the midair collision but the target aircraft does not.

The analysis is conducted in reverse sequence for simplicity. The 500 ft separation 

distance is required and is used as the final time. The preceding times (and distances) for 

ownship to achieve this separation standard are calculated as shown in the following 

figure. The final condition is sketched in Figure 10. 

Figure 10: God's Eye View of Final Encounter Position 

To achieve this final position, constant altitude turns using 3 values of normal 

acceleration for ownship are used as a separation maneuver. The normal acceleration 

values are 1.25g, 1.50g, and 2.0g – not atypical for transport category aircraft. For 

constant altitude, constant speed conditions, the turn rate and turn radius are computed 

Intruder

Ownship

500 ft
22 

from kinematics. Ownship is flying at 250 KIAS which equates to 488 ft per second true 

airspeed (v). The bank angle () required to achieve the target normal acceleration values 

are:

g levels Bank Angle ()

1.25 g 37 deg 

1.50 g 48 deg 

2.00 g 60 deg 

By kinematics, the respective turn rates (	), equal to 

v

g tan()

, are as follows: 

g levels Turn Rate (	)

1.25 g 2.8 deg/sec 

1.50 g 4.2 deg/sec 

2.00 g 6.5 deg/sec 

With the respective turn radii (R), equal to tan( )

2

g 

v , being: 

g levels Turn Radius (R)

1.25 g 9850 ft 

1.50 g 6613 ft 

2.00 g 4270 ft 

The final geometry for the separation between ownship and the non-maneuvering target 

is sketched in Figure 11. A 500 ft radius circle is assumed around the target at the final 

position. Ownship has been turning at a constant altitude, constant speed, and constant 

turn rate. The turn rate and radius of turn, R, depended upon the g-level of the 

separation, as shown in the data above. 
23 

Figure 11: God's Eye View of Encounter Geometry 

For this final position, the two circles are tangential to each other. Two circles with 

centers at   1 1 x , y and   2 2 x , y with radii of r1 and r2, are mutually tangent if 

    2

1 2

2

1 2

2

1 2 x  x  y  y  r  r (Weisstein, 2009). For the target aircraft, the 

separation circle is x1 = y1 = 0 with r1 = 500 ft. Knowing that y2 = R, the position x2 is 

derived as  2 2

x2  500  R  R .

The heading angle (
) that ownship must turn to create this separation geometry is 

computed as arctan(x2/R). The time to compute this turn is computed using the turn rate 

for each g-loading, as follows: 

g levels Turn Angle (
) Time to Turn 


1.25 g 17.9 deg 6.3 sec 

1.50 g 21.6 deg 5.1 sec 

2.00 g 26.5 deg 4.0 sec 

To simplify the analysis, it was assumed that 3 seconds prior to starting this separation 

maneuver, the ownship aircraft begins rolling to the target bank angle (either 37, 48, or 

60 degrees) but without generating any turn rate or lateral separation. This assumption is 

pessimistic and admittedly not realistic; however, it makes the calculations trivial. 

The time from the beginning the rolling maneuver to achieve a 500 ft separation and the 

distance between ownship and a non-maneuvering target aircraft is given in Table V.

Table V: Time and Distance to Achieve 500 ft Separation in Head-On Scenario

g-Level Time to Complete Distance Between 

r2= 500 ft

3 seconds

Time Required To 

Transverse Angle 


3 sec

r1 = R 


Top view – not to scale

x

y

(x2,y2)

(x1=0,y1=0)
24 

Separation 

Maneuver 

Aircraft

1.25 9.3 sec 9237 ft (1.5 nm) 

1.5 8.1 sec 8096 ft (1.3 nm) 

2.0 7.0 sec 7061 ft (1.2 nm) 

This simple analysis shows that between 9.3 and 7.0 seconds are required to achieve 500 

ft separation in a worst-case, mid-collision scenario. The data of Table V also show that 

the higher g-levels create the required separation in shorter times – as one would logically 

expect. 

More importantly, these times are significantly longer than the 2.4 seconds required in 

AC90-48C (Table III) to begin the separation maneuver. At best, 4.6 seconds more time 

is needed to create the 500 ft separation than stipulated in AC90-48C. This extra 4.6 

seconds – for a total of 17.1 seconds for the see-and-avoid process - equates to 4494 ft 

additional separation between the two aircraft required in the detection and orientation 

phase. These new see-and-avoid distances suggest that that the XVS resolution would 

have to be closer to 98 ppd or 84 ppd for successful detection and orientation, 

respectively, of the “small” aircraft’s most critical dimension (instead of the 71.8 ppd and 

50.0 ppd shown in Table IV).

Formal safety analysis, based on various sources to mathematically model and predict 

human and sensor performance (i.e., determine a probability of detection) in “see-andavoid” environments, should be performed. These types of analyses (for instance, a MidAir Collision Assessment Tool (MARCAT®)) have generated probabilistic risk 

assessment, providing objective data on collision scenarios and risk mitigating avoidance 

strategies (see Schaefer, 2004).

The analyses of Schaefer (2004) suggest that the human vision process for see-and-avoid 

is inadequate to prevent mid-air collisions. This work and others (Drumm et al, 2004) 

suggest that 12.5 seconds may not be sufficient for detection and maneuvering to create a 

500 ft separation. The analyses performed herein and the data of Table V corroborate 

these modeling efforts. 

While the inadequacies of human vision (and vigilance) for see-and-avoid are welldocumented (Australian Transport Safety Board, 1991; Graham, 1989), it should be 

noted, however, that the analytical models of Schaefer (2004) are not supported by 

accident data statistics. The accident statistics are not as bad as implied by the analytical 

tool results. 

In summary, the see-and-avoid analyses show that: 

See-and-avoid is problematic – human observers are neither very efficient nor 

continually vigilant – but the accident data, especially with the advent of 

automatic surveillance and alerting services, such as Traffic alert and Collision 

Avoidance System, and increased pilot training and awareness does not indicate 
25 

that it is problematic. In any event, see-and-avoid is a regulatory requirement as 

per 14 CFR Part 91.113. 

Target detection primarily depends upon the displayed target size and contrast 

compared to its background and the atmospheric conditions through which the 

target is observed. A simple, rough order of magnitude approximation to quantify 

an observer’s ability to detect a target, determine its orientation and direction of 

flight, recognize its type, and identify the target is provided by Johnson’s criteria 

(Table II). 

AC90-48C offers times required for target detection and determining its collision 

course in time to avert a mid-air collision (Table III). A worst-case, head-on midair course was assumed and, using Johnson’s criteria, shows that 72 ppd for XVS 

camera and display resolution are required. 

Analyses for a head-on collision also shows that the times shown in AC90-48C 

are optimistic. To create a 500 ft separation, a target should be detected almost 17 

seconds – not 12.5 seconds - before the collision point. The XVS resolution to 

detect the target at this range is almost 98 ppd. This requirement, however, is 

unsubstantiated by present accident data statistics or certification advisory 

material. 
26 

6. Visual Acuity for “See-to-Follow”

“See-and-avoid” is a regulatory requirement applicable to all aircraft, in all airspace 

classes. On the contrary, “see-to-follow” is an operational capability – one of the tenets 

of VFR operations. It might be possible to operate an XVS-equipped aircraft strictly on 

an IFR basis; however, its economic viability could be marginal. For this reason, the 

display/sensor resolution requirements for “see-to-follow” operations are briefly 

examined in the following. 

The applicability of Cockpit Display of Traffic Information (CDTI) for aiding pilot 

separation in this task is not analyzed in the following analyses. Obviously, these 

electronic aides (e.g., Prinzo and Hendrix, 2003, McAnulty and Zingale, 2004, and Bone 

et al, 2004) would significantly ease the task, at a minimum, and might eliminate the need 

for natural vision in the future. But since these new operations are not yet operational nor 

can it always be assured that these types of non-visual separation procedures would be 

used, the “worst-case” scenario of using actual vision (albeit provided by an XVS 

display) will be analyzed.

Minimum separation and spacing requirements for today’s “see-to-follow” operations can 

be derived from current wake turbulence standards and procedures. As per US FARs, a 

pilot, when sshe has accepted ANSP instructions to visually follow another aircraft, is 

responsible to maintain in-trail separation and to accept responsibility for wake 

turbulence separation. Standard separation from other than a “heavy/B757” aircraft is 3 

miles. Wake turbulence separation of a small aircraft from a “heavy/B757” is 5 miles. 

Johnson’s criteria is again used as a rough, first-order approximation to the visual task 

requirements for “see-to-follow.” Unlike the visual target acquisition task, the corollary 

between Johnson’s criteria using detection, orientation, recognition, and identification 

phases (Table II) to the see-to-follow task is not obvious. 

In this analysis, the following assumptions are used: 

The search is likely a directed search; that is, the flight crew knows the general 

direction in which to look for the traffic. This directed search is most often 

obtained from direct ANSP radio communication (e.g., “follow traffic on base, 3 

miles at your 2 o’clock, level at two thousand”). Since directed searches are up to 

9 times more effective than un-alerted searches (Andrews, 1991 and Boff and 

Lincoln, 1988), the detection task is not included in this analysis. (It is assumed 

to be successful.) In addition, this task may occur at distances much greater than 

the 3 to 5 mile separations. 

The flight crew, focused on a detected object, must correctly determine that the 

object is an aircraft and it must correctly determine its trajectory. This task 

should correspond to Johnson’s “recognition” task. 

Following recognition, the flight crew should positively identify that the aircraft it 

is following is indeed the correct aircraft. This task should correspond to 
27 

Johnson’s “identification” task, whereby the flight crew determines that the 

aircraft is a C-172, and not a Beech King Air. 

Under these assumptions, the XVS resolution requirements are derived by using the 

worst-case requirements – the smallest aircraft at the greatest distances. These are 

approximated by using the “small” C-172 dimensional data at a 3 mile separation 

standard and the “big” B-757 dimensional data at a 5 mile separation standard (see Figure 

9). The results of these analyses are shown in Table VI. 

Table VI: See-to-Follow-Based XVS Resolution Requirements

Recognition Identification 

Vehicle Range 15840 ft 15840 ft 

Resolution Reqr’d 9 pixels 14 pixels 

C-172 Height (8.9 ft) 300 ppd 467 ppd 

Length (27.1 ft) 90 ppd 140 ppd 

 Span (36.1 ft) 69 ppd 108 ppd 

Recognition Identification 

Vehicle Range 26400 ft 26400 ft 

Resolution Reqr’d 9 pixels 14 pixels 

B-757 Height (44.5 ft) 90 ppd 140 ppd 

Length (178.6 ft) 23 ppd 36 ppd 

 Span (124.8 ft) 33 ppd 52 ppd 

The minimum dimension for both a B-757 and C-172 is its height. Using these numbers, 

the XVS resolution requirements to recognize and identify a C-172 at 3 miles requires 

300 to 467 ppd, respectively. These resolution requirements are 5 and 8 times the 60 ppd 

equivalent of Snellen visual acuity. Pilot opinion suggests that a C-172 cannot be seen 

head-on at 3 miles. Consequently, these resolution requirements don’t seem to 

reasonably approximate human vision equivalent. They far surpass them and hence, 

would unreasonably tax the design requirements for an XVS. 

More likely, the recognition and identification requirements for see-to-follow depend on 

the perception of the length and span of the aircraft. A target aircraft, once off of the pure 

head-on/tail-on aspect, begins to show significant visual area (i.e., its wing and body).

Using the smaller dimension of the length or span, the worst-case XVS display resolution 

requirement requires between 69 to 140 ppd to recognize and identify a C-172 at 3 miles. 

The B-757 at 5 miles requires less display resolution since its much larger visual area 

more than offsets the additional 2 miles of range between it and the ownship vehicle. 
28 

7. Technology Trends and Associated Research Needs

The preceding analyses show that: 

a) 60 ppd may meet a Snellen visual acuity equivalence but other human visual 

properties require higher resolution. 60 ppd and Snellen equivalence to 20:20 

vision should be an absolute minimum requirement. 

b) Assuming see-and-avoid scenarios and applying associated AC material, higher 

resolutions, nearing 100 ppd, are required; and,

c) See-to-follow capabilities require even higher display/sensor resolutions 

because greater distances are involved. A rough, first-order approximation shows 

that up to 140 ppd is required to meet see-to-follow operations. 

Under the HSR program, designs to achieve 60 ppd XVS display were investigated.

Methods to achieve these resolutions were extremely difficult, given the hardware 

available at that time. 

HDTV equipment was in its infancy during the time period of the HSR program. 

Prototype HDTV equipment was procured and flight-tested. As shown in Figure 12, a 

Cathode Ray Tube HDTV monitor, weighting approximately 175 lbs, was installed in the 

Total In-Flight Simulator (TIFS) flight test vehicle. The monitor was 22.75" wide x 

12.75" high, providing approximately 49o

 H x 29o V field-of-view from a 25" design eye 

reference point (DERP) view. This monitor provided approximately 40 ppd. With this 

resolution, a Snellen acuity of approximately 20/30 should be yielded. Field-tests of 

Snellen acuity showed between 20/40 and 20/50 performance. The differences between 

theoretical and actual acuity is likely due to imperfection in the video performance of the 

HDTV monitor and camera systems – i.e., the system was not “loss-less.” MTF tests 

were not performed. (In addition to the forward field-of-view HDTV monitor, an inboard FOV monitor was also installed to meet foreseen FOV requirements – see 

Summers, 1998.) During this flight test, see-to-follow or see-and-avoid tasks were not 

performed. The evaluations primarily emphasized self-navigation tasks. Self-navigation 

with this Snellen acuity was not a problem. 
29 

Figure 12: TIFS Flight Test HDTV XVS Display 

Following this flight test, a prototype XVS was flight-tested. This prototype hardware is 

shown in Figure 13. The desired 60 ppd resolution display was met, albeit only over a 

small field-of-view. The overall size of the display met HSR field-of-view requirements. 

The monitor provided 40o

 H x 50o V field-of-view from a 25" DERP, using three (3) 

tiled, 1280x1024 projectors with a rear-screen projection. The 60 ppd inset was only 21o

H x 17o V – smaller than today’s typical Head-Up Display field-of-view. The display 

resolution outside of this inset was only about 33 ppd. Symbology was overlaid on the 

XVS video by use of chroma-keying. 

Figure 13: FL5 Flight Test XVS Prototype Display 

During this flight test (the so-called “FL5” flight test) a fairly elaborate series of see-tofollow and see-and-avoid flight test maneuvers were flown. Unfortunately, significant 

portions of the data, especially the human performance aspects of this test were not 

analyzed because the HSR program ended just at the conclusion of the FL5 flight test.

However, this work did feed Image Object Detection (IOD) algorithm development as 

part of the “see-and-avoid” functionality for HSR (see Gandhi et al (2003) and Yang 

Circa 1999: 60 pixels/deg Inset

Projector-Inset 

Projector-Bottom 

Projector-Top 
30 

(2000)) which continues today as essentially part of the UAV “sense-and-avoid” 

program. 

This flight test also provided valuable development and validation of flight test 

maneuvers to operationally characterize and assess the performance of an XVS (or other 

vision-based system devices) for see-and-avoid and see-to-follow operations. In 

Appendix A, the most pertinent of these tests maneuvers are described. 

Since the NASA HSR program, display technology has significantly progressed in areas 

which impact resolution. These advances have primarily been driven from four thrusts. 

1. HDTV has emerged as a commercial standard. As such, HDTV resolutions in 

many monitor sizes are now available at very affordable pricing. 

2. Video refresh rates for these HDTV systems are often designed to meet the 

demanding video-gaming / entertainment user, not just the 24 Hz cinematic 

requirement. 

3. The medical industry is moving toward eliminating film and using digital record 

archival. This change necessitates that digital storage and display resolutions 

approach that of X-Ray films. 

4. The cost of Field Programmable Gate Arrays and Digital Signal Processors has 

dropped by orders of magnitudes and programming of these devices is now a 

relatively easy and inexpensive task. Custom video hardware and software 

applications are now an “everyday” event so affordable merging, tiling, and 

seamless integration of video streams is no longer a technological hurdle. It’s just 

an engineering task.

The state-of-the-art in today’s technology is so-called “Quad-HD” – that is, four times 

HDTV resolution or 3840x2400 pixels. Numerous venders are releasing commercially 

available Quad-HD monitors in various sizes, but so far, none have been in the XVS 

required form-factor. 

In 2001, a limited run of Quad-HD resolution monitors – the IBM T221 (Figure 14) – 

was made. The flat panel displays provide over 90 ppd resolution across an approximate 

40o

 H x 26o V FOV, when viewed from 25 inches DERP. 
31 

Figure 14: IBM T221 Quad-HD Resolution Display 

The T221 monitor came in different versions and was sold by various vendors. The 

versions differed by their installed video cards and digital video interfaces which 

influenced the resultant resolution and associated refresh rates. The native refresh rate 

was reported to be 48 Hz. The refresh rates could vary from 13 Hz up to 48 Hz. An 

Iiyama AQU5611D liquid crystal display, which is based on the IBM T221 Model 9503-

DG3 monitor, was used as the display for an XVS proof-of-concept evaluation. 

Under Department of Defense funding, two cameras were specially built to drive the IBM 

T221 monitor directly, exercising their native resolution. This Quad-HD camera (Figure 

15) utilizes custom field programmable gate array circuitry to generate and drive the IBM 

display with real-time video. Unfortunately, MTF or other similar test data have not been 

found documenting the results of using this camera and display system. 

If the native resolution of the display and a “loss-less” video system is assumed, better 

than 20/15 Snellen acuity could be provided by this system. This monitor and display 

system would nearly meet the projected XVS requirements for see-and-avoid and see-tofollow. 

One limitation with this camera and display combination for an XVS application is the 

inability to insert head-up symbology onto the video. The camera directly drives the 

display so chroma-keying (i.e., symbology insertion) cannot be easily performed. The 

camera would have to be mounted behind a collimated display (e.g., a Head-Up Display) 

to create overlaid symbology. 
32 

Figure 15: Quad-HD Camera 

7.1 XVS Proof-of-Concept Evaluation

An alternate means of creating an XVS display using the T221 Quad-HD monitor was 

developed. The goal was to quickly assess the problem of adding symbology while 

achieving display resolutions near those required for an XVS. 

Four separate HDTV video cameras (Sony Model EV1-HD1) were used to drive the four 

HDTV inputs to the Quad-HD monitor. (For laboratory testing purposes at this time, the 

problem of aligning these four separation cameras to make a seamless single image is not 

important to this analysis.) 

To add symbology to this monitor, four HDTV frame grabbers (Foresight Accustream 

170) were installed into a PC. The sketch of this system is shown in Figure 16 

Figure 16: Quad-HD XVS Proof-of-Concept System 

Snellen acuity tests were conducted with this system. Snellen acuity of 20/20 was 

obtained. These data are slightly worse than expectations but not terribly so. Rigorous 

analysis of the resolution results, such as the computations of the MTF, has not yet been 

conducted. The cameras used in this set-up are not state-of-the-art by any means. They 

were selected to provide a convenient 1920x1080 video source. 

Sony EVI-HD1 Camera (4)

Foresight Accustream 170 

frame grabber

Symbology

Foresight Accustream 170 

frame grabber

IBM QuadHD T221 Monitor

Personal Computer (PC) (Driving Entire Display)
33 

While the primary objective of this test was an XVS resolution proof-of-concept, a 

critical constraint to this design is the system latency. A Snellen acuity test is a static 

image test, however, the content of the XVS display is dynamic. Camera systems have 

latencies associated with acquiring an image and sending the video signal of the scene. In 

addition, the monitor has refresh updates and video bandwidth constraints. Displaying 

symbology on the XVS image can also introduce another source of lag or latency as the 

symbology is computed and merged with a frame grabber image. The combined effect of 

all these time delays can have a significant effect on visual perception and pilot 

performance during use of the XVS. The latencies must be considered in determining the 

overall effectiveness of the XVS system. If latencies are too high, the system can be unflyable.

An end-to-end latency test (camera acquisition, transmission, capture, process via frame 

grabber, symbology overlay and display interface and refresh) was conducted. Timer 

software was written and displayed on the Quad-HD monitor. This timer is then viewed 

by the camera system and displayed on the same computer monitor via frame grabber 

software. The camera image is captured by a frame grabber card. The total camera system 

latency can be determined by subtracting the computer displayed time minus the frame 

capture time. 

In Figure 17, a screen capture of the computer timer and the same timer captured and 

rendered by the camera and display system are shown. The T221 monitor was configured 

for a display resolution of 3840x2400 at 13Hz. Each camera was outputting a 1080i, 

60Hz format signal that was captured by a frame grabber. The camera latency was 

measured as the difference between the two time displays. The system latency was 

between 300 and 330 milliseconds. This latency has been found to be unacceptable 

(Bailey, Arthur, and Williams, 2004). A reasonable requirement of 100 msec latency 

maximum for an XVS is suggested. 

This same latency test method was used to diagnose the prominent latency sources in our 

proof-of-concept XVS.

First, four parallel video streams were reduced to a single video input. The video 

capture process was still used as our symbology generation method. With this 

set-up, the latency was reduced to 230 msec. This result indicates that there was 

approximately 100 msec of latency associated with the overhead of running 4 

parallel capture cards and displaying this Quad-HD video on the T221 monitor.

Next, the Quad-HD monitor was replaced with a commercial off-the-shelf 

Samsung (Model LN32A550) HDTV monitor, running 1920x1080 resolution.

With this set-up the latency was reduced to 200 msec. This result indicates there 

was only 30 msec of additional latency associated with the T221 monitor 

compared to a present-day liquid crystal HDTV monitor.

Finally, the video capture cards were bypassed and the HDTV video cameras 

directly drove the Samsung HDTV monitor, running 1920x1080 resolution. With 

this set-up the latency was reduced to 155 msec. This result indicates that 

approximately 45 msec of latency is associated with the frame grabbing process. 
34 

Figure 17: XVS Proof-of-Concept Quad-HD End-to-End Latency Test 

Detailed investigations into the exact processes that are causing these latencies have not 

yet been undertaken. The data suggest that two significant factors are influencing these 

results. 

First, the HDTV cameras being used are not providing a sufficient real-time video source. 

The latency for just the camera and HDTV monitor is 155 msec. Even if the camera was 

running as slow as a 24 Hz frame rate, the minimum expected latency would be on the 

order of 41.67 msec. Since the measured latency is so much greater than this minimum, 

it indicates that there are significant other factors included in the camera system (e.g., 

frame buffering, sensor element integration and processing, etc.) that degrade real-time 

performance. 

In addition, the T221 monitor used in this proof-of-concept employs a limited refresh 

rate. While the data does not indicate that the monitor is a major latency source, the new 

Quad-HD monitors expected in the near future will have significantly improved video 

refresh rates and video processing circuitry which should reduce the latency contribution.

Before diagnosing and creating remedies for this XVS proof-of-concept, better cameras 

and monitors should be procured. Once this happens, the hardware and software 

processes which control the video frame grabbers will have to be addressed to overcome 

the 100+ msec of latency that they contribute.

Laptop with Timer Display

Quad-HQ Image of Laptop Timer
35 

8. Concluding Remarks 

This report provides the background for conceptual XVS design standards for display and 

sensor resolution.

The resolution requirements were derived from the basis of equivalent safety and 

performance to forward-facing windows currently used and certificated for 

“conventional” aircraft. Three measures for equivalent safety and performance were 

investigated: a) human vision performance equivalence; b) see-and-avoid performance 

and safety; and c) see-to-follow performance. 

From these three factors, a minimum but perhaps not sufficient resolution requirement of 

60 pixels per degree was shown for human vision equivalence. However, see-and-avoid 

and see-to-follow performance requirements are nearly double. These results need to be 

verified and validated by in-flight testing. 

XVS display and sensor resolution requirements should be developed using modulation 

transfer function-based criteria instead of simple pixel per degree measures. The MTF 

measures would more accurately capture the true system performance and more 

accurately reflect in-flight performance. 

This report also reviewed historical XVS testing. The trends in commercial video 

systems are very favorable such that emerging technology should be available to meet the 

needs of an XVS system. Proof-of-concept testing has identified one significant 

limitation however. The latency of the XVS can be a significant technical hurdle. 

Methods for commercial off the shelf architectures are being explored, but custom-video 

processing may be needed instead. Fortunately, custom video work is no longer a 

technological hurdle, but merely an engineering task. This work should be done to 

increase the Technology Readiness Levels for XVS technology so the lack of forwardfacing windows will not be a risk-factor in the successful launch of commercial and 

business supersonic aircraft.
36 

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39 

Appendix A

Recommended XVS Flight Test and Evaluation Maneuvers 

The following maneuvers are a subset of evaluation tests recommended for the research, 

development, test and evaluation of XVS and other “visionic” systems. The term 

“visionics” is used as the name for a generic device which provides a pictorial 

representation of the external scene (i.e., the environment in which the aircraft is flying), 

generated by electronic means such as electro-optical sensors, radars and/or a database 

(synthetic vision). In this case, the XVS (or “visionic” system) is used to replicate the 

functionality of a forward-facing window for see-and-avoid and see-to-follow. 

These maneuvers are a subset of those which have been previously used during the HSR 

program in the initial development of an XVS (Yang et al, 2000, Gandhi et al, 2003) and 

are also being used for “sense-and-avoid” testing for UAVs (Shakernia et al , 2007). 

For each of the test maneuvers that follow, some brief operational background and 

specific test objectives is given. The test scenario is then described. Extensions and 

modifications to these maneuvers can be easily made to accommodate different 

operational contexts. Some of these are briefly discussed in the “operational 

background” for each task. 

The general term – “visionics” – is used in an attempt to remain technology agnostic. 

“See-and-Avoid” – Visual Target Detection

1. Operational Background

Visionics devices may provide visual flight reference information sufficient to allow 

operational capabilities enabling equivalent Visual Flight Rules operations; that is, the 

ability to see-and-avoid other aircraft. The ability of the pilot (operator) to see other 

aircraft (i.e., “targets”) is critical for the purposes of self-separation and see-to-follow. 

Comparative testing is used to quantify the ability of the pilot, when using a visionics 

device, to meet this see-and-avoid / see-to-follow requirement. Comparative testing is 

necessary since absolute criteria are not available.

2. Test Objectives:

The test objective is to measure the ability of the visionics device to provide the visual 

cues to enable the evaluation pilot to detect, recognize, and identify other aircraft traffic. 

Pure “expansion” (i.e., the target is represented by an increasing number of pixels without 

a translation or change in the position of the target on the display) of a target is typically 

tested as it represents the most difficult recognition task. 

3. Test Scenario Definition
40 

The following maneuver highlights one of numerous verification tests which may be used 

for comparative evaluation against laboratory or simulation tests for in-flight 

determination of traffic (target) detection, recognition, and identification (Figure A-1). 

This test emphasizes visual traffic (i.e., “target”) detection by the evaluation pilot using 

“expansion” along a horizontal trajectory away from the evaluation pilot. This maneuver 

should also be conducted in the reverse direction (i.e., ownship overtaking the target) to 

test for “expectancy” effects on the part of the evaluation pilot. 

Other trajectories, across path, along path, with and without vertical trajectories, may also 

be performed to evaluate (in-flight) visionics performance and operator detection (of 

“just noticeable differences”) such as Pure Expansion with Pitch Trajectories, False 

Expansion, Pure Translation Expansion with Pitch, and Expanding Background 

Expansion with Translation.

Figure A-1: Visual Target Detection - Expansion With Horizontal Trajectories 

The intent of this scenario is to evaluate the visual (acuity and contrast) performance of 

the visionics system in-flight for target detection, recognition and identification. The 

target aircraft (labeled “Target” in Figure A-1) presents a traffic situation to the 

evaluation pilot who is flying ownship aircraft:

The task begins with the target aircraft flying in loose formation to the left or 

right and behind ownship.

When the evaluation pilot with the visionics device is ready, the pilot will radio 

the target aircraft to move into the task starting position ahead of ownship on the 

same heading. 

The target aircraft begins the task at the same speed, altitude and heading of 

ownship and positioned nominally 100 feet ahead (Time 1 in Figure A-1). 

Ownship maintains constant altitude, speed, and heading.

On a mark from the evaluation pilot (i.e., data “event marker” triggered and radio 

transmission of "Start Maneuver"), the target aircraft accelerates to a constant 

.

62 13 85

target ownship

v1 v2

x0

.

62 13 85

target ownship

v1 v2

x2

Time - 2

Time - 1
41 

airspeed and pulls away from ownship (Time 2), maintaining heading and 

altitude.

The first test-point occurs when the target is no longer discernible to the 

evaluation pilot using the visionics device. 

If comparative testing can be done in real-time, a second test-point occurs when 

the target is no longer discernible to another observer using natural vision.

Weather, atmospheric obscurants and conditions, and time-of-day variations may 

significantly affect the results of these tests; thus, measurements of these conditions 

should be made, as possible and practical, and repeated measurements taken for statistical 

reliability. The results will also be influenced by the observers’ visual acuity so 

individual testing should be made and numerous observers used. 

Important performance metrics are as follows: 

Aircraft separation distance when target aircraft is no longer visible, with and 

without the visionics device.

Comparison against “theoretical” visual target recognition, and identification 

criteria. 

“See-and-Avoid” – Departure Level-off

1. Operational Background

The following maneuver is analogous to the “See-and-Avoid – Visual target detection” 

task with the notable exception that ground clutter is a significant component to the target 

identification and recognition. 

This task is also critical in that it is an all-too-often operational scenario. In this case, the 

“ownship” aircraft is being vectored toward an airport and the target aircraft is departing 

on opposite headings. The target aircraft on a departure climb-out errs and flies through 

an Air Traffic Control “level-off” altitude with ownship flying a constant altitude. This 

maneuver tests the ability of the visionics system to support pilot detection of the target 

aircraft blunder.

2. Test Objectives:

The test objective is to measure the ability of the visionics device to provide the visual 

cues to enable the pilot to identify other aircraft traffic, follow that traffic, and maintain 

separation in the landing pattern in the presence of ground clutter. 

3. Test Scenario Definition

The following maneuver highlights one of numerous verification tests which may be used 

for comparative evaluation against laboratory or simulation tests for in-flight 

determination of traffic (target) detection, recognition, and identification (Figure A-2). 
42 

This task emphasizes visual traffic (i.e., “target”) detection by the evaluation pilot using 

“expansion” toward the evaluation pilot in ground clutter. This maneuver should also be 

conducted in the reverse direction (i.e , target aircraft flying away from ownship) to test 

for “expectancy” effects on the part of the evaluation pilot. 

Other trajectories, across path, along path, with and without vertical trajectories, may also 

be performed to evaluate (in-flight) the visionics performance and operator detection (of 

“just noticeable differences”) such as Pure Expansion with Pitch Trajectories, False 

Expansion, Pure Translation Expansion with Pitch, and Expanding Background 

Expansion with Translation but these are lower priority. 

Figure A-2: Visual Target Detection - Departure Level-Off 

The intent of this scenario is to evaluate the visual (acuity and contrast) performance of 

the visionics system in-flight for target detection, recognition and identification. The 

target aircraft (labeled B-200 in Figure A-2) presents a traffic situation to the evaluation 

pilot who is flying ownship aircraft:
43 

The task begins with the target aircraft (labeled “B-200” in Figure A-2) and 

ownship (labeled “TIFS” in Figure A-2) flying in opposite directions, separated in 

altitude (labeled “Time 1”). 

When the evaluation pilot with the visionics device is ready, the pilot will radio 

the target aircraft to begin the task. 

The target aircraft begins the task at the same speed but opposite heading of 

ownship. The altitude separation is based on geometry (as follows) and flight test 

considerations. 

The target aircraft begins a climb (“Time 2” in Figure A-2) to achieve a constant 

angle climb as seen by ownship. Airspeed and climb rate are determined by flight 

test considerations to maintain this constant angle on the visionics device 

(“Expansion” testing without target translation).

The target aircraft levels-off the climb to maintain an altitude buffer as required 

for safety of flight. 

The first test-point occurs when the target is detected (and/or recognized, 

identified) by the evaluation pilot using the visionics device. 

If comparative testing can be done in real-time, a second test-point occurs when 

the target is detected (and/or recognized, identified) by another observer using 

natural vision. 

Weather, atmospheric obscurants and conditions, and time-of-day variations may 

significantly affect the results of these tests; thus, measurements of these conditions 

should be made, as possible and practical, and repeated measurements taken for statistical 

reliability. The results will also be influenced by the observers visual acuity so individual 

testing should be made and numerous observers used. 

Important performance metrics are as follows: 

Aircraft separation distance when target aircraft is no longer visible, with and 

without the visionics device.

Comparison against “theoretical” visual target recognition, and identification 

criteria. 

See-to-Follow Performance – Self-Separation and Flight-Following 

1. Operational Background

The following outlines a methodology – one of several – for evaluation (validation) of 

visionics devices as a way to evaluate equivalent visual operations, in this case, for “seeto-follow” capabilities. 

2. Test Objectives:

The test objective is to measure the ability of the visionics device to provide the visual 

cues to enable the pilot to identify other aircraft traffic, follow that traffic, and maintain 

separation in the landing pattern. 
44 

The following maneuver highlights one of numerous “typical” VFR operational problems 

associated with traffic following and self-separation (Figure A-3). This one emphasizing 

traffic positions inboard of the evaluation pilot who in this case is seated in the left-hand 

seat. 

3. Test Scenario Definition

Figure A-3: See-to-Follow Equivalent Visual Operation 

The intent of this scenario is to have the target aircraft (labeled “Target” in Figure A-3) 

present a traffic situation to the evaluation pilot who is flying ownship aircraft during the 

entry of the downwind leg of a nominal approach to landing which requires close 

monitoring of the incoming traffic to ensure adequate separation. The evaluation pilot 

must identify and observe the target aircraft’s movement to enter the traffic pattern 

behind it. 

The ownship aircraft begins a circling holding pattern in a loiter area abeam the 

downwind leg.

The target aircraft (“Target”) begins on a downwind leg of a right-hand traffic 

pattern, at 1500 ft above field level (AFL), holding a constant approach airspeed. 

The evaluation pilot will maneuver the ownship aircraft to maintain a constant intrail spacing (e.g., 3 nm, or 5 nm) spacing on the target aircraft and follow it to 

landing (Time 2). 

On the base turn, the target will descend to 1000 ft AFL. The target will hold this 

altitude until glideslope intercept. Ownship will maintain 1500 ft AFL until 

glideslope intercept. 
45 

Both aircraft continue the landing approach through Time 3 and Time 4. The 

target aircraft will execute a go-around at 200 ft AFL, ending the evaluation. 

This same scenario may also be conducted for a left-hand approach, emphasizing 

traffic positions outboard of the evaluation pilot who is seated in the left-hand 

seat. 

The test data should reflect when the target is detected, recognized, and identified by the 

evaluation pilot using the visionics device. If comparative testing can be done in realtime, second test-points occur when the target is detected, recognized, and identified by 

another observer using natural vision. The ability to maintain separation (spacing) should 

be recorded. Different targets also provided different levels of visionics performance 

testing. 

Weather, atmospheric obscurants and conditions, and time-of-day variations may 

significantly affect the results of these tests; thus, measurements of these conditions 

should be made and repeated measurements taken for statistical reliability. The results 

will also be influenced by the observers visual acuity so individual testing should be 

made and numerous observers used. 

See-and-Avoid / See-to-Follow Performance – Parallel Runway Ops 

1. Operational Background

The following outlines a methodology – one of several – for evaluation of visionics 

devices as a way to evaluate equivalent visual operations, in this case, for “see-to-follow” 

capabilities during parallel runway operations.

2. Test Objectives:

The test objective is to measure the ability of the visionics device to provide the visual 

cues to enable the pilot to identify other aircraft traffic, follow that traffic, and maintain 

separation in the landing pattern. 

The following maneuver highlights one of numerous “typical” VFR operational problems 

associated with see-and-avoid and see-to-follow in VFR operations, in this case, during 

parallel runway operations (Figure A-4). This one emphasizes traffic positions forward 

and inboard of the evaluation pilot who is seated in the left-hand seat (until the final 

approach).

3. Test Scenario Definition

The intent of this scenario is to have the target aircraft (labeled “Target” in Figure A-4) 

present a traffic situation to the evaluation pilot who is flying ownship aircraft during 

opposite traffic patterns, serving parallel runway approach and landings. The task 

requires close monitoring of the opposite direction traffic to ensure adequate separation.

The evaluation pilot must identify and observe the target aircraft to maintain safe 

separation and monitor for potential “blunders.”
46 

Figure A-4: See-and-Avoid / See-to-Follow Performance – Parallel Runway Ops 

Both the target and ownship aircraft begin on downwind legs to parallel runways 

using left- and right-hand patterns, respectively. The target aircraft ownship can 

be a “virtual runway” to ease operational constraints (e.g., by using waypoints for 

vectors). The spacings between the parallel runways should be varied to test 

visionics performance. 

The size and aircraft spacings in the traffic pattern are designed based on flight 

test considerations, as follows.

The target aircraft (“Target”) begins on a downwind leg ahead of ownship in the 

pattern (“Time 1”) so that: a) some reasonable amount of time is provided to the 

evaluation pilot flying ownship to visually acquire the target while on opposite 

facing base legs (“Time 2”); and, b) if executed in a timely fashion, the ownship 

and target aircraft will have 2 nm, 3 nm or 5 nm spacings (experimentally varied) 

on final to their respective runways (“Time 3”). 

The evaluation pilot will maneuver the ownship aircraft to maintain a constant intrail spacing (e.g., 2 nm, 3 nm, or 5 nm) spacing on the target aircraft and follow it 

to landing (“Time 4”). 

A test for awareness may be experimentally varied by having the target aircraft, 

unbeknownst to the evaluation pilot flying ownship, fly to the wrong parallel 

runway (i.e., incurring onto ownship’s runway). 

This same scenario may also be conducted for a right-hand approach by ownship. 

Time 1

Runway

Time 4

Virtual Runway

Time 1

Time 2

Time 2

Time 3

Ownship

Target 
47 

The test data should reflect when the target is detected, recognized, and identified by the 

evaluation pilot using the visionics device. If comparative testing can be done in realtime, second test-points occur when the target is detected, recognized, and identified by 

another observer using natural vision. The ability to maintain separation (spacing) should 

be recorded. Finally, separation spacing and runway spacing can be varied to assess 

visual acuity and contrast performance. Different targets should also be used to test 

different levels of visionics performance. 

Weather, atmospheric obscurants and conditions, and time-of-day variations may 

significantly affect the results of these tests; thus, measurements of these conditions 

should be made and repeated measurements taken for statistical reliability. The results 

will also be influenced by the observers visual acuity so individual testing should be 

made and numerous observers used. 
REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

2. REPORT TYPE 

Technical Memorandum

 4. TITLE AND SUBTITLE

Conceptual Design Standards for eXternal Visibility System (XVS) Sensor 

and Display Resolution 

5a. CONTRACT NUMBER

 6. AUTHOR(S)

Bailey, Randall E.; Wilz, Susan J.; Arthur, Jarvis (Trey) J., III

 7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

NASA Langley Research Center

Hampton, VA 23681-2199

 9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

 8. PERFORMING ORGANIZATION

 REPORT NUMBER

L-19921

10. SPONSOR/MONITOR'S ACRONYM(S)

NASA

13. SUPPLEMENTARY NOTES

12. DISTRIBUTION/AVAILABILITY STATEMENT

Unclassified - Unlimited

Subject Category 03

Availability: NASA CASI (443) 757-5802

19a. NAME OF RESPONSIBLE PERSON

STI Help Desk (email: help@sti.nasa.gov)

14. ABSTRACT

NASA is investigating eXternal Visibility Systems (XVS) concepts which are a combination of sensor and display 

technologies designed to achieve an equivalent level of safety and performance to that provided by forward-facing windows in 

today’s subsonic aircraft. This report provides the background for conceptual XVS design standards for display and sensor 

resolution. XVS resolution requirements were derived from the basis of equivalent performance.

Three measures were investigated: a) human vision performance; b) see-and-avoid performance and safety; and c) 

see-to-follow performance. From these three factors, a minimum but perhaps not sufficient resolution requirement of 60 pixels 

per degree was shown for human vision equivalence. However, see-and-avoid and see-to-follow performance requirements are 

nearly double. This report also reviewed historical XVS testing. 

15. SUBJECT TERMS

Aviation safety; Enhanced visibility; NextGen; Synthetic vision

18. NUMBER

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56

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(443) 757-5802

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3. DATES COVERED (From - To)

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

609866.02.07.07.02

11. SPONSOR/MONITOR'S REPORT

 NUMBER(S)

NASA/TM-2012-217340

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1. REPORT DATE (DD-MM-YYYY)

01- 02 - 2012