DOT/FAA/AM-00/4

U.S. Department

of Transpor tation

Federal Aviation

Administration

Kurt M. Joseph

Civil Aeromedical Institute

Federal Aviation Administration

Oklahoma City, Oklahoma 73125

Dieter W. Jahns

SynerTech Associates

Everson, WA 98247

February 2000

Final Report

This document is available to the public

through the National Technical Information

Service, Springfield, Virginia 22161.

Enhancing GPS Receiver

Certification by Examining Relevant

Pilot-Performance Databases

Office of Aviation Medicine

Washington, D.C. 20591
NOTICE

This document is disseminated under the sponsorship of

the U.S. Department of Transportation in the interest of

information exchange. The United States Government

assumes no liability for the contents thereof.
i

Technical Report Documentation Page

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.

DOT/FAA/AM-00/4

4. Title and Subtitle 5. Report Date

Enhancing GPS Receiver Certification by Examining Relevant PilotPerformance DatabasesFebruary 2000

6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No.

Joseph, K.M.1, and Jahns, D.W.2

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)

11. Contract or Grant No.

1

FAA Civil Aeromedical Institute, P.O. Box 25082, Oklahoma City, OK 73125

2

 SynerTech Associates, Everson, WA 98247

12. Sponsoring Agency name and Address 13. Type of Report and Period Covered

Office of Aviation Medicine

Federal Aviation Administration 14. Sponsoring Agency Code

800 Independence Ave., S.W.

Washington, DC 20591

15. Supplemental Notes

Work was accomplished under approved task AAM-A-00-HRR-514

16. Abstract

The rapid introduction of Global Positioning System (GPS) receivers for airborne navigation has outpaced

the capacity of international aviation authorities to resolve human factors issues that concern safe and

efficient use of such devices. Current certification technical standards appear to have had little impact on

promoting the design of standardized receiver architectures, interfaces and operating manuals- despite

evidence from a variety of sources that lack of standardization may undermine safety. This paper explores

the relationship between existing human factors data relevant to GPS-interface design and incident/accident

databases, which are a rich source of information and serve to highlight the safety-critical nature of GPSreceiver interface issues. An approach to expanding the role of human factors assessments in the certification

of GPS receivers is briefly summarized.

17. Key Words 18. Distribution Statement

GPS Receivers, Certification, Human Factors,

General Aviation

Document is available to the public through

the National Technical Information Service,

Springfield, Virginia 22161

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price

Unclassified Unclassified 11

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

The authors thank Mike Nendick, University of Newcastle in Callaghan, NSW, Australia,

for permission to use the GPS User Survey 1997. Howard Harris, FAA-CAMI, created a database of the ASRS reports used in this report.

ACKNOWLEDGMENTS

iii

1

ENHANCING GPS RECEIVER CERTIFICATION BY EXAMINING

RELEVANT PILOT-PERFORMANCE DATABASES

INTRODUCTION

The accelerated development and introduction of

Global Positioning System (GPS) receivers for use in

airborne navigation has outpaced the capacity of

international aviation authorities to fully implement

regulations and guidance for the safe and efficient use

of such devices (Nendick & St. George, 1996). Technical Standard Order (TSO) C129 A1 is currently

used to certify standalone, Instrument Flight Rules

(IFR) GPS receivers for installation in the United

States and Canada and is accepted as the certification

standard elsewhere, including Australia and New

Zealand. However, it appears to have had little influence on standardizing receiver architectures, interfaces, and operating manuals (Heron, Krolak, &

Coyle, 1997). In addition, there is no standard for the

design of hand-held GPS receivers despite their prevalence among General Aviation (GA) pilots as a supplemental aid to Visual Flight Rules (VFR) navigation

with an approved primary means of navigation (e.g.,

dead reckoning, pilotage, and/or electronic navigation). RTCA, Inc. (1993) has published guidance

material for the use of such receivers, which are

considered portable electronic devices under Federal

Aviation Regulation Part 91.21. Furthermore, the

Aircraft Owners and Pilots Association (AOPA) is

currently publishing guidance material for use with

GPS technology (AOPA, in press).

At least two human factors references directly

support TSO C129 A1. McAnulty’s (1994) review of

human factors principles and guidelines for the design of controls and displays for standalone, IFR GPS

and Long Range Navigation (LORAN) receivers is

germane to regulatory requirements. In fact, this

review is the basis for the other reference that supports TSO C129 A1: the aircraft certification human

factors and operations checklist for standalone, IFR

GPS receivers (Huntley, Turner, Donovan, &

Madigan, 1995). The checklist includes a bench test

and a flight test, both of which are designed to assist

certification personnel and manufacturers in evaluating

the characteristics of GPS receivers in accordance

with TSO C129 A. These tests focus on GPS-receiver

controls, displays, and operating characteristics.

The guidelines proposed by McAnulty (1994) and

the checklist developed by Huntley et al. (1995)

represent important progress in resolving some of the

safety-critical interface issues associated with GPS

receivers and their certification. However, many issues still remain. This paper serves a dual purpose, the

first of which is to review unresolved interface issues

by summarizing existing corroborative evidence from

a variety of independent sources. The second is to

illustrate the safety-critical nature of these issues by

analyzing evidence from several incident and accident report databases.

SUMMARY OF EXISTING EVIDENCE

Analysis of GPS Receivers using System Design

Principles

O’Hare and St. George’s (1994) and Heron et al.’s

(1997) use of system design principles to analyze

existing GPS-receiver interfaces illustrates how complex receiver architectures and cumbersome receiver

operations can combine to impair pilot performance.

The myriad of functions supported by receiver architectures necessitates multiple modes, pages and subpages, which quickly overwhelm pilot information

processing resources, especially memory. In addition, receiver operations are constrained inappropriately by unintuitive logic, control knobs and buttons

that induce data-entry errors, and displays that are

not optimized for legibility and intelligibility. Heron

et al. also point out that the databases used by GPS

receivers occasionally contain erroneous or missing

data and anomalous identifiers.
2

Flight Tests of GPS Receivers

 Specific observations made during FAA flight

tests of Terminal Instrument Procedures using IFRapproved GPS receivers support the general results of

Heron et al.’s analysis. Winter and Jackson (1996)

and Williams (1998a) summarized these observations by giving examples of GPS-receiver design

problems. These examples illustrated overly complex

receiver operations, inadequate feedback, inconsistent labeling and placement of control knobs and

buttons, procedural problems involving alternate airport selection and waypoint sequencing, and differences in receiver functioning attributable to methods

used for installation. Winter and Jackson (1996) also

cited instances where GPS receivers affected pilot

performance because they did not support flight

functions appropriately during critical flight phases

(e.g., intermediate or final approach segment). In

particular, they noted increased pilot workload and

delays in communication when Air Traffic Control

(ATC) requested information about the distance of

the aircraft from the airport. The GPS receivers did

not allow easy access to such information, and pilots

were forced to calculate distance manually, or to

access distance information by exiting the current

function page, entering a different page, and then

returning to the previous page to continue the approach segment. The latter procedure required at

least four keystrokes when done correctly and as

many as nine if done incorrectly (i.e., reprogramming

the approach).

Flight Simulation Tests of GPS Receivers

Flight simulation research at the Federal Aviation

Administration Civil Aeromedical Institute (FAA/

CAMI) indicates that some features of hand-held

GPS-receiver interfaces can compromise their effective use and perhaps undermine safety. Wreggit and

Marsh’s (1998) systematic examination of a typical

hand-held receiver’s interface design serves as a benchmark for usability testing of GPS receivers. Initially,

they used flow diagrams of GPS menu structures to

familiarize and train pilots with no GPS receiver

experience. After this training phase, pilots observed

a demonstration of the receiver’s features and procedures, and practiced with the receiver until they

passed proficiency tests. Pilots then performed 37

GPS-related tasks during an hour-long flight simulation. The tasks required waypoint setting, navigation,

and data entry and retrieval. Pilot performance was

affected by several menu structures that slowed data

entry, editing of stored data, and activation of functions. Pilots frequently exceeded the minimum number of keystrokes necessary to accomplish a given task

and spent a significant amount of time “head-down”

while programming the GPS receiver. The average

length of a head-down glance while working on a

GPS-related task was 10 seconds, whereas the average

head-down time (i.e., sum of head-down glances)

necessary to complete each of 28 GPS-related tasks

ranged from approximately 10-75 seconds, with a

median time of nearly 24 seconds. Wreggit and

Marsh (1998) concluded that several factors had the

potential to negatively affect pilot performance (e.g.,

excess keystrokes and head-down time). Among these

were the constraints imposed by the logic of receiver

menu structures, pilots’ understanding of receiver

controls, the difficulty of recovering from erroneous

inputs, lack of appropriate feedback, and inconsistent mapping of controls to functions.

Other FAA/CAMI flight simulation research has

illustrated that specific GPS receiver functions can be

designed to better support pilots as they perform inflight tasks using GPS. Williams (1998b) has used

empirical data to make a compelling argument against

the use of existing text-only, tabular displays of

nearest airport information, especially during emergencies. His work indicates that pilots using such

displays are three times more likely to misjudge the

relative direction of the nearest airport, and are five

seconds slower, on average, than pilots who use map

or enhanced-text GPS displays. Consistent with

Wreggit and Marsh (1998), Williams (1998b) also

found that most pilots did not cross-reference GPS

receiver information with other instruments (e.g.,

heading indicator) that could help orient them. The

failure to cross-reference information may indicate

that GPS receivers trap pilots’ attention, thereby

disrupting their scan.

Questionnaires about GPS-Receiver Interface

Design

Nendick (1994) developed a comprehensive questionnaire (viz., GPS User Survey) to gather responses

from 227 New Zealand pilots in an attempt to

identify GPS-receiver interface design and operational issues that eroded flight safety. The 125-item

GPS User Survey elicited pilot perceptions of and

experiences with GPS receivers by asking questions

about controls and displays, operating logic, functions,
3

operations, operating procedures, navigation performance, pilot attitudes, and training. Joseph, Jahns,

Nendick, and St. George (1998) used an expanded

version of this survey (i.e., 163 items) to collect

responses from 308 American pilots, and Nendick

and St. George are currently using it to compile

responses from Australian and New Zealand pilots.

Several recommendations that are relevant to the

present discussion can be drawn from the GPS User

Survey data. Nendick (1994) suggested that:

•the design of future GPS-receiver interfaces, especially controls and displays, would benefit from

strict adherence to published guidelines (cf.,

McAnulty, 1994);

•receiver operating logic could be improved through

standardization of various models;

•serious consideration should be given to the inclusion of receiver features that reduce and possibly eliminate over-reliance and complacency;

•improvements in the content, layout and indexing of GPS operating manuals were necessary;

and,

• pilots should be required to undergo some type of

formal GPS training.

Finally, Joseph et al.’s (1998) analysis of GPS User

Survey data identified numerous GPS-interface design and operational issues that are not addressed by

the TSO C129 A1 human factors checklist and could

be used as a basis for revising or supplementing

existing GPS receiver certification standards.

ANALYSIS OF INCIDENT AND ACCIDENT

DATABASES

ASRS and FAA Incident Databases

O’Hare and St. George (1994) discussed the implications and speculated about possible unintended

consequences associated with the use of GPS in

aviation. These authors introduced their discussion

by expressing the need for “...an awareness of the

relevant human factors issues amongst pilots and

controllers before a GPS incident/accident database

has developed.” Similarly, they concluded their discussion by hoping that the many human factors

issues associated with GPS would be “...carefully

considered by the regulatory authorities, and widely

discussed by potential users before these latent

problems manifest themselves in operational experience.” The statements made by O’Hare and St. George

(1994) presaged the trend shown in Figure 1, which

illustrates an accelerated increase in the number of

Aviation Safety Reporting System (ASRS) and FAA

Incident Database reports containing the term “GPS.”

The information in Figure 1 is based on an analysis

of ASRS and FAA incident reports from January

1989 through September 1998. A search of the databases for this period produced a sample of 468

incident reports in which the term “GPS” appeared in

the report narrative. Four of these reports were from

the FAA incident database. Further analysis of each

report narrative revealed that the use of GPS contributed as a factor in 162 of the 468 incidents. The term

“GPS” was used only to describe the type of navigation in the remaining 306 incident reports. Although

Figure 1 shows a steady increase in the frequency of

incident reports containing the term “GPS” and

those involving GPS as a contributing factor, the

ratio between the former and the latter type of incident reports has remained relatively constant from

1995 to 1997. The data for 1998 are incomplete. The

ratio is represented in Figure 1 by the line with the

filled circles and is expressed as a percentage on the

right ordinate.

The 162 incident reports involving GPS were

categorized based on the 24 non-orthogonal, GPSinterface design and operational issues identified by

Joseph et al. (1998). Frequency counts are listed for

categories, as are percentages, which have been

rounded to the nearest whole number. Six issues

accounted for 77% (n=124) of the incident reports.

Thirty percent (n=49) of the 162 reports described

incidents that were associated with changes in pilot

workload due to the use of GPS. For example, pilot

interaction with GPS receivers changed mental

workload, increased head-down time, and reduced

the use of charts. Fifteen percent (n=25) of the

incident reports were characterized as operational

errors committed by the pilot. That is, the incidents

were associated with incorrectly entering data into

the GPS receiver, misreading the receiver display, or

being unaware of the active receiver mode.

Nine percent (n=16) of the reports described incidents in which pilots were overly dependent on GPS

and became complacent or relied solely on it for

operations. Eight percent (n=13) of the incidents

were associated with GPS signal reception; specifically, the signal lacked integrity or was not reliable.

Seven percent of incident reports (n=11) involved
4

problems with receiver accessories. For instance, pilots reported unreliable power supplies in the form of

spent batteries or loosely connected cords, and poorly

mounted antennas. Finally, despite Heron et al.’s

(1997) call for more comprehensive, accurate and

standardized GPS-receiver databases, 6% (n=10) of

the incident reports involved errors, missing data, or

anomalous identifiers in such databases.

Of the 162 ASRS incident reports involving GPS,

58% (n=93) resulted in deviations from assigned

clearances or unauthorized entries into restricted

airspace. Deviations from an assigned altitude clearance were very common, as were unauthorized entries

into terminal areas and special use airspace. Ten

percent (n=15) of the incident reports described

instances where database or programming errors were

discovered and corrected before safety was jeopardized. Fifteen percent (n=24) of incidents involving

GPS were divided equally among deviations or diversions from the planned route of flight, deviations

from standard approach procedures, and unintended

landings at the wrong airport.

NTSB Aviation Accident/Incident Database

A search of the National Transportation Safety

Board (NTSB) Aviation Accident/Incident Database

from 1989-1998 revealed nine accident reports involving GPS receivers. Although only one of these

reports explicitly stated that the use of a GPS receiver

was a probable cause for the accident, each of the

remaining reports listed at least one probable cause

(e.g., diverted attention) that was associated with

GPS receiver use. Accidents were not categorized

because there were too few. Six accidents were characterized as either an in-flight collision with terrain or loss

of control on ground/collision, one accident was a midair collision, one was a gear-up landing, and another was

a forced landing/collision with trees. The accidents

resulted in one minor injury and one fatality.

GPS User Survey

GA pilots’ written responses to four open-ended

questions from the GPS User Survey administered by

Joseph et al. (1998) provide another source of data on

the operational experiences and design preferences of

0

20

40

60

80

100

120

140

89 90 91 92 93 94 95 96 97 98*

Year

Number of ASRS Reports

0.0%

20.0%

40.0%

60.0%

80.0%

100.0%

Percenta

ge of Incidents to Re

ports

ASRS reports citing the term 'GPS'

ASRS reports of incidents involving GPS

Proportion of incidents to reports

* Jan-Sep

Figure 1. ASRS and FAA Incident Report Databases- Reports citing the term

"GPS" and reports of incidents involving GPS.
5

pilots using GPS receivers. Responses to the four

questions were categorized based on the 24 nonorthogonal, GPS-interface design and operational

issues identified by Joseph et al. The questions and

results of the categorization are discussed in turn.

Question 1. This question asked pilots what they

found difficult about using GPS. Of the 308 pilots

who returned a completed survey, 185 provided at

least one response to this question. Several pilots gave

more than one response to this question; hence, 192

responses were counted. Twenty-eight percent (n=53)

indicated that GPS receiver operations were inordinately complex, that they quickly consumed available pilot memory resources, and required significant

amounts of practice to achieve and maintain proficiency. Another 24% (n=46) noted that receiver

programming demands were burdensome. Many of

these pilots suggested that programming and reviewing a route, and selecting or entering waypoints

should be simplified. They also favored a standard set

of functions for all GPS receiver interfaces. Several

other issues accounted for the remaining pilot responses. Those listed most frequently by pilots included difficulty reading information on receiver

displays (9%, n=17), lack of knowledge and experience with receiver operations (6%, n=12), trouble

with receiver accessories and installation (6%, n=11),

and difficulty entering and modifying data (6%, n=11).

Question 2. This question asked pilots what problems they have had using GPS. At least one response

was provided by each of 199 pilots. As with Question

1, several pilots gave more than one response to this

question; hence, 208 responses were counted. Thirtyone percent (n=65) of pilots stated that they had

problems with signal reception or the integrity of

receiver information. Sixteen percent (n=33) remarked

about problems created by overly complex receiver

operations. A majority of the remaining responses to

this question focused on four issues: problems with

accessories and installation (9%, n=19); problems

with errors, missing data and anomalous identifiers

in the receiver database (8%, n=16); burdensome

receiver programming demands (7%, n=14); and

problems reading information on receiver displays

(7%, n=14).

Question 3. This question asked pilots if they had

examples of “hazards” or “traps” that may catch GPS

users off guard. At least one response was provided by

each of 129 pilots. As with Question 1, several pilots

gave more than one response to this question; hence,

134 responses were counted. Pilots most frequently

(17%, n=23) gave examples of how their sole dependence on GPS receivers for navigation gave way to

complacency and lapses in awareness during flights.

Sixteen percent (n=22) cited instances where unreliable signals or lack of signal integrity prevented them

from using GPS. Consistent with the results of Wreggit

and Marsh (1998) and Williams (1998b), 12% (n=16)

of pilots noted that the use of GPS consumed valuable information processing resources, thereby reducing the amount of time spent on other flight tasks

such as “see-and-avoid” lookout and scanning of

instruments. Finally, 8% (n=11) of pilots cited examples of how errors, missing data, and anomalous

identifiers in the receiver database could cause unintended consequences.

Question 4. This question asked pilots if they had

views on how GPS should be developed further. At

least one response was provided by each of 149 pilots.

As with Question 1, several pilots gave more than one

response to this question; hence, 171 responses were

counted. Nineteen percent (n=33) of pilots stated

that GPS should be developed further by easing

programming demands and standardizing receiver

interfaces. An almost equal number (19%, n=32)

wanted GPS infrastructure improvements to the

National Airspace System (NAS). Such improvements included additional GPS approaches, the elimination of intentional signal error, and availability of

integrated terrain and weather information for use

with GPS receivers. Many of the remaining responses

were nearly equally divided among four issues. These

were improvements in availability and reliability of

GPS signals (12%, n=21), improvements in GPSreceiver accessories (e.g., antenna) and installations

(12%, n=21), enhancements in receiver display legibility (12%, n=20), and corrections to GPS-receiver

databases (12%, n=20).
6

EXPANDING THE ROLE OF HUMAN

FACTORS IN GPS RECEIVER

CERTIFICATION

One of the many challenges for the FAA and

aviation community as they progress towards sole

reliance on augmented, satellite-based navigation is

to enhance TSO C129 A1 and future technical standard orders with human factors specifications for

GPS-receiver interfaces. Such specifications should

ensure usability by providing a standard set of receiver functions without prohibiting GPS-receiver

manufacturers from adding new features to their GPS

devices. The evidence in favor of expanding the role

of human factors in GPS receiver certification is

compelling and reliable. The previous sections of this

paper showed that research and analyses of GPSreceiver interfaces and operations in several different

environments consistently yield similar results. For

example, GA pilots, who compose the largest and

most diverse user group in the NAS, prefer GPSreceiver interfaces that include a standard set of

functions, are easier to program, are more reliable,

and have more legible displays. These preferences are

consistent with the views of the AOPA Air Safety

Foundation, whose executive director suggested in a

personal communication that the design of basic

IFR-approved, GPS receivers should include simple

programming that allows navigation to and from a

waypoint on a selected bearing; holding, using a

waypoint as a reference; execution of an approach;

and a missed approach (B. Landsberg, personal communication, April 9, 1999). According to GA avionics equipment estimates 42% of the 187, 312 active

GA aircraft were equipped with some type of GPS

receiver (AOPA, 1998). Hence, expanding the role of

human factors in the GPS receiver certification process would benefit tens of thousands of pilots.

One possible approach for expanding the role of

human factors in the GPS receiver certification process begins with a collaborative review of the existing

TSO C129 A1 human factors supplement by a team

of FAA certification personnel, manufacturers, and

human factors specialists. The addition of new functions has made GPS receivers more complex, and the

1995 supplement may not adequately support presentday, certification tasks. Joseph et al.’s (1998) analysis

of GPS User Survey data identified numerous GPSinterface design and operational issues that do not

appear to be adequately addressed by the TSO C129

A1 human factors supplement and could be used to

enhance it. After reviewing the current standards, the

team would develop a list of functions common to all

GPS receivers. The team also would develop a list of

bench and flight usability tests for each function. The

usability testing procedure might resemble the bench

and flight-simulation tests used by Wreggit and Marsh

(1998). All GPS receivers then would be tested on the

selected set of functions and the test results would be

used to establish standards for receiver-interface design. Performance-based standards for the selected

set of functions could be defined by using cutoff

scores, which would be based on point estimates from

score distributions for each function. For example, if

the waypoint function of a receiver is to be certified,

x percent of a representative sample of pilots must be

able to select or program a waypoint in y seconds or

less. These standards would be applied to new receiver models only, and they could be reviewed and

revised periodically to reflect incremental improvements in receiver interfaces. Finally, manufacturers

could use the results of such performance-based comparisons to demonstrate the capability of their receivers.

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