DOT/FAA/AM-99/13

Office ofAviation Medicine

Washington, D.C. 20591

GPS User-Interface

Design Problems

Kevin W. Williams

Civil Aeromedical Institute

Federal Aviation Administration

Oklahoma City, Oklahoma 73125

April 1999

Final Report

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through the National Technical Information

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19990521 003

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Technical Report Documentation Page

1. Report No.

DOT/FAA/AM-99/13

2. Government Accession No.

4. Title and Subtitle

GPS User-Interface Design Problems

7. Author©

Williams, K.W.

9. Performing Organization Name and Address

FAA Civil Aeromedical Institute

P.O. Box 25082

Oklahoma City, OK 73125

12. Sponsoring Agency name and Address

FAA Office ofAviation Medicine

Federal Aviation Administration

800 Independence Avenue, SW.

Washington, DC 20591

15. Supplemental Notes

3. Recipient's Catalog No.

5. Report Date

April, 1999

6. Performing Organization Code

8. Performing Organization Report No.

10. Work Unit No. (TRA1S)

11. Contract or Grant No.

13. Type of Report and Period Covered

14. Sponsoring Agency Code

16. Abstract

This paper is a review of human factors problems associated with the user-interface design of a set of Global Positioning

System (GPS) receivers, certified for use in aircraft for instrument non-precision approaches. The paper focuses on

design problems associated with the interfaces and specific inconsistencies across the set of interfaces that could cause

confusion or errors during operation. Some specific problems addressed involve the layout and design of knobs and

buttons; control labeling inconsistencies across units; the placement and use of warnings; feedback, or the lack thereof;

and the integration ofspecific flying tasks while using the receivers. Recommendations for solving some ofthe problems

are provided, as well as suggestions to the FAA, GPS manufacturers, and pilots regarding the future development and

use ofthese products.

17. Keywords

Global Positioning System, Human-Computer Interface,

Aircraft Displays, Applied Psychology

19. Security Classif. (of this report)

Unclassified

Form DOT F 1700.7 (8-72)

18. Distribution Statement

Document is available to the public through the

National Technical Information Service,

Springfield, Virginia 22161

20. Security Classif. (of this page)

Unclassified

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11

Reproduction of completed page authorized

22. Price
ACKNOWLEDGMENTS

This research is part of the Civil Aeromedical Institute's General Aviation Human Factors

Research Program. Program direction is provided by the Aircraft Certification (Small Aircraft

Directorate, ACE-100) and General Aviation and Commercial Division (AFS-800) sponsors.

The author expresses his appreciation to SteveJackson and SteveWinterofthe FAA'sTechnical

Programs Division, Flight Procedure Standards Branch (AFS-420) fortheirinvaluable assistance

in providing information about the many GPS products currently on the market.
GPS USER-INTERFACE DESIGN PROBLEMS

INTRODUCTION

This paper is a review ofhuman factors problems

associated with the user-interface design of a set of

Global Positioning System (GPS) receivers, certified

for use in aircraft for instrument non-precision approaches. No GPS products will be mentioned by

name, since the aim ofthe paper is not to criticize a

particular GPS manufacturer. Instead, the paper will

focus on design problems associated with the interfaces and specific inconsistencies across the set of

interfaces that could cause confusion or errors during

operation. Some specific problems to be addressed

involve the layout and design of knobs and buttons;

controllabeling inconsistencies across units; the placement and use of warnings; feedback, or the lack

thereof; and the integration of specific flying tasks

while using the receivers. Recommendations forsolving some of the problems are provided, as well as

suggestions to the FAA, GPS manufacturers, and

pilots regarding the future development and use of

these products.

Data Collection

Most of the human factors problems reported in

this paper were obtained from interviews with subject

matter experts from the Federal Aviation Administration's (FAA's) Technical Programs Division,

Flight Procedure Standards Branch (AFS-420). Additionally, data were collected through FAA internal

memoranda (i.e., personal communications, S. Jackson, August, 1997; S.Jackson, February 18,1998; S.

Jackson, May 22, 1998, S. Winter, September 25,

1996), as well as from an FAA technical report

(Winter & Jackson, 1996). Finally, data were taken

from observation logs from a recently conducted

operational test of a GPS Wide Area Augmentation

System (WAAS), and from personal observation from

the WAAS test.

Human Factors Guidelines

Initially, the FAA Aircraft Certification Human

Factors and Operations Checklist for Standalone

GPS Receivers (FAA, 1995) was the primary human

factors reference source for GPS receiver design. The

human factors and operations checklist is hereafter

referred to as "the checklist." The checklist is a useful

source of human factors design guidelines and includes references to several other commonly used

guideline documents from both the military and

civilian sectors.

DESIGN PROBLEMS

The following design problems are not intended to

be an exhaustive list. Many problems could not be

included because of time and space constraints. Also,

with the exception ofcomplexity, the problems are not

presented in order ofimportance since the importance

ofany particular problem cannot usually be determined

without looking at how often it occurs and its effect on

pilot workload and performance. Further research is

required to make those determinations.

Complexity

Probably the mostsignificant feature ofGPS units,

as far as the potential for user errors is concerned, is

the sheer complexity involved in their operation. Itis

notthatthe manufacturers ofthese devices purposely

designed units to be difficult to operate. Instead, the

primary reason for their complexity is thatthey allow

the performance of a large number of tasks using a

limited number of controls and a small display area.

One indicator of the complexity of a GPS unit is

the size of its instruction manual. Manuals of between 100 and 300 pages are common. In addition to

the large number oftasks that can be performed by a

GPS unit, there is a relatively small area for the

display and the controls —the buttons, switches, and

knobs — needed to access GPS functionality. For

this reason, most of the controls serve to activate

multiple functions.

Taken together, the large number of accessible

functions and limited number ofcontrols will lead to

the inadvertent activation of unwanted functions.

When this happens, the pilot may not be familiar

enough with the display configuration to correctly
recover from the mistake without accessing the operations manual. The end result is confusion for the

pilot, increased head-down time, and possible airspace blunders. It is unlikely that anyone would be

familiar with the entire operations manual for a

particular device, and it is also unlikely that a pilot

would be able to know what to do in every situation

without referring to the manual. One possible fix for

this problem is to require an "undo" button/function

on the unit. This would allow a pilot to return to the

previous display configuration and proceed with the

intended operation.

Knobs and Dials

The following problems have been noted for more

than one GPS unit currently used for instrument

approaches.

ACTIVATION FEEDBACK

The checklist suggests that all knobs and buttons

should provide adequate activation feedback to the

user. Although this suggestion is part ofthe checklist's

bench test section, it is important that activation

feedback be sufficient during flight, when noise and

vibration are present. At least one ofthe units on the

market provided no tactile or auditory feedback indicating that a button had been activated. There was no

noticeable tactile or auditory feedback when pressing

the button, and visual feedback occurred only after a

delay, when the screen display changed. A lack of

feedback can lead to multiple button presses and the

activation of displays and functions that were not

intended.

BUTTON PLACEMENT

The checklist indicates thatthe risk ofinadvertent

activation or deactivation of GPS functions should

be minimized. An example of a violation of this

principle is shown in Figure 1 and concerns the

placement of 3 buttons.

UM

. u 1 1 CLR IE^^H

Figure 1: Example of poor button placement

The figure portrays a portion ofthe control interface of one of the more popular GPS units on the

market. Three buttons are shown and located at the

bottom of the display.

They include the "direct-to" (capital D with an

arrow), "CLR" (clear), and "ENT" (enter) buttons.

While performing an approach, ifthe pilot elects to

execute a missed approach, the procedure for this

GPS unit (and most others) is to push the "direct-to"

button and then the "enter" button. The placement

ofthe clear (CLR) button between the direct-to and

enter buttons increases the likelihood of accidentally

pressing the clear (CLR) button during the execution

of the missed approach procedure. In so doing, the

display changes to one thatis not expected, leading to

confusion about how to recover. Furthermore, recovery may require extensive reprogramming ofthe GPS

unit. This would certainly add to the already high

degree ofstress involved in executing an unplanned

missed approach. Maintaining a specified minimum

altitude and course, while trying to reprogram the

GPS unit, could create a visually and mentally overloading situation. Repositioning the receiver buttons

is warranted, though again, one possible solution to

this problem would be an "undo" button/function.

Knob Issues

Many ofthe GPS units reviewed required the use

of a rotary-type knob to select the information required for an operation. One example is in the selection of airport identification codes. These codes

consist of three or four alphanumeric characters of

the form F28, KSTL, OKC, etc., thatidentify various

airports, VORs, and other navigational waypoints.

Some receivers do not allow the input to "wrap

around" from A, back to Z, using a single counterclockwise turn. Instead, to go from A to Z, the user

must dial forward (clockwise) through the entire list.

Also, ifthe users inadvertently dial past the intended

character, theymust continue to dial forward through

the list until again reaching the correct character.

This arrangement leads to significantly increased

head-down time while using the receiver, a problem

that has been mentioned in recent GPS studies (Williams, 1998; Wreggit & Marsh, 1998).

A second major issue concerning the use of knobs

is that some receivers allow the knob to be in one of

two physical positions, either out or in. Pulling the

knob out enables different functionality than when

the knob is pushed in. Using the knob in the alternate
position (pulled out) is not required to operate the

receiver, and the pilot may not be aware that it

functions in this manner. A serious problem with this

arrangementis thatitis difficult to know whether the

knob is out or in without first testing it. None ofthe

units provide an indication of the knob's position,

such as a warning light, on the display. As one might

be in the habit ofperforming a certain function with

the knob at a particular pointin a flight (for example,

during the set up for an approach), it is unlikely that

the correct knob position will be ascertained. The

unit could fail to respond in the expected manner,

leading to confusion and requiring a correction.

Using a procedural checklist would avoid this type of

error; however, referencing a checklist would significantly increase the head-down time of the pilot.

Labeling

ANNUNCIATOR VARIATIONS

Some GPS units have an associated annunciator

panel to provide indications of GPS status and satellite availability to the pilot. The annunciator panel, if

used, is not necessarily co-located with the GPS unit

and depends on room available on the cockpitinstrument panel. If the annunciator panel is not located

near the GPS unit, or the navigational instrument

used during an instrument approach, there is a possibility that the pilot will miss critical information

displayed on the various instruments.

Also, annunciator panels can be made by a third

party manufacturer, so that the panel for a particular

unit can vary from airplane to airplane. The labeling

of GPS modes varied across the units reviewed. Figure 2 shows two ofthe several different GPS annunciator panels reviewed.

In particular, notice the third box of each annunciator panel in Figure 2, labeled unit A and unit B

respectively. TSO-C129a (FAA, 1996), Par

(a)(3)(xi)(2), states that, "The equipment shall provide the capability for accomplishment of holding

patterns and procedure turns. Activation ofthis function shall at least: a. Change automatic waypoint

sequencing to manual. ... d. Permit the pilot to

readily return to automatic waypoint sequencing at

any time prior to the designated fix ('TO' waypoint)

and continue with the existing flight plan."

UNIT A

Switch and light Light only Switch and light Switch and light

UNIT B

GPS APR

ARM/ACTV

Switch and light Switch and light

Figure 2: Two examples of annunciator panels

GPS SEQ

AUTO/HOLD

Switch and light

GPS MSG

GPS WPT

Light only
For unit A, the annunciator button for manual

interruption ofautosequencing, for purposes ofholding and procedure turns is called GPS SEQ, and

includes two modes labeled AUTO and HOLD.

Unit B labels their annunciator button GPS CRS,

with the two modes labeled OBS and LEG. A third

brand, not shown in the figure, does not have an

external annunciator button and automatically enters the "hold" function when outbound on an approach and enters an "auto" sequence function when

inbound. As will be discussed below, there can be

confusion over the labeling and operation of a panel

and receiver if a pilot is required to suspend and

restart autosequencing.

BUTTON LABELS

The number and variety of buttons included on

each GPS display is different for every unit on the

market. In addition, buttons that perform the same

type of task on different units can have different

labels. Table 1 presents several popular brands of

GPS units, along with a listing of the buttons and

their labels that are present on each of the units.

In addition to the buttons listed, most of these

units have one or two ganged knobs. A ganged knob

consists of an outer, shorter knob, surrounding an

inner, taller knob. As was mentioned earlier, the

inner knob can be pulled out or pushed in to provide

additional functionality for some of the units.

Keep in mind that all ofthe units reviewed provide

essentially the same set offunctions but require accessing specific functions in different ways. Some functions

that are accessed through a knob on one unit may

require a button press on a different unit. While most

units have an "enter" (ENT) button to initiate a selection, Unit E in Table 1, for example, uses a button

labeled ACK for this task. This lack ofconsistency and

operational vagaries make it very difficult for a pilot to

transition from one type of unit to another.

Procedural Problems

SELECTING AN ALTERNATE AIRPORT

The selection ofan alternate airport requires many

steps, since approaches can only be selected from the

active flight plan and only one approach can be stored

at one time. Approaches cannot be stored in inactive

flight plans. The selection of an approach to an

alternate is done after the decision to go to the

alternate airport has been made. If the alternate

airport is fairly close to the primary airport, very little

time is available to accomplish the program change,

as well as other required tasks. Many GPS units allow

setting up a second flight plan. This feature could be

used to establish a route to an alternate airport but

not the approach itself. Ifthe procedure for selecting

a second flight plan is not often used, it might be

difficult for a pilot to recall the details, should the

situation warrant. Minimizing workload in this situation requires the pilot to be well rehearsed in the

procedure beforehand. This advice, of course, holds

for many ofthe problems discussed in this paper. The

pilot could also program additional waypoints after

the destination, if the receiver allows. However, as

was determined during WAAS receiver testing (Winter & Jackson, 1996), some pilots do not take the

time to input a complete flight plan into their GPS

receiver, much less a flight plan plus additional

waypoints to serve as an alternate destination.

AUTOMATIC VS. MANUAL WAYPOINT SEQUENCING

One of the most often cited problems occurring

during the operational testing of these systems, to

date, involved either placing the receiver in a mode

where it automatically sequences from one waypoint

to the next during the approach or in a non-sequencing mode. The non-sequencing mode is required for

many overlay-type approaches (and some stand-alone

approaches) in which a procedure turn, or hold-inlieu-of procedure turn, is accomplished before

Table 1: Button Labels for GPS Units

Unit Number Labels

A 7 buttons CRSR (2), MSG, ALT, D->, CLR, ENT

B 10 buttons CRSR MSG, D->, CLR, ENT, NRST, SET, RTE, WPT, NAV

C 9 buttons MSG, D->, ENT, WPT, NAV, FPL CALC, AUX, APT/VOR

D 10 buttons MSG, EMG, NAV, DB, FPL SYS, D->, SFJL, INFO, ENT

E 6 buttons CRSR (2), D->, ALRT, MSG, ACK
establishing the aircraft on the final approach. In

recently completed operational tests of these receivers (Winter & Jackson, 1996), it was reported that

subject-pilots frequendy forgotto take the GPS receiver

out ofthe "hold" function after completing the procedure turn. Pilots were often unable to proceed to the

next approach fix because the receiver was still in the

"hold" function, and the pilot was often unable to

determine the problem. Usually, the safety pilot had to

prompt the subject-pilots about the failure, so as to

prevent them from flying about aimlessly. Pilots were

also sometimes unable to suspend autosequencing because the unit had already sequenced to the following

waypoint before the pilot had taken the required actions.

Other testing found that the procedure for suspending and re-establishing automatic waypoint sequencing differed from unit to unit. For many of

them, a button or switch on the annunciator panel

was used to suspend and renew sequencing; however,

for one unit, suspension and re-establishment of

waypoint sequencing was primarily automatic. While

this generally decreased pilot workload, if the pilot

was required to suspend waypoint sequencing with

this unit, the complex procedure was as follows:

• Press the direct-to button;

• Turn outer knob one click counterclockwise;

• Turn inner knob until proper course is selected;

• Press direct-to button again.

the same approach procedure. In this configuration,

the GPS unit will present a message to the pilot each

time a new heading should be selected. One possible

scenario for pilots who rent aircraft is a situation in

which they are given an identical airplane, with

identical GPS equipment, butthe instruments behave

differently during the approach because ofdifferencesin

the way the equipment was installed. In this situation,

it may be difficult to determine ifthe aircraft has autoslewing capability until after it is airborne.

A second example concerns the selection and deselection ofthe GPS unit during an approach. Some

receivers automatically de-select the GPS unit when

an Instrument Landing System (ILS) frequency is

dialed in to the navigation radio. Under this condition, the ILS display is receiving its information

solely from the ILS ground station. For other receivers, the GPS signal is not de-selected unless it is done

manually by pushing a button on the annunciator

panel. Whether the de-selection is automatic or

manual is determined solely by the installation ofthe

GPS unit.

A situation can arise where the pilot believes that

the GPS has been de-selected when it has not. This

will result in a case in which the glide slope indicator

on the ILS display is controlled by the ILS signal, but

the course indicator on the same display is controlled

from the GPS.

To then re-establish waypoint sequencing, the

pilot was required to press the direct-to button twice.

In addition to being non-intuitive, this procedure

demonstrates the use of a button to accomplish a

function for which it was not labeled and not originally designed.

INCONSISTENT FUNCTIONALITY

One final human factors problem to be discussed

concerns differences in the way each GPS receiver

functioned, depending on how it was installed in the

aircraft. One example is found with the auto-slewing

function for receivers connected to a horizontal situation indicator (HSI) used during a Distance Measuring Equipment (DME) Arc approach. GPS

receivers can be installed in such a way that the HSI

will automatically slew from one heading to the next

during the execution of an approach procedure. A

different installation with the same GPS unit will

result in a configuration requiring the pilot to manually dial in new headings on the HSI while executing

CONCLUSIONS

This paper was not intended to be overly critical of

GPS manufacturers or currently used GPS equipment. However, it is important, given the problems

stated in this paper, that efforts are made to discover

the frequency of occurrence of these problems and

what their effects are on pilot workload and performance. As was stated in the introduction, the primary

problem with most receivers is that they have a large

number ofavailable functions, but a limited number

ofcontrols for activating those functions. Also problematic is the manner in which GPS functionality is

implemented. Different procedures are required for

the same functions for every receiver on the marketeven some units made by the same manufacturer.

It is doubtful that easy solutions exist for all ofthe

problems mentioned in this paper. However, opportunities for GPS improvements are presented below

for the FAA, GPS manufacturers, and the end users -

the pilots.
FAA

One suggested solution to the problem ofreceiver

complexity is to reduce the number ofdifferent kinds

of GPS approaches that receivers need to accommodate. The elimination of approaches (overlays and

GPS approaches containing procedure turns) that

require suspension of automatic sequencing of

waypoints would eliminate the need for the extra

functionality required to accomplish this task. Most

ofthe approaches requiring suspension of automatic

sequencing are overlay approaches. Overlay approaches are approaches that were previously established VOR or NDB approaches that have been

redefined by the FAA as GPS approaches. Steve

Jackson, of the FAA Flight Standards Division, has

suggested that, "The GPS 'T approach must be

established as the standard to maximize GPS receiver

capabilities" (personal communication, February 18,

1998). Operationally, a "T" approach would eliminate the need to suspend waypoint sequencing, and

would likely reduce pilot workload during a highworkload portion offlight.

Manufacturers

Designing an "undo" function on all of these

receivers would keep the pilot from becoming lost

after making an entry error and reduce control inputs

during critical phases of flight. Whether standards

are imposed by the FAA, or volunteered by manufacturers, some standardization ofbutton labels, annunciator panels, and displays is needed. At a minimum,

a core set of GPS functions should be performed in

essentially the same way for every unit on the market.

John Steuernagle, ofthe Airplane Owners and Pilots

Association Air Safety Foundation, has recommended

that standardized procedures be designed for the

following set of GPS functions (J. Steurenagle, personal communication, February 3, 1998):

• Selecting a waypoint

•Establishing a course to or from a waypoint

• Selecting and activating an approach

• Transitioning to a missed approach procedure

• Interruption of autosequencing.

Based on findings stated earlier, two items that

could be added to this list are the re-activation of

automatic waypoint sequencing, and the selection

and activation ofan approach to an alternate airport.

Pilots

The Aeronautical Information Manual (FAA,

1998, p. 1-1-49) recommends that, before using any

particular receiver for instrument flight, pilots should

practice GPS approaches under visual meteorological

conditions until thoroughly proficient with all aspects of their equipment (receiver and installation).

The pilot should practice:

• Utilizing the receiver autonomous integrity monitoring (RAIM) prediction function

• Inserting a Standard Instrument Departure (SID)

into the flight plan, including setting terminal

Course Direction Indicator (CDI) sensitivity, if

required, and the conditions under which terminal RAIM is available for departure (some receivers are not SID or STAR capable)

• Programming the destination airport

• Programming and flying the overlay approaches

(especially procedure turns and arc)

• Changing to another approach after selecting an

approach

•Programming and flying "direct" missed approaches

•Programming and flying "routed" missed approaches

•Entering, flying and exiting holding patterns,

particularly on overlay approaches with a second

waypoint in the holding pattern

• Programming and flying a "route" from a holding

pattern

• Programming and flying an approach with radar

vectors to the intermediate segment

• Indication ofthe actions required for RAIM failure both before and after the Final Approach

Waypoint (FAWP)

• Programming a radial and distance from a VOR

(often used in departure instructions).
In addition, Steve Winter, of the FAA Flight

Standards Division recommends familiarization with

the following additional procedures (S. Winter, personal communication, September 25, 1996):

•Recovering from flying past a waypoint where

holding was intended, after failing to place the

receiver in the hold mode;

•Adding another waypoint prior to the Initial

Approach Fix (IAF) waypoint after entering the

approach procedure data into the flight plan;

• Rejoining the course between two waypoints after

being cleared and proceeding directly to another

waypoint.

Pilots should never assume that familiarity with

one GPS unit will facilitate learning to use another

unit. During the course ofthe operational tests mentioned in this paper, there were several instances of

pilots having difficulty transitioning to the units

used in the test, despite familiarity with their own

GPS units. Pilots should make certain that they are

comfortable and proficient with the unit thatis to be

used for that flight. Also, pilots should not assume

that a familiar type of GPS unit will interact with all

avionics displays as expected, due to possible differences in installation procedures.

Finally, if a pilot finds himself/herselftotally lost

and unfamiliar with what is seen on the GPS display,

they should have a backup procedure ready to implement. Ifall else fails, pilots should be prepared at any

time during the flight to simply turn the unit off.

They shouldn'tfollow their GPS unitinto the ground.

REFERENCES

FAA (1995). FAA aircraft certification human factors

and operations checklist for standalone GPS receivers (TSO C129 Class A). DOT/FAA/AAR95/3. U.S. Department ofTransportation, Federal Aviation Administration, Washington, DC.

FAA (1996). TSO-Cl29a,Airbornesupplementalnavigation equipment using the global positioning

system (GPS). U.S. Department of Transportation, Federal Aviation Administration, Aircraft

Certification Service, Washington, DC.

FAA (1998). Aeronautical information manual: Official

guide to basic Eightinformation andATCprocedures. U.S. Department ofTransportation, Federal Aviation Administration, Washington, DC.

Williams, K.W. (1998). GPS design considerations:

Displaying nearest airportinformation. U.S. Department of Transportation, Federal Aviation

Administration, Office of Aviation Medicine,

Washington, DC. DOT/FAA/AM-98/12. NTIS

# ADA346043.

Winter, S. & Jackson, S. (1996). GPS Issues U.S.

Department ofTransportation, Federal Aviation

Administration, Standards Development Branch,

Oklahoma City, OK. DOT/FAA/AFS-450.

Wreggit, S.S. & Marsh, D.K., II. (1998). Cockpit

integration of GPS: Initial assessment - menu

formats and procedures. U.S. Department of

Transportation, FederalAviation Administration,

Office of Aviation Medicine, Washington, DC.

DOT/FAA/AM-98/09. NTIS # ADA341122.