Wide Area Augmentation Systems
The 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-wideweb 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.
1
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.
2
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
3
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
____________________
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.
1
4
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
5
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/DOD4650.5
6
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 jetliners. 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
____________________
5
Gebre-Egziabher Demo A DME Based Area Navigation Systems for GPS/WAAS Interference Mitigation in General Aviation
Applications
7
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 wideranging 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
8
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
_____________________
7
GARMIN GPS Beginner’s Guide July 2008 Part Number 190-00224-00
9
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
Operations
of %
of
Total
Delayed Minutes
Operations
%
of
Total
Delayed Minutes
On Time
35,924,747
76.98%
N/A
N/A
Air 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 3,571,311
Delay
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/A
Diverted
102,375
0.22%
N/A
N/A
Total 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.
_____________________
8
Research and Innovative Technology Administration Bureau of Transportation Statistics
10
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
_____________________
9
Sparks, Jim. Aircraft Maintenance Technology, Jan/Feb2009, Vol. 20 Issue 3, p18-21, 4p,
10
Dillingham, Gerald L.Problems Plaguing the Wide Area Augmentation System and FAA’s Actions to Address Them GAO Reports,
6/29/2000, p1, 17p;
11
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
12
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
13
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.
14
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
15