GAO 

United States General Accounting Office 

Report to the Chairman, Committee on 

Science, Space, and Technology, House 

of Representatives 

October 1989 AVIATION WEATHER 

FAA Needs to Resolve 

Questions Involving 

the Use of New Radars 

GAO/RCED-90-17 
United States 

General Accounting Office 

Washington, D.C. 20548 

Resources, Community, and 

Economic Development Division 

H-22221 7 

October 12, 1989 

The Honorable Robert A. Roe 

Chairman, Committee on Science, 

Space, and Technology 

IIouse of Representatives 

Dear Mr. Chairman: 

In response to your August 1988 request, we evaluated specific aspects 

of three weather-related systems: the Airport Surveillance Radar (ASH-S) 

weather channel, the Terminal Doppler Weather Radar (TDWR), and the 

Aeronautical Data Link. Developed as part of its National Airspace System (NAS) Plan, these three systems are designed to enhance the Federal 

Aviation Administration’s (FAA) ability to disseminate hazardous 

weather information to pilots.’ 

Specifically, the ASK-9 radar will support air traffic control functions, 

but unlike previous surveillance radars, it has the capability to provide 

more precise information on precipitation location and intensity. The 

TDW~~ will detect low-altitude wind shear and other hazardous weather 

conditions in airport terminal areas so that pilots can avoid or prepare 

for encounters with such events. The Aeronautical Data Link will initially be used to enable pilots to directly access weather information 

from FAA ground-based systems, instead of receiving this information by 

voice communication from controllers. 

The availability of weather information from these systems is vital to 

the safety of aviation. In 1986, the most recent year that data are available, weather was cited as a factor in almost 44 percent of all major 

airline accidents. 

As agreed with your office, the objectives of our evaluation were to (1) 

describe FAA'S progress in preparing operational procedures for using 

and disseminating weather information from the ASIt-9 and the TDWR and 

(2) assess the status and availability of initial data link weather 

services. 

‘These systems arc‘ disrussed in further detail in Aviation Weather: Status of FAA’s New Ikudrdous 

Weather Detection and Dissemination Systems, GAO/RCED-87-208, Sept. 29, 1987. 

Page 1 GAO/RCED-!jO-17 Aviation Weather 
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Results in Brief We found that 

. FAA has not established formal procedures for sending ASK-9 weather 

data from air traffic controllers to pilots, although the first radar is now 

operational and additional radars will soon be deployed. FAA believes 

that before implementing formal procedures, controllers need to experience basic changes in the system’s precipitation detection capabilities. 

Moreover, a policy question regarding whether to route aircraft around 

storms using ASK-9 weather data will not be answered until FAA learns 

more about precipitation effects on aircraft and the work load effects on 

controllers. 

9 FAA is currently evaluating procedures for disseminating TDWR weather 

information. Operational tests have shown that the TDWR can detect 

wind shear and microbursts. Unresolved is how this new information 

will be integrated within existing air traffic control procedures. 

Instances of pilots misinterpreting advisories from controllers occurred 

during operational testing. Several pilots appeared to key on the part of 

the controller message authorizing them to land, not on the warning of 

intense wind shifts. Although FAA has considered some changes to the 

structure and content of these advisory messages, it has not yet established how this will be eventually performed. In addition, FAA's reluctance to establish procedures for rerouting aircraft during periods of 

wind shear stems from uncertainties about added controller work load, 

as well as the policy implications this would have on pilot and controller 

responsibilities. 

9 Initial weather services, such as hazardous weather advisories, which 

FAA plans to provide aircraft through the Aeronautical Data Link, may 

be delayed for more than 2 years-from December 1990 to March 

1993-because of significant slippage in the schedule for a major interrelated system. Additionally, that delay will require FAA to conduct 

another set of operational tests after data link processors are installed 

in the field. Any problems uncovered in that testing could result in additional program costs, as well as further delays in the schedule for the 

initial weather services. 

Background 

u 

Hazardous weather is a factor in many aviation accidents, and wind 

shear can be particularly disastrous for aircraft, especially when 

encountered below 1,000 feet. Microbursts, the most hazardous form of 

wind shear, produce more powerful and concentrated down drafts than 

other wind shear events. Between 1975 and 1985, wind shear and 

microbursts contributed to 14 major aircraft accidents, resulting in over 

400 deaths. In addition to being a safety concern, hazardous weather 

Page 2 GAO/RCED-90-17 Aviation Weather 
H-222217 

-_-_ - ^_,_ -.^-_.-___I_. 

conditions affect the air traffic control system’s operational efficiency. 

A recent FAA report on system delays states that 65 percent of airline 

delays were attributable to weather. To help detect hazardous weather 

and provide such vital information to pilots, FAA has designed the AM-9 

weather channel, the TDWR, and the Aeronautical Data Link. 

ASR-9 Operational 

Procedures 

Although the initial AsI<-9 radar is now operational at one field location 

and more units will follow, FAA has not implemented new operational 

procedures for the interpretation and dissemination of precipitation 

information from the system’s weather channel. Operational procedures 

provide specific guidance describing how the equipment and data are to 

be used in conjunction with air traffic control functions. FAA originally 

planned to test and develop operational procedures before installation 

of the radars. However, FAA officials now believe that a good understanding of the ASR-Q'S ability to detect hazardous weather is necessary 

before they prepare ironclad operational procedures and assess whether 

controllers can use the system to redirect traffic. The new radar provides air traffic controllers with a fundamentally new tool for identifying precipitation location and intensity. Previous airport radars 

provided some low quality data on precipitation location but little information on storm intensity. Because the ASR-9 weather channel provides 

such a new capability over and above that available with previous surveillance radars, FAA officials believe that controllers should experience 

the system before procedures are made final. As a result, FAA has not 

implemented new operational procedures, although the first ASK-~ is now 

deployed in the field, and additional radars will soon become 

operational. 

FAA officials do not believe that the absence of operational procedures 

hinders controllers’ use of the radar’s weather capabilities. For example, 

controllers at the first ASR-9 operational site are now providing advisory 

voice messages on precipitation intensity to pilots, Controllers had previously issued this information based on printed reports received from 

the National Weather Service. FAA officials believe that an evaluation of 

this method is necessary before they decide how to change procedures. 

They also pointed out that the absence of operational procedures does 

not affect the radar’s primary use-identifying aircraft for safe separation-since that is a separate function of the radar. However, the 

agency has not set time frames for implementing ASR-9 weather channel 

procedures. Without operational procedures, controllers have no guidance on how often to use the AsR-9 weather channel, or how to interpret 

the precipitation display. 

Page 3 GAO/RCED-90-17 Aviation Weather 
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AsIt-9 weather capabilities raise a policy question of whether FAA and the 

airline industry should modify the advisory nature of weather information provided by the agency. Currently, FAA does not require controllers 

to use weather information to reroute planes, although in practice controllers may at times choose to redirect aircraft around severe weather, 

such as thunderstorms, or suggest alternative routes. Weather radar 

systems now aboard many aircraft can provide information superior to 

that available from previous airport radars. Further, pilots are responsible for their flight’s safety and often contact controllers for permission 

to reroute when they encounter adverse weather. These requests can 

result in hectic situations for controllers, possibly slowing air traffic. An 

FAA official said that controllers would prefer to project the traffic flow, 

rather than react as it happens. 

The ASIUJ'S improved weather detection could enhance national airspace 

system efficiency and safety by helping controllers anticipate rerouting 

requests or by giving them the information they need to reroute planes 

around adverse weather. However, according to FAA, it needs to learn 

more about the hazards of intense precipitation. For example, it needs to 

evaluate the effect of various levels of precipitation on different aircraft 

before it can instruct controllers on when to route planes around such 

weather. (Additional details on the ASR-9 are provided in app. I.) 

TDWR Operational 

Procedures 

FM plans to have the first TDWR reach operational status by June 1993, 

with a total of 47 radars scheduled to be installed over the following 3 

years. As part of its testing of the new radar, FAA is evaluating procedures for disseminating hazardous weather data from the TDWR to pilots. 

Operational tests in Denver during 1988 showed that the radar can reliably detect most wind shear and microburst phenomena. Despite this 

efficacy at locating wind shear events, FAA and aviation industry 

experts are concerned about the adequacy of using procedures developed for a less capable weather system to alert pilots to events identified by the more sophisticated TDWH. Instead of establishing new 

operational procedures for TDWK testing, FAA had controllers use the 

same method of relaying reports of microburst activity to pilots as they 

use for the current ground-based system, the Enhanced Low Level Wind 

Shear Alert System (ELLWAS).~ In other words, controllers issued the 

microburst alert, together with wind speed and direction, as part of a 

:!Thc ISLLWAS is an upgraded version of a ground-based system used to detect wind shear and 

microbursts. The I%I,WAS consists of 1 1 wind sensors placed around an airport that detect changes in 

wind speed and direction for dissemination to controllers and pilots. 

Page 4 GAO/RCED-90-17 Aviation Weather 
B-222217 

longer air traffic control voice message. FAA used existing procedures to 

avoid confusion on the part of controllers and pilots, especially since the 

T~WR was operational for only part of the day. 

Ilowever, the use of existing procedures was not completely effective, as 

highlighted by an incident during the testing. Four commercial airline 

pilots from one airline flew into microburst activity, despite prior warning by the controllers and explicit airline policy to avoid microburst 

alert areas. Post-occurrence analysis by the airline concluded that the 

crews either did not hear clearly or did not know the meaning of the 

term “microburst alert.” The crews for three of these flights could not 

recall hearing microburst alert upon initial contact with the tower. IIowever, two of these crews did recall their clearance to land from the same 

transmission. The pilots’ not recognizing the warnings has been attributed at least in part to the structure of the microburst alert message, 

which is inserted in the middle of routine communications providing 

clearance to land. Pilots reportedly keyed into the phrase that concludes 

with the words: “clear to land.” This problem points to the need for FAA 

to refine procedures for disseminating microburst alerts during future 

testing. 

FAA has considered some alternatives to deal with this cognitive problem, such as providing microburst alerts as a distinctly separate advisory, or replacing the clear to land ending of the message with the new 

instruction, “say intention.” In the latter case, the pilot would be 

required to respond whether he intended to land or terminate his final 

approach. During TDWR testing to be conducted at Kansas City this summer, FAA plans to move wind speed and direction information away from 

the alert message to reduce pilot confusion. 

Alternatively, FAA could substantially reduce the potential of missing an 

alert message by requiring that controllers redirect planes from runways with microbursts. For example, during the 1988 Denver tests, controllers adopted the practice of routing aircraft away from runways 

experiencing a microburst alert. Under existing policy, such decisions 

are left up to the pilots. While not established as a formal policy, controllers also routinely reroute planes because of thunderstorms. IJsing 

this approach for microburst alerts could offer several advantages over 

existing procedures, including (1) providing controllers with a more stable situation in which they can better predict traffic flow, (2) providing 

earlier warnings and more time for pilot response, and (3) eliminating 

potential problems with pilots not heeding a warning. Notwithstanding 

Page 5 GAO/RCED-90-17 Aviation Weather 
B-222217 

the policy implication this would have on controller and pilot responsibilities, FAA is reluctant to institute rerouting as a formal procedure 

without analyzing the resultant work load on its controllers. FAA has no 

current plans to test the feasibility of this alternative. 

To help define concerns and develop potential solutions to implementation problems, FAA established a user group specifically associated with 

the TDWR. The group consists of pilots, controllers, engineers, and meteorologists from both government and private industry that have been 

instrumental in helping to develop the user and display requirements for 

the system. We believe that the concept of a user group could serve as a 

model for coordinating system design and user needs on other FAA 

projects, including the ASK% (Additional details on the TDWR are provided in app. II.) 

Status of Aeronautical Weather services are the first planned use of the Aeronautical Data 

Data Link Weather 

Services 

Link. These initial services may not be available at the first field site 

until March 1993, well beyond FAA'S original schedule of December 1990. 

Services at all 22 sites are planned to be operational by March 1996. The 

Aeronautical Data Link’s initial weather services rely on the development of interdependent systems. In addition to needing the data link 

processor-which will retrieve data from other weather systems and 

prepare it for transmission to controllers and pilots-the use of the system depends heavily on integration of the Mode S.” The Mode S system 

will be the primary means through which FAA actually transmits 

weather messages to the aircraft. Although the data link processor is on 

schedule, the Mode S program has suffered significant delays with its 

first system now scheduled to be operational by December 1992. As a 

result, the initial weather services that FAA intends to have available at 

the first operational site may be significantly delayed. 

Additionally, the slippage in the Mode S program will require FAA to conduct an additional set of operational tests. FAA will initially test the data 

link processor with a simulated Mode S, since an actual version of that 

system will not be available. If those tests are successful, the processors 

will be installed at field locations, After the Mode S is delivered to FAA'S 

testing facility, a separate round of operational tests will be conducted 

to assure that the two systems interface properly. According to FAA officials, using simulations will not provide FAA as much confidence as using 

“The Mode S system consists of sensors and antennae on the ground for receiving and transmitting 

information from and to aircraft. Mode S will replace existing radar beacon systems aboard aircraft. 

Page 6 GAO/RCED-90-17 Aviation Weather 
--.... All.“--.. 

B222217 

real sys tems. Therefore, some risk exis ts that changes will need to be 

made to the data link processors after they are ins talled in the field. If 

that occurred, ~ti would incur additional costs and the schedule for the 

initial weather services would be further delayed. The Aeronautical 

Data Link program manager believes that FAA must proceed with ins talling the data link processor, because substantial additional costs would 

be incurred on the contract if it was delayed to wait for an actual Mode 

s . 

E’AA expects the quality of the initial weather services to improve as new 

data acquis ition, processing, and communication sys tems are integrated 

into the sys tem. For example, upgrades to the automated weather 

observing sys tem will update surface observation information for data 

link every minute, as compared with the initially planned hourly 

updates. Additionally , FAA has identified enhanced services, such as air 

traffic control functions and wind shear alerts from the TDWH, that could 

be phased-in after the initial weather services. However, FAA has no 

plans to provide ASR-9 weather channel data v ia the Aeronautical Data 

Link . (Additional details on the Aeronautical Data Link are provided in 

app. III.) 

Conclus ions In recent tes ts , FAA has demonstrated that the ASR-9 and TDWR sys tems 

will provide air traffic controllers with improved information on precipitation and wind shear location and intens ity . These capabilities , combined with effec tive operational procedures describing how the 

equipment is to be used and how the information will be disseminated, 

could reduce the number of weather-related accidents and save lives. 

Furthermore, the Aeronautical Data Link could upgrade the quality of 

weather information available to pilots . 

Although the firs t AXI-9 is now operational, FAA is deferring the establishment of firm operational procedures until controllers are more familiar with the sys tem. However, the agency has set no time frames for 

developing those procedures. W hile deferring final guidance may be justified to gain greater experience, we believe that the impending full 

deployment of the AS&~ requires FAA to promptly issue operational 

guidelines that provide interim direc tion to controllers operating its 

weather channel, particularly if such a deferral will be lengthy . The 

implementation of operational procedures would maximize use of the 

ASK-~'S new capability and ensure that controllers use it consis tently . In 

the longer term, to achieve the greatest benefit from the radar’s weather 

channel, IJAA needs to address how this information will be used and to 

P&l@?7 GAO/RCED-90-17 Aviation W eather 
- -_-~ -- 

R-222217 

evaluate the effects of the various alternatives, such as redirecting 

flights, on national airspace safety and efficiency. Resolving the fundamental safety and policy questions involving ASRQ weather data use will 

require advice from air traffic controllers, meteorologists, airlines, 

pilots, and aircraft manufacturers, since those groups all play an important role in the national airspace system. FAA should expedite the process of answering these questions and implementing effective 

procedures by replicating its successful TDWR experience of assembling 

and achieving consensus from a working group made up of experts from 

these varied interests. 

While deployment of the TDWR is not scheduled until 1993, concerns 

about operational procedures for the new radar also need to be resolved. 

?‘I)WR operational testing to date has shown that the procedures used to 

warn pilots of wind shear and microbursts were not always effective. 

Analysis of 1988 testing showed that pilots may have misunderstood or 

not heard the type of warning issued using existing procedures. Because 

the first 'I‘DWR will not be operational until 1993 and therefore testing 

will continue for the next 4 years, FAA should use that time to thoroughly evaluate the impact and effectiveness of the full range of alternative procedures for TDWR, including the potential for redirecting 

flights around microbursts. 

One of the major challenges to achieving the full promise of FAA'S 

weather-related systems is the integration of their new capabilities into 

the existing air traffic control environment. While technical capabilities 

to detect hazardous weather information have been developed, effective 

dissemination of this information will likely require a policy decision 

concerning the controller’s authority to reroute traffic during periods of 

inclement weather. We recognize that if controllers are to use hazardous 

weather warnings as more than just advisory information for pilots, this 

will entail a change in the way FAA controls aircraft and will also require 

fundamental changes by users of the national airspace system. The current problems associated with developing operational procedures for the 

ASR-9 and the TDWH reflect FAA'S difficulty in tackling this issue. 

Although a long-term resolution of this policy issue is required for FAA to 

obtain maximum benefits from these new systems and capabilities, 

action should be taken now to enhance the basis from which such a decision will be made. 

Page 8 GAO/RCED-90-17 Aviation Weather 
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B-222217 

Recommendations To ensure the consis tent dissemination and use of new weather data 

available from the ASR-9, we recommend that the Secretary of Transportation direc t the Adminis trator, FAA, to (1) establish time frames for 

issuing A%-9 operational procedures, (2) convene a joint government/ 

indus try user group, s imilar to that used for the TDWR, to resolve uncertainties regarding the use and dissemination of ASR-9 weather data, and 

(3) issue interim guidelines if s ignificant ASR-9 implementation will occur 

before final procedures are developed. 

W e also recommend that the Secretary of Transportation direc t the 

Adminis trator, FAA, to evaluate, during subsequent operational tes ts of 

the TDWR, the impact and effic iency of having controllers direc t aircraft 

around microbursts. The agency could then resolve the policy question 

concerning the dissemination of microburst warnings and therefore 

implement the most effec tive operational procedures. 

Views of Agency 

Offic ials 

FAA and Department of Transportation offic ials provided oral comments 

on a draft of this report. These offic ials agreed with our conclus ions and 

recommendations. The offic ials did suggest some technical changes to 

the report which we incorporated as appropriate. 

In conducting our review, we examined pertinent sys tem planning documents, tes t results , interv iewed FAA and Martin Marietta offic ials , and 

partic ipated in a sess ion of the TDWR working group. (See app. IV for 

details on our scope and methodology.) Our review was conducted 

between August 1988 and July 1989 in accordance with generally 

accepted government auditing s tandards. 

As agreed with your office, unless you public ly announce its contents 

earlier, we plan no further dis tribution of this report until 7 days from 

the date of this letter. At that time we will send copies to the Secretary 

of Transportation and the Adminis trator, FAA. This work was done 

Page 9 GAO/RCED-90-17 Aviation W eather 
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-- 

under the direction of Kenneth M. Mead, Director, Transportation 

Issues, who may be reached at (202) 275-1000, if you or staff have 

any questions. Other contributors are listed in appendix V. 

Sincerely yours, 

c/ Assistant Comptroller General 

Page 10 GAO/RCED-So-17 Aviation Weather 
Page 11 GAO/RCED-90-17 Aviation Weather 
-- 

Contents 

*’ 

Letter 1 

Appendix I 

Information on the 

Airport Surveillance 

Radar Weather 

Channel 

Weather Channel Capabilities 

System Status and Development Plans 

Appendix II 

Information on the 

Terminal Doppler 

Weather Radar 

TDWIZ Products 

System Status and Development Plans 

16 

16 

16 

Appendix III 

Information on the 

Aeronautical Data 

Link System and 

Services 

Data Link Component Systems 

Data Link Integration and Interdependence 

Descriptions of Initial Weather Services 

18 

18 

18 

20 

Appendix IV 

Objectives, Scope, and 

Methodology 

23 

Appendix V 

Major Contributors to Resources, Community and Economic Development 

This Report Division, Washington, D.C. 

24 

24 

Tables 

Figure I 

Table III. 1: Data Link Component Systems Schedule 

Table 111.2: Initial Data Link Weather Services 

Table 111.3: Enhanced Data Link Services 

Figure III. 1: Interdependencies of Data Link Component 

Systems 

Page 12 GAO/RCEDBO-17 Aviation Weather 
____ r-” ..-. ._ ..----- 

Contents 

Abbreviations 

ASR-9 Airport Surveillance Radar - 9 

DLP Data Link Processor 

ELLWAS Enhanced Low Level Wind Shear Alert System 

rJAA Federal Aviation Administration 

GAO General Accounting Office 

NAS National Airspace System 

Tl)Wli Terminal Doppler Weather Radar 

Page 13 GAO/RCED-90-17 Aviation Weather 
Information on the Airport Surveillance RadarWeather Channel 

~.---- -.__. -.- 

The Federal Aviation Administration’s (FAA) new airport surveillance 

radar (As%9) is primarily a short-range, highly accurate radar system 

for monitoring aircraft movement and position within a radius of 60 

nautical miles from the airport terminal. Air traffic controllers use the 

radar’s aircraft position data to keep aircraft safely separated and to 

control their movements into and out of the airport. 

Capabilities of radars that uses digital technology to do more sophisticated weather 

processing than previous radars. ASR9 has a separate capability to provide information on precipitation location and intensity. Previous airport radars provided some low quality data on precipitation location but 

little information on intensity. FAA acknowledged from the onset of system design that precipitation intensity information does not always correlate with areas of potentially hazardous turbulence. FAA argues, 

however, that the system will improve FAA’S weather detection capability. FAA has maintained that the radar’s improved detection of precipitation intensity has the potential to reduce the number of aviation 

accidents in which severe weather is a cause or contributing factor. 

FXA has been evaluating a Doppler conversion for the ASR system. Doppler capability’ would give the ASR the ability to detect microburst 

activity. Doppler conversion may be part of FAA'S planned ASR-10, the 

next-generation surveillance radar. According to FAA officials, this capability could be provided to the ASR-9 system with additional processors 

and software. If As&I), when used in conjunction with the Doppler package, provides acceptable wind shear detection, it might be able to provide wind shear detection for airports that are not among the original 47 

airports receiving TDWR. 

System Status and 

Development Plans in September 1983 for the production of 101 ASR-QS. The program has 

experienced substantial delays primarily because of technical problems 

during the development of the system’s hardware and software. As a 

result, the first ASR-Q was commissioned at Huntsville, Alabama, in May 

1989, about 2-l/2 years behind the 1983 contract schedule. FAA believes 

that all technical problems have been solved based on testing done at 

‘Doppler radars produce detailed, velocity structures of storms and have been used in wind shear 

efforts in the past. 

Page 14 GAO/RCED-90-17 Aviation Weather 
Appendix I 

Iufonuation on the Airport Surveillauce 

Radar Weather Chauuel 

Huntsville prior to commissioning. Five systems had been delivered as 

of June 15, 1989. Plans call for shipment at a rate of 3 per month for the 

remainder of the contract. 

Page 16 GAO/RCED!KL17 Aviation Weather 
mcndix 11 -____-..----______- -.- 

Information on the Terminal Doppler 

Weather Radar 

In 1986, VXA added the Terminal Doppler Weather Radar (TDWR) project 

to the National Airspace System (NAS) Plan in response to overwhelming 

scientific evidence that low-altitude wind shear had caused several 

major air carrier accidents. The program is designed to develop a reliable automated system for detecting low-altitude wind shear in airport 

terminal areas. The system will be used to provide warnings to help 

pilots avoid or prepare for these events on approach and departure. In 

addition, TDWR will provide alerts of other hazardous weather conditions 

in the terminal area and provide advance notice of changing wind conditions in order to permit timely change of active runways. 

TDWR Products Although microburst detection is the primary concern of TDWR research 

and development, other weather information is also of interest. Additional TDWH products offered during operational tests were gust front 

detection,’ wind shift prediction, and precipitation information. Preliminary TDWR testing indicated that aircraft operational efficiency would 

benefit substantially if air traffic control supervisors could receive 

advance warning of wind shifts-caused by gust fronts-that would 

result in a change in runway operations. 

System Status and 

Development Plans 

In the summer of 1988, operational testing at Denver of a TDWH prototype demonstrated to FAA that the system’s overall microburst detection 

rate met its requirements. On the basis of that demonstration, FAA proceeded with procurement of the initial 47 systems. 

FAA awarded the TDWR production contract to Raytheon in November 

1988. The contract provides an option for an additional 55 TDWRS, 

increasing the total to 102 systems. The first production system is 

planned for delivery to the E’AA Technical Center in late 1991 for testing. 

An operational system is scheduled to be delivered in late 1992. Subsequent TI)WIZS are planned for delivery at the rate of three per month 

beginning in early 1993. 

TDWH testing will continue this summer at Kansas City, Missouri, 

although FAA believes that all testing needed to justify initial dcployment has been completed, Further testing is intended to determine any 

environmental and climatological effects on the radar, and also to allow 

controllers to gain experience using the system. 

- 

‘A gust front is the leading edge of cold air outflow from a thunderstorm, Wind shear and turbulence 

along the gust front are potentially hazardous to landing or departing aircraft. 

Page16 GAO/RCED-90-17AviationWeather 
’ . 

-..-. .._________ 

Appendix II 

Information on the Tetial Doppler 

Weather Radar 

Future TDWR development is planned to include technical improvements 

such as predicting microburst strength and detecting weaker gust fronts. 

WA also plans to improve flight crew training in response to microburst 

alerts and the effectiveness of information transfer to controllers and 

pilots from the system. 

Page 17 GAO/RCEDSO-17 Aviation Weather 
Appendix III .- 

Information on the Aeronautical Data Link 

System and Services 

The Aeronautical Data Link is planned to provide pilots with direct 

access to weather services and other flight services, thereby reducing 

the burden on air traffic controllers and flight service specialists. 

Twenty-four data link systems will be installed, including systems at 22 

Air Route Traffic Control Centers, 1 at the FAA Aeronautical Center for 

training, and 1 at the FAA Technical Center for development and testing. 

Data Link Component To provide initial weather services, data link depends on the integration 

Systems of interdependent subsystems (see fig. 111.1). The data link processor is 

the central component of the data link system. It will interface with 

Mode S sensors to receive requests for weather services from pilots and 

route responses for t,ransmission. In order to respond promptly to pilot 

requests, the data link processor will maintain its own database made 

up of weather products received from the Weather Message Switching 

Center, an existing system that disseminates information to flight centers and towers across the country. 

Mode S is planned to provide a communications channel between aircraft and ground systems. It will replace the current Air Traffic Control 

Radar Beacon System at most terminal and en-route surveillance sites 

and also perform aircraft surveillance functions. On board the aircraft, 

the airborne data link processor will enable the pilot to communicate 

with the ground via Mode S. 

Data Link Integration Initial weather services availability depends on the development and 

and Interdependence deployment of data link component systems. Each site must have all 

component systems on-line and properly integrated to have a functioning data link system. Similarly, improvements to the initial weather services depend on the availability of future systems, such as the Weather 

Message Switching Center Replacement and the Automated Weather 

Observing System’s Data Acquisition System.’ The future enhanced services will only be available when Terminal Doppler Weather Radar and 

other systems are operational and properly integrated with the data link 

processor. 

‘The Automated Weather Observing System obtains weather data via sensors and processes the data 

for dissemination. Its Data Acquisition System will function as a message concentrator for the! 

.smsor.s. 

Page 18 GAO/RCED-90-17 Aviation Weather 
Appendix III 

Information on the Aeronautical Data Link 

System and Services 

Figure 111.1: interdependencies of Data Link Component Systems 

[‘- 

~--- 

Airborne 

Data 

Link 

Processor 

(ADLP) 

1 Mode S 

System 

Mode S Sensors 

Weather Message 

Switching Center 

(Replacement) 

WMSC(R) 

Data Link 

Processor 

0-P) 

AWOS Data 

Acquisition 

System + 

(ADAS) 

AWOS 

Automated 

Weather 

Observing 

System 

Page 19 GAO/RCED-90-17 Aviation Weather 
Appendix III 

Information on the Aeronautical Data Link 

System and Services 

A smooth integration of complex data link component systems poses a 

significant challenge to FAA, directly impacting the availability of initial 

weather services. FAA'S data link test plans state the need for adequate 

integration between Mode S and the data link processor, noting that 

these systems are intimately related and neither system can be adequately tested and evaluated without reference to the other. Because of 

delays in Mode S, testing of the data link processor will continue with 

simulators for the Mode S. After data link processors are installed at 

field locations, FAA will conduct a separate integration test with the 

processor and an actual Mode S system at its Technical Center in 

Pomona, New Jersey. 

The development of data link component systems is interdependent (see 

table III. 1 for the implementation schedule for data link component systems). For example, the data link processor requires at least one Mode S 

sensor to provide data link services, and this creates schedule interdependencies and relationships between the two projects. If the Mode S 

schedule changes, the data link processor schedule is re-evaluated for 

impacts on implementation. The data link processor and Mode S programs have been beset by delays primarily due to delays by the Mode S 

contractor in completing the detailed system design and in ensuring adequate system capacity. 

. _- ~. ..-....-___. ^--

Table 111.1: Data Link Component 

Systems Schedule First on-site 

implementation date 

Systema 1988 NAS Plan 1989 NAS Plan 

Data Link Processor 12190 3193 

Mode S 9190 1 Z/92 

~~ -- ~~~.~~ ~.~ - ~..._~~ -..~ - ~~- ~~ ~~~-~-~~ -- ~~~ 

Weather M&sage Switching Center 

Replacement 5192 12192 

Automated Weather Observing System Data 

Acquisition System IO/91 10/91 

National Airspace Dais Interchange Network l/93 I /93 

Terminal Doppler Weather Radar 6193 6193 

“The data llnk processor and Mode S are needed to provide the inrtial weather servrces. The other 

component systems will support improvements and enhancements to the initial messages. 

Descriptions of Initial Table 111.2 lists descriptions of the six initial weather services FAA chose 

Weather Services from a list of 38 candidate services. These services are meant to enhance 

safety, benefit users, or improve productivity. PAA chose these services 

for both apparent utility and compatibility with present-day operations. 

In addition, FAA believes these services will establish the credibility of 

Page 20 GAO/ICED-90-17 Aviation Weather 
Appendix III 

Information on the Aeronautical Data Lii 

System and Services 

data link and encourage airlines and general aviation pilots to voluntarily install data link equipment. 

Table 111.2: Initial Data Link Weather Services 

Services 

Surface Obscrvatlons 

Pilot Woathor Reports 

Terrntrral Forecasts 

Winds and lcmperatures Aloft 

Forecasts 

Radar Summanes 

IHazardous Weather Advlsones 

Descriptions 

Evaluations of one or moremeteorological elements that describe the state of the atmosphere at the 

location where the observation is taken. 

Reports of weather conditions observed by pilots in-flight. 

Twenty-four hour forecasts of surface weather conditions within the immediate vicinity of airports. 

Forecasts for over 130 specific locations in the contiguous United States. 

Low-resolution graphic representations of precipitation intensities using characters. 

In-flight advisories, issued by th;~National Aviation Weather Advisory Unit to warn en-route aircraft of the 

develooment of Dotentiallv hazardous weather conditions. 

Improvements to the 

Initial Weather Services 

Enhanced Data Link 

Serviws 

FAA plans to add new systems and upgrade data link component systems 

in order to improve initial weather services. The Weather Message 

Switching Center Replacement will provide all the weather services provided by the Weather Message Switching Center but with added graphics capabilities through state-of-the-art technology. The Automated 

Weather Observing System’s Data Acquisition System will provide minute-by-minute surface observation data rather than the initial hourly 

updates. The addition of the National Airspace Data Interchange Network communication system2 will increase capacity, flexibility, and service availability for data link. 

After the initial weather services are in place, FAA plans to add enhancements, For example, wind shear alerts will be part of the enhanced 

weather services. Additionally, FAA plans to add air traffic control functions. Table III.3 describes the enhanced data link services currently 

planned by MA. 

‘The Nalional Airspace IMa Int,crclrangc! Network is an FAA communications system planned to providc grcatcT cqJ;lbility to l.ransfW mcssagcs and at a faster speed. 

Page 2 1 GAO/RCED-M-17 Aviation Weather 
Appendix III 

Information on the Aeronautical Data Link 

System and Services 

Table 111.3: Enhanced Data Link Services 

Services Descriptions 

Transfer.of Communications 

l_l ..____. ---.-.-- -..-- ___... -... -_.--- 

Delivers a computer-generated message which informs the pilot which radio frequency is needed to 

contact the receiving controller. The service may be initiated either automatically or manually by the 

controller makina a soecific kevboard inout. 

_____-.....-.---IL__.- ’ 

Altrtude Assianment Cor%&~~%+livers a computer-oenerated%~~~&%~t messaae to the pilot, which confirms the controller 

.._ ..-. -_- 

Menu Text/i&e Text 

Automatic Terminal Information 

Service 

Wind Shear Alerts 

voice assignbent. ‘ihe data link message-would automafically be generated when the altitude 

information is entered into the air traffic control computer. 

-_____ ___._--____- 

Permits a controller to pre-store air traffic control instructions he desires to send to multiple aircraft. The 

free text service would allow the controller to enter any desired text that would then be sent to the 

specific aircraft desired. 

-_____-____ --___ --.-__l__ 

Transmits weather and airport conditions to pilots. The DLP will receive these new reports every hour. A 

pilot would enter a request, via data link, for the service and the DLP will return the current report for 

the requested location. ____----.- 

Generates alphanumeric wind shear alert messages from the Terminal Doppler Weather Radar and 

sends them to the DLP which will maintain a database of all current wind shear alerts. When a wind 

shear alert is received, the DLP will automatically generate a data link message to all aircraft that had 

oreviouslv reauested weather advisorv information for that airport. 

Notlces to Airmen 

Center Weather Ad&&ies 

Maintains a national database of recently issued Notices to Airmen, received from the Weather Message 

Switching Center Replacement. Notices to Airmen include such information as airport and runway 

closures and changes in the status of navigational aids. A pilot would request these messages via 

data link and the DLP would generate the response message. 

___-.-_-- .___- 

Stores the current Center Weather Advisories as received from the Weather Message Switching Center 

Replacement. These messages would be provided in response to a request for hazardous weather 

advisories. 

Page 22 GAO/RCED-90-17 Aviation Weather 
Appendix IV 

Objectives, Scope, and Methodology 

.-._..... ._.... - _..... --~ 

On August 10,1988, Chairman Robert Roe of the House Committee on 

Science, Space, and Technology asked us to provide information on FAA'S 

progress in disseminating hazardous weather data from three new 

weather-related systems: (1) the AsR-9, (2) the TDWR, and (3) the Aeronautical Data Link. In subsequent discussions with the Chairman’s 

office, we agreed to 

. describe FAA'S progress in preparing the operational procedures needed 

to transmit hazardous weather information from controllers to pilots for 

the ASR-9 and the TDWR and 

. assess the status and availability of weather services FAA intends to provide through the Aeronautical Data Link. 

To determine the status and issues concerning the development of operational procedures for the ASR-9, we interviewed officials in FAA’S air 

traffic organization, as well as the program manager. We also met with 

officials from the Lincoln Laboratories, which was tasked by FAA with 

evaluating the performance of the A,%-9 weather channel and providing 

input to the agency on developing operational procedures. 

We addressed the same objective for the TDWR through discussions with 

FAA's program office and reviewed test plans and other program documents We also attended a meeting of the TDWR user group in Boulder, 

Colorado. We discussed the results of the initial operational tests and 

alternative operational procedures with the members of that group, 

including representatives from FAA, the National Center for Atmospheric 

Research, and aviation user groups. We also reviewed United Airlines’ 

evaluation of the initial operational tests performed at Denver in 1988. 

To review the status of the data link program, we met with the program 

manager as well as representatives from FAA's Systems Engineering and 

Integration Contractor for the NAS Plan. We also reviewed test plans and 

other relevant program documents. Additionally, we discussed the use 

of simulations in testing, as well as proposed improvements and 

enhancements to initial data link services, with officials at FAA'S Technical Center in Pomona, New Jersey. We also discussed the status of the 

Mode S with the program manager for that system. 

Our review was conducted between August 1988 and ,July 1989 in 

accordance with generally accepted government auditing standards. 

Page 23 GAO/RCED-90-17 Aviation Weather 
Appendix V 

Major Contributors to This Report 

* 

Resources~ Community Allen Li Assistint Director 

and Economic Martin bertel, Evaluator-in-Charge 

Development Division’ 

Robert Wurster Evaluator-in-Charge 

Washington, D.C. Andrew Vogel&g Evaluator 

Arthur A. Shantz, Advisor 

(841217) Page 24 GAO/RCED-90-1Aviation ’7 Weather 
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