Wide Area Augmentation SystemsThe W.A.A.S. of the Future

Michael A. Clarke

For The

Virginia Space Grant Consortium

Sponsored by the National Aeronautics and Space Administration

About The Author

Michael Clarke is a senior in the Aviation Department of the College of Science and Technology at Hampton University. He is a commercial instrument rated pilot with over 250 flight hours. This paper is the result of his extensive personal research of the National Airspace System (NAS). His experience in researching the National Airspace System has greatly developed his interest in aviation including the functions of: Air Traffic Control, Airport Management, and Airport Operations.

Virginia Space Grant Consortium

The Virginia Space Grant Consortium (VSGC) is a coalition of five Virginia colleges and universities, NASA, state educational agencies, Virginia's Center for Innovative Technology, and other institutions representing diverse aerospace education and research.

The VSGC acts as an umbrella organization, coordinating and developing aerospace-related and high technology educational and research efforts throughout the Commonwealth and connecting Virginia's effort to a national community of shared aerospace interests. The students work under the guidance of prominent engineering professor. For more information about the VSGC program, visit the VSGC world-wide-web page at http://www.vsgc.odu.edu/

Acknowledgements

The author would like to thank the members and officers of the Virginia Space Grant Consortium Program for the opportunity to research Wide Area Augmentation Systems under the guidance of the program. Programs such as the Virginia Space Grant Consortium offer an additional educational experience involving both academia and society. It also helps to enhance the National Transportation System.

Special thanks to Mrs. Margaret Browning of Hampton University for her knowledge and expertise of the National Airspace System which helped to build the foundation of this research paper. Thanks also to Mr. Stephen Prater of the Federal Aviation Administration who helped to gather sources on the topic of Wide Area Augmentation Systems.

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Abstract

Wide Area Augmentation System (WAAS) is a system of satellites and ground

stations that gives the user of GPSs more accurate readings. Wide Area Augmentation

System is a new navigational aid system being used by the Federal Aviation

Administration (FAA ) to provide aircrafts the ability to use GPS satellites in all phases

of flight; allowing for signal corrections and giving the user better position accuracy than

any other current aerial navigation aid. WAAS is being implemented to enhance the

National Airspace System (NAS) to make the congestion due to the demand within

aviation more manageable.

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Table of Contents

About the Author 1

Virginia Space Grant Consortium 1

Acknowledgements 1

Abstract 2

Analysis 4

How WAAS Works 4

Traditional Navigational Aids 5

Scope 7

Next Generation Satellite Navigation 8

Global Navigation Satellite System 8

Global Positioning System 8

GPS Elements 9

Flight Delay Statistics 10

Wide Area Augmentation Systems 10

WAAS Synopsis 11

List of Abbreviations 15

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Analysis

Analysis in this paper discusses the need for enhanced aerial navigational aids

(NAVAIDs), as the commercial demand for navigational accuracy and flexibility in

aviation increases. This paper will also discuss the Federal Aviation Administration’s

(FAA) plan to modernize the National Airspace System (NAS) with the implementation

of the Next Generation Air Transportation System (NextGen) which will substantially

reduce aerial congestion pressures experienced by Air Traffic Controllers (ATC). Current

NAVAIDs are technologically antiquated; updates are absolutely necessary, to ensure the

increase in congestion will not overwhelm the NAS causing greater ground delays and

restrict air travel within the commercial aviation community.

How WAAS Works

The Wide Area Augmentation System (WAAS) uses a series of 38 receiver sites

throughout North America. Each site receives signals from all GPS satellites in view. The

site then develops a correction message, which is transmitted to two geostationary

satellites (GEOS). The GEOS re-transmit the correction message to the WAAS-enabled

aircraft receiver, which applies the correction. While basic GPS typically has an error of

approximately 25 meters (horizontally), the corrected WAAS position calculation is

usually within two or three meters1. These corrections permit a user's receiver to compute

a more accurate position, often to better than 1 meter horizontally and 2 meters vertically,

with a 95% confidence.2

In addition to the correction message, the GEOs broadcast a positioning message

that can be used by a WAAS-enabled receiver. This means the user has two additional

____________________1 Larry O. Oliver FAA Aviation News March/April 2009 pg 12 “GPS with Vertical Guidance: The Lowdown on Going

Low” 2 Timothy R. Schempp GPS World January 2008 pg 62 “Good Better Best: Expanding the Wide Area Augmentation

System” view over North America.

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satellites, always in the system with sufficient positioning information. WAAS has no

such requirement because of the additional GEOS and the number of GPS satellites that

are assured in view.1

Traditional Ground Based Navigational Aids

Flying under Instrument Flight Regulations (IFR) the traditional radio NAVAIDs

used by commercial pilots are: Very High Frequency Omni-directional Range Radio,

Distance Measuring Equipment, Non-Directional Beacon and Instrument Landing

System. Each of these NAVAIDs have their own set of vertical and lateral limitations and

is not perfectly precise. These NAVAIDs have given ATC specialists the ability to

provide services to pilots who fly using aerial navigation such as; victor, and jet-route

airways. Victor and Jet-Route airways are interstate highway systems in the sky, which

are used in conjunction with the VHF Omni-directional Range (VOR) systems.

VHF Omni-directional Range (VOR) Radio is the primary NAVAID used by

civil aviation in the NAS. The VOR ground station is oriented to magnetic north and

transmits azimuth information to the aircraft, providing 360 courses TO or FROM the

VOR station. When distance measuring equipment (DME), is installed with a VOR, it is

referred to as a VOR/DME and provides both azimuth and distance information.3

Distance Measuring Equipment (DME) is used in conjunction with a VOR

system, DME makes it possible for pilots to determine an accurate geographic position of

the aircraft, including the bearing and distance TO or FROM the station. The aircraft

DME transmits interrogating radio frequency pulses, the timing of the pulses are received

by the ground facility which converts the pulses into distance measurements allowing the

user to identify his position.3

____________________3 U.S. Department of Transportation Instrument Flying Handbook FAA-H-8081-15A 2008

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Non-Directional Beacon (NDB) is a ground-based radio transmitter that transmits

radio energy in all directions. The Automatic Direction Finder (ADF), when used with an

NDB, determines the bearing from the aircraft to the transmitting station. The ADF

needle points to the NDB ground station to determine the relative bearing (RB) to the

transmitting station. It is the number of degrees measured clockwise between the

aircraft’s heading and the direction from which the bearing is taken. The aircraft’s

magnetic heading is the direction the aircraft is pointed with respect to magnetic north.

The magnetic bearing is the direction to or from a radio transmitting station measured

relative to magnetic north.3

Instrument Landing Systems (ILS) is a precision approach system normally

consisting of a localizer facility, a glide slope facility, and associated VHF marker

beacons. It provides vertical and horizontal navigation information during the approach to

landing at an airport runway. Instrument Landing Systems are 98.6 percent reliable.

However, terrain and other factors may impose limitations upon the use of the ILS

signal.4

ILS approaches are categorized into three different types of approaches (Cat I, II,

and III) based on the equipment and the experience level of the pilot. Localizer and glide

slope facilities give azimuth and vertical guidance information to a pilot. While on the

glide slope, the VHF marker beacons sends a signal to the ILS receiver to verify the user

of his/her position.

____________________4 U.S Department of Transportation and Department of Defense 2001 Radio Navigation Systems DOT-VNTSC-RSPA-01-3.1/DOD-

4650.5

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Scope

Handling aviation safety, flight delay, and cost issues are the main priorities that

are being addressed by the FAA with the implementation of NextGen and WAAS.

Current NAVAIDs used within aviation are not keeping up with the increasing demand

for commercial aviation flights. These slow responses are causing flight delays that are

becoming a problem for the entire aviation commercial industry. Augmenting a higher

availability and continuity for GPS systems will increase the integrity and efficiency

needed for a safety-of-life navigation system.

Today’s NAS is one of the safest means of transportation, it has evolved into a

large, complex, distributed, and loosely integrated network of systems, procedures, and

infrastructure without the benefit of seamless information exchange. The process of

control is primarily through the use of surveillance radars, voice radio systems, limited

computer support systems, and numerous complex procedures. Today’s system has

severe limitations on operational flexibility and overall capacity.

The FAA is leading the NAS modernization effort, in part by supplanting

traditional air traffic services with GPS aided by WAAS. Making GPS the sole means of

navigation will enhance safety, flexibility and efficiency of operations for all aircraft

ranging from the single engine general aviation aircraft to the complex commercial jet-

liners. This transformation of the NAS will be gradual and the build-up to a sole GPS

capability is expected to occur concurrently with the decommissioning of a significant

number of existing ground-based navigational facilities.5

____________________5Gebre-Egziabher Demo A DME Based Area Navigation Systems for GPS/WAAS Interference Mitigation in General Aviation Applications

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Next Generation Satellite Navigation

The Next Generation Air Transportation System is a transformation of the entire

NAS. The transformation is due across the United States in stages between now and

2018. This system will significantly impact the way airports operate. Through this wide-

ranging initiative the NAS will transition from ground-based navigation to a dynamic,

satellite-based system capable of handling future aviation demand. Through the

implementation of new technologies, standards, procedures, and infrastructure

development, the new system will accommodate demand in a safe, efficient, and

environmentally-friendly manner.

Global Navigation Satellite System

Global Navigation Satellite System (GNSS) is the term for satellite navigation

systems that provide positioning with global coverage. A GNSS allow small electronic

receivers to determine their location (longitude, latitude, and altitude) to within a few

meters using time signals transmitted along a line of sight by radio from satellites. During

the past decade, GNSS have been playing a more important role in surveying and other

position sensitive disciplines, such as transportation, personal location and

telecommunications.6 Aircraft navigation systems usually display a "moving map" and

are often connected to the autopilot for en-route navigation. GNSS receivers and glass

cockpits are appearing in general aviation aircraft which use WAAS to increase accuracy.

Global Positioning System (GPS)

Global Positioning System (GPS) is a satellite-based radio

navigation system, which broadcasts a signal that is used by receivers

to determine precise position anywhere in the world.

_____________________6 Steve Hewitson GNSS Receiver Autonomous Integrity Monitoring: A Separability Analysis

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The receiver tracks multiple satellites and determines a

measurement that is then used to determine the end-user’s location. GPS consists of three

distinct functional elements: space, control, and user.7

GPS Elements

The space element consists of 24 Navigation System using Timing and Ranging

(NAVSTAR) satellites in 6 orbital planes. The satellites in each plane are spaced 60°

apart for complete coverage and are located approximately 11,000 miles above the Earth.

The control element consists of a network of ground-based GPS monitoring and control

stations that ensure the accuracy of satellite positions and their clocks. Presently, there

are six monitoring stations, three ground antennas, and a master control station. The

monitoring stations are unmanned and constantly sends and receives information from

the GPS satellites and then sends the orbital and clock information to the master clock

system (MCS). The MCS make precise corrections to the data as necessary, and sends the

data to the GPS satellites, ground antennas, and end-user. The user element consists of

antennas and receiver/processors on board the aircraft that provide positioning, velocity,

and precise timing to the user. GPS equipment must meet the airworthiness installation

requirements and must be “approved” for that type of IFR operation and be operated in

accordance with the applicable POH/AFM or flight manual supplements.5

Flight Delay Statistics

The Bureau of Transportation Statistics (BTS) summarized the Airline On-Time

Performance of approximately 20 major airline carriers between 2003 and 2010. These

statistics were taken. A flight is considered delayed if it arrived at (or departed) the gate 15

minutes or more after the scheduled arrival (departure) time as reflected in the Computerized

_____________________7GARMIN GPS Beginner’s Guide July 2008 Part Number 190-00224-00

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Reservation System. The information is based on data submitted by reporting carriers. Between

June 2003 and January 2010 the 20 airline carriers were delayed by the NAS by

approximately 7.65%. Delays and cancellations attributable to the NAS refer to a broad

set of conditions, such as non-extreme weather conditions, inoperative NAVAIDs, airport

operations, heavy traffic volume, and air traffic control.8

Number of Operations

% of Total Operations Delayed Minutes    % of Total

Delayed Minutes

On Time 35,924,747 76.98% N/A N/AAir Carrier Delay 2,682,790 5.75% 146,056,833 27.66%Weather Delay 405,852 0.87% 30,519,902 5.78%National Airspace System Delay

3,571,311 7.65% 162,002,860 30.68%

Security Delay 27,487 0.06% 1,014,523 0.19%Aircraft Arriving Late 3,113,511 6.67% 188,452,019 35.69%Cancelled 836,798 1.79% N/A N/ADiverted 102,375 0.22% N/A N/ATotal Operations 46,664,863 100.00% 528,046,137 100.00%

The chart produced by the BTS show that the NAS is the highest cause of airline

carrier delays and the problems within the system need to be solved to make air travel

more efficient.

Wide Area Augmentation System (WAAS)

While traditional NAVAIDs have standardized ranges, wide area augmentation

systems cover nearly all of the NAS. Currently, WAAS satellite coverage is only

available in North America. WAAS also provides horizontal and vertical navigation for

approach operations for all users at all locations.

_____________________8Research and Innovative Technology Administration Bureau of Transportation Statistics

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As of March 2008, there are 31 actively broadcasting satellites with those above

the original 24 intended to improve the precision of GPS receiver calculations by

providing redundant measurements. The increased number of satellites changed the

constellation to a non-uniform arrangement; this arrangement was shown to improve

reliability and availability of the system, relative to a uniform system, when multiple

satellites fail.9

The system takes advantage of ground-based reference stations in North America

and Hawaii to measure the satellites' signals and determine position error. Information

from the reference stations are routed to master stations, which queue the received

deviation correction and send the correction messages to geostationary WAAS satellites

in a timely manner (at least every five seconds or better). Those satellites broadcast the

correction messages back to Earth, where WAAS enabled GPS receivers use the

corrections while computing their positions to improve accuracy.10

WAAS Synopsis

WAAS Eliminates the Impacts of Global Terrain

WAAS Provides a Nationwide Navigation Signal

WAAS Provides a High Quality Positioning Signal

WAAS Enables Safe Navigation at Low Altitudes

WAAS Increases Runway Availability by Enabling Lower Minimums

WAAS Provides Significant Capability for Relatively Low Cost

_____________________9Sparks, Jim. Aircraft Maintenance Technology, Jan/Feb2009, Vol. 20 Issue 3, p18-21, 4p,10Dillingham, Gerald L.Problems Plaguing the Wide Area Augmentation System and FAA’s Actions to Address Them GAO Reports, 6/29/2000, p1, 17p;

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WAAS signals broadcast from above and not from the ground via a line-of-sight

broadcast, pilots do not need to worry about losing the signal behind mountainous terrain

or other obstacles. This feature makes WAAS very valuable when flying in terrain where

such obstacles are prevalent. For example, one pilot sited difficulties faced in areas of

Arizona where mountains often blocked reception of VOR broadcasts. Both GPS and

WAAS signals would not be blocked in this situation. WAAS also adds the requisite

accuracy and integrity to support instrument flight operations.

The WAAS signal covers the United States, effectively providing navigation

capability in nearly all areas of the United States. Due to the nature of the WAAS signal,

pilots are not constrained by the location of ground-based NAVAIDS when planning

their flight routes. Additionally, due to the high levels of reliability and availability built

into the WAAS, pilots can feel a greater sense of confidence that the navigation signal

will be there when they need it within the extensive WAAS coverage area. One piece of

equipment (WAAS TSO receiver) can provide the aviator with reliable navigation

anywhere within the WAAS coverage area.

WAAS has been heralded as the best source of high-quality positioning

information available today. WAAS has been producing accuracies of 2 - 3 meters

vertically and 1 - 2 meters horizontally. Although, by specification, WAAS is required

only to produce accuracies to 7.6 meters vertically and horizontally, constant monitoring

by the FAA Technical Center and other organizations has shown that WAAS exceeds

these requirements on a regular basis. Additionally, WAAS operates under very stringent

integrity and availability requirements.

The highly-accurate position determination and robust integrity of WAAS

provides increased situational awareness in the air and enables low-altitude routes. Such

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routes provide protection from icing and positive guidance in obstacle-rich terrain. One

example of how this benefit is making a significant impact is in Alaska where general

aviation is a common mode of transportation. Capstone, an Alaska aviation project, is

using GPS and WAAS to support such low-altitude route structures. Special Federal

Aviation Regulation (SFAR) 97 allows trained pilots equipped with GPS/WAAS

equipment to fly on lower than usual altitudes. The initial application of this SFAR has

opened up 41, 000 feet of usable airspace spread over 1,521 nautical miles of existing

routes in Southeast Alaska. Although this application of WAAS is just in the early stages

of maturity, the benefits to be gained stand to be significant.

Since the commissioning of WAAS, certified avionics are beginning to make their

way on to the market. There are currently a few pieces of certified avionics equipment on

the market using WAAS for navigation. These units range in price from approximately

$7K to $12K; however, they also provide much more than just WAAS navigation,

including moving maps and terrain awareness warning systems. As additional receivers

find their way to market, it is anticipated that options will grow to include avionics

ranging from very simple, less expensive models to highly complex, more expensive

ones. Although these units may not yet be in the price range affordable to all, these most

recent units are still significantly less costly than comparable navigation equipment used

on airliners. Additionally, WAAS will require a lot less real estate than the more complex

avionics used by airlines. WAAS can provide en route IFR capability and also enable

precision approach capability anywhere in the U.S. where supporting procedures exist.

WAAS does not enable quite the same minimums as ILS, but comes very close. WAAS

provides a highly-accurate and reliable vertical navigation position not provided by GPS.

This vertical accuracy, combined with WAAS reliability, and the extensive WAAS

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coverage area, offers a unique opportunity to enable lower minimums at runways

throughout the U.S. WAAS supports LNAV/VNAV and LPV approaches, both providing

vertical guidance. Pilots using WAAS can fly to lower approach minima, in many cases

down to 250 ft., without additional augmentation in the aircraft or on the ground to

provide for safer vertical guidance on landing. WAAS gives aviation users the ability to

make vertical guidance approaches at smaller airports with and without air traffic control

towers, and where ground-based navigation equipment may not exist to provide

vertically-guided approaches alleviating the pressures on the NAS.

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List of Abbreviations

AFM- Airplane Flight Manual

ATC – Air Traffic Controller

BTS - Bureau of Transportation Statistics

GEOS - Geostationary Satellites

GNSS - Global Navigation Satellite System

GPS- Global Positioning System

IFR - Instrument Flight Regulations

ILS -Instrument Landing Systems

LNAV-Lateral Navigation

LPV- Localizer Performance with Vertical guidance

MCS - Master Clock System

NAS - National Airspace System

NAVAID - Navigational Aid

NAVSTAR - Navigation System using Timing and Ranging

NDB - Non-Directional Beacon

NextGen - Next Generation Air Transportation System

POH - Pilot Operating Handbook

SFAR - Special Federal Aviation Regulation

VHF- Very High Frequency

VNAV - Vertical Navigation

VOR - VHF Omni-directional Range

WAAS - Wide Area Augmentation System

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