Wide Area Augmentation System

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WAAS Operation

The Wide Area Augmentation System (WAAS) is an extremely accurate navigation system developed for civil aviation by the Federal Aviation Administration (FAA) in conjunction with the United States Department of Transportation (DOT). Its accuracy is less than 3 meters 95% of the time, and it provides integrity information equivalent to or better than receiver autonomous integrity monitoring (RAIM). This is achieved through 25 ground stations throughout the US and Alaska which measure the difference between their surveyed location and the GPS signal. These ground stations send the measured difference to a master relay station which sends the corrections to two geostationary satellites at the same longitudes as the East and West coasts. Those satellites beam the correction signal back to Earth, where WAAS-enabled GPS receivers apply the correction to their computed GPS position.

Before WAAS, the U.S. National Airspace System (NAS) did not have the ability to provide horizontal and vertical navigation for precision approaches for all users at all locations, as ground-based systems are quite expensive. WAAS provides service for all classes of aircraft in all flight operations, including en route navigation, airport departures, and airport arrivals, including all-weather precision approaches throughout the NAS.

Europe and Asia are conducting parallel efforts by way of the European Geostationary Navigation Overlay System (EGNOS) and the Japanese Multi-Functional Satellite Augmentation System (MSAS), respectively. John Deere also operated a similar commercial service known as StarFire. The International Civil Aviation Organization (ICAO) calls this type of system a Satellite Based Augmentation System (SBAS).

History

FAA WAAS logo.

Long before WAAS was first commissioned in July of 2003, the U.S. Coast Guard developed a Maritime Differential GPS system (MDGPS) in the late 1980s as an aid to navigation in and around harbors. The MDGPS was gradually fielded over the succeeding decade, and achieved 100% of planned coverage in 1999. While the two are similar in concept, DGPS is confined to the area in the vicinity of the DGPS transmitter, whereas WAAS covers most of North America.

WAAS was jointly developed by the United States Department of Transportation (DOT) and the Federal Aviation Administration (FAA), beginning in 1995, to provide precision approach capability for all aircraft possessing the appropriately certified equipment. Without WAAS, ionospheric disturbances, clock drift (timing), and satellite orbit errors create too much error in the GPS signal for aircraft to perform a precision approach. A "precision approach" is one that is always aligned with the runway. It provides course guidance, distance from the runway, and elevation information at all points along the approach, usually down to lower altitudes and weather minimums than non-precision approaches.

The traditional system for precision approaches is the instrument landing system, which used a series of radio transmitters each broadcasting a single signal to the aircraft. This complex series of radios needs to be installed at every runway end, some offsite along a line extended from the runway centerline, making the implementation of a precision approach both difficult and very expensive. Additionally, it required redundant radios in every aircraft to receive the signals, although these radios generally were also used for voice communications and often required anyway.

For some time the FAA and NASA developed a much improved system, the microwave landing system (MLS). The entire MLS system for a particular approach was isolated in a single box (sometimes two) located beside the runway, dramatically reducing the cost of implementation. MLS also offered a number of practical advantages that eased traffic considerations, both for aircraft and radio channels. Unfortunately, MLS would also require every airport and aircraft to upgrade their equipment, and while this was taking place, ILS would have to be maintained as well. Nevertheless the FAA was convinced this had to happen, and in the early 1990s was planning to turn off existing ILS systems in 2010.

During the development of MLS, however, consumer GPS receivers of various quality started appearing. GPS offered a huge number of advantages to the pilot, combining all of an aircraft's long-distance navigation systems into a single easy-to-use system, often small enough to be hand held. Deploying an aircraft navigation system based on GPS was largely a problem of developing new techniques and standards, as opposed to new equipment. The FAA started planning to shut down their existing long-distance systems (VOR and NDBs) in favour of GPS. This left the problem of approaches, however. GPS is simply not accurate enough to replace ILS systems. Typical accuracy is about 15 meters, whereas even a "CAT I" approach, the least demanding, requires a vertical accuracy of 4m.

This inaccuracy in GPS is mostly due to large "billows" in the ionosphere, which slow the radio signal from the satellites by a random amount. Since GPS relies on timing the signals to measure distances, this slowing of the signal makes the satellite appear farther away. The billows move slowly, and can be characterized using a variety of methods from the ground, or by examining the GPS signals themselves. By broadcasting this information to GPS receivers every minute or so, accuracy can be greatly improved.

This led to the concept of Differential GPS, which used separate radio systems to broadcast the correction signal to receivers. Aircraft could then install a receiver which would be plugged into the GPS unit, the signal being broadcast on a variety of frequencies for different users (FM radio for cars, longwave for ships, etc). Unfortunately broadcasters of the required power generally cluster around larger cities, making such DGPS systems less useful for wide-area navigation. Additionally, most radio signals are either line-of-sight, or can be distorted by the ground, which made DGPS difficult to use as a precision approach system or when flying low for other reasons.

The FAA considered systems that could allow the same correction signals to be broadcast over a much wider area, leading directly to WAAS. Since a GPS unit already consists of a satellite receiver, it made much more sense to send out the correction signals on these frequencies than to use an entirely separate system and thereby double the probability of failure. Existing GPS satellites did not have any additional channels that could be used for this feature, so instead it was planned to add broadcasters to existing communications satellites. In addition to lowering implementation costs by "piggybacking" on a planned launch, this also allowed the signal to be broadcast from geostationary orbit, which meant a small number of satellites could cover all of North America.

Operation

WAAS Reference Station Barrow, Alaska.

WAAS has twenty-five Wide-area Reference Stations positioned throughout the United States which compare the GPS signal with known (surveyed) coordinates. Future improvements to WAAS include integration of four new WRS sites in Alaska, five new WRS sites in Mexico, and four new WRS sites in Canada expanding Localizer Performance with Vertical Guidance (LPV) coverage.Federal Aviation Administration. [SatNav News]. April 2006. Accessed June 13, 2006. These ground stations send their findings to one of three Wide-area Master Stations (WMS) using a land-based communications network.

The WMSs divide the country into a grid and builds ionospheric correction information for each cell. The WMSs then broadcast a correction signal to the two WAAS satellites covering the US, which in turn broadcast that correction signal on a per-satellite basis to each WAAS-enabled GPS receiver. At present there are two geostationary satellites serving WAAS, both Inmarsat IIIs, the POR (Pacific Ocean Region) and AOR-W (Atlantic Ocean Region-West). Two additional satellites that will be operational later in 2006.

The GPS receiver calculates which grid it falls into using normal GPS calculations, and then applies the correction signal for that grid. Data can be updated every minute if necessary. Ephemeris errors and ionosphere errors do not change this frequently, so they are only updated every two minutes and are considered valid for up to six minutes. "Clock and ephemeris data is specific to a satellite but ionospheric errors are specific to your location therefore they must be sent separately".gpsinformation.net. [Differential GPS]. Accessed June 12, 2006.

The WMS's also monitor the signal from the WAAS geostationary satellites, providing integrity information for them as well. "Further, the WAAS system was designed to the strictest of safety standards – users are notified within six seconds of any issuance of hazardously misleading information that would cause an error in the GPS position estimate".Federal Aviation Administration. [WAAS Benefits]. Accessed June 12, 2006.

Accuracy

The accuracy of WAAS is between one and two meters horizontally and between two to three meters vertically throughout most of the continental United States and large parts of Canada and Alaska. It's also been stated that "Preliminary tests indicate individual errors are less than seven meters 95 percent of the time, and average errors when collecting for 30 minutes are between one and three meters."Bolstad, Paul, GIS Fundamentals: A first text on geographic information systems. Eider Press. June 2003. p.139. ISBN 0971764719

The following table lists the accuracy of the historical GPS systems:

100 meters: This is the advertised accuracy of the GPS system with the Selective Availability (SA) option turned on. SA was an imposed error designed to thwart an enemy's use of GPS for its own purposes. SA was employed by the U.S. Government until May 1, 2000 but has not been used since. According to the Inter Agency GPS Executive Board (IGEB),

The United States has no intent to ever use SA again. To ensure that potential adversaries do not use GPS, the military is dedicated to the development and deployment of regional denial capabilities in lieu of global degradation.National Space-Based PNT Executive Committee. [Frequently Asked Questions About SA Termination]. June 12, 2006.

≤ 10 meters: This is the Differential GPS (DGPS) accuracy. According to the 2001 Federal Radionavigation Systems (FRS) report published jointly by the U.S. DOT and Department of Defense (DoD), accuracy degrades with distance from the facility; it can be < 1 m but will normally be < 10 m. Maritime DGPS was implemented in the 1990's, and is used in various seaports and inland waterways to provide pinpoint navigation for shipping. It has been superseded by the National DGPS (NDGPS) program. NDGPS will expand the existing system for railway and highway usage. NDGPS is stated to have accuracy of < 1 m with high end equipment and < 10 m with standard equipment.

< 3 meters: This is the figure currently being given for WAAS accuracy in the vertical plane. WAAS accuracy in the horizontal plane is less than 2 meters. WAAS is capable of achieving Category I precision approach accuracy of 16 m laterally and 4 m vertically.

< 1 meter: Local Area Augmentation System (LAAS). As of 2001, LAAS was capable of achieving a Category I ILS accuracy of 16 m laterally and 4 m vertically. The goal of the LAAS program is to provide Category III ILS capability. This allows aircraft to land with zero visibility utilizing 'autoland' systems and indicates a very high accuracy of < 1 m.

Benefits

WAAS Ground uplink station (GUS) facility in Napa, California.

WAAS covers all of the "navigation problem", providing highly accurate positioning that is extremely easy to use, for the cost of a single receiver installed on the aircraft. Ground- and space-based infrastructure is relatively limited, and no on-airport system is needed. WAAS allows a precision approach to be published for any airport, for the cost of developing the procedures and publishing the new approach plates. This means that almost any airport can have a precision approach, the cost of implementation is dramatically reduced.

Additionally WAAS works just as well between airports. This allows the aircraft to fly directly from one airport to another, as opposed to following routes based on ground-based signals. This can cut route distances considerably in some cases, saving both time and fuel. In addition, because of its ability to provide information on the accuracy of each GPS satellite's information, aircraft equipped with WAAS are permitted to fly at lower en-route altitudes than was possible with ground-based systems, which were often blocked by terrain of varying elevation. This enables pilots to safely fly at lower altitudes, not having to rely on ground-based systems. For unpressurized aircraft, this conserves oxygen and enhances safety.

Drawbacks

The Future of WAAS

In March 2005, the FAA finalized the Geostationary Satellite Communications Control Segment contract with Lockheed Martin for WAAS geostationary satellite leased services through 2016. Two additional satellites, PanAmSat Galaxy XVPanAmSat. [Galaxy 15]. Accessed June 14, 2006. and Telesat Anik F1R, were launched in 2005 and plan to be operational in late 2006. The Telesat was launched September 9, 2005. This will enhance coverage of North America and all but the northwest part of Alaska. PanAmSat is located at 133W and ANIK F1R is at 107W. In early 2006, the AOR-W satellite was repositioned from 54W to 142W, while the POR satellite remains at 178E. This left a gap in WAAS coverage in New England, which should be filled by the end of 2006 when PanAmSat is operational.Federal Aviation Administration. [Information for Pilots]. Accessed June 12, 2006.

In 2007, WAAS vertical guidance is projected to be available nearly all the time (greater than 99%), and its coverage will encompass the full continental U.S. and most of Alaska. Horizontal service (RNP 0.3 and better) is already available throughout the U.S. airspace.Federal Aviation Administration. [WAAS 200’ Minimum Related Questions and Answers]. Accessed June 12, 2006.

At that time, the accuracy of WAAS will meet or exceed the requirements for Category 1 ILS approaches, namely, three-dimensional position information down to 200 feet above touchdown zone elevation. An even more capable system, known as Local Area Augmentation System (LAAS) is currently under development, and will provide CAT II/III equivalent precision at most major airports. The military is working on a parallel system with additional capabilities called the Joint Precision Approach and Landing System, or JPALS.

Timeline

Date	Summary	Action
March 2006	WAAS Approved for new, lower minimums	WAAS was approved to provide guidance down to 200 feet above an airport’s surface for LPV instrument approaches.
Sep-Oct 2005	Additional WAAS Geostationary Earth Orbit (GEO) satellites launched	Telesat Anik F1R and PanAmSat Galaxy 15 satellites were launched and scheduled for WAAS operation in 2006.
June 2005	WAAS Reference Station Installed in Gander, New Newfoundland, Canada	WAAS achieves a major milestone toward Full LPV approach capability throughout the continental United States with the installation of the first international WAAS reference station in Canada.
October 2004	First WAAS LPV Receiver Hits Market	The FAA approves the first WAAS-equipped avionics for LPV approach operations, the Garmin 480.
September 2004	Site Surveys Complete to Extend WAAS North	Site surveys required for new WAAS reference stations (WRS) in Alaska and Canada are completed.
May 2004	WAAS Program Re-Baselined	The WAAS program completes a total rebaselining effort to replace the 1999 WAAS baseline.
July 10, 2003 (12:01AM)	WAAS is Commissioned	The FAA commissioned the Wide Area Augmentation System (WAAS)
March 31, 2003	First GPS/WAAS Receiver Certified	Capstone conducted the first commercial flight with a TSO-145 GPS/WAAS receiver. Already, this has produced an additional 41,000 feet of airspace along 1,521 nautical miles of the existing route structure in Southeast Alaska.
March 5, 2003	WAAS GEO Contract Awarded	The FAA leased up to three Geostationary Earth Orbit (GEO) satellites to deliver the WAAS signal and contracted the development of accompanying ground stations.

(Source Federal Aviation Administration. [WAAS Current news]. Accessed June 12, 2006.)

See also

• Local Area Augmentation System
• Joint Precision Approach and Landing System
• Distance Measuring Equipment (DME)
• Instrument flight rules (IFR)
• Instrument Landing System (ILS)
• LORAN
• Microwave Landing System (MLS)
• Non-Directional Beacon (NDB)
• Tactical Air Navigation (TACAN)
• Transponder Landing System (TLS)
• VHF Omni-directional Range (VOR)

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References