Wide Area Augmentation System

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). The stated accuracy is within 3 meters of the true position 95% of the time, and it provides integrity information equivalent to or better than receiver autonomous integrity monitoring (RAIM). This is achieved via a network of ground stations located throughout the north-western hemisphere which monitor the GPS signal and measure the difference between that signal and their surveyed location. These ground stations send the measured differences to a master relay station which sends corrections to two geostationary satellites over the East and West coasts. Those satellites transmit the correction values 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 precision approaches throughout the NAS.

Europe and Asia are conducting parallel efforts by way of the European Geostationary Navigation Overlay Service (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, and satellite orbit errors create too much error in the GPS signal for aircraft to perform a precision approach. A "precision approach" includes altitude information, instead of simply course alignment information, and with sufficient precision to allow aircraft to descend to a low enough point properly lined up with the runway to safely make a landing in most weather conditions. 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.

On July 10, 2003, the WAAS signal was activated for general aviation, covering 95% of the United States, and portions of Alaska offering 350 ft minimums.

Operation

WAAS Reference Station Barrow, Alaska.

As with GPS in general, WAAS is composed of three main segments; the Ground segment, the Space segment, and the Receiver segment.

Ground Segment

As of November 2006, the ground segment consists of a total of 29 Wide-area Reference Stations (WRS); 19 in the Conterminous United States (CONUS), six in Alaska, one in Hawaii, one in Puerto Rico.^[1] These ground stations compare the GPS signal with known (surveyed) coordinates and 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 Space segment for rebroadcast to the Receiver segment.

The WMSs 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".^[2]

Space Segment

The Space segment currently consists of up to four geosynchronous artificial satellites which transmit the correction signal to the Receiver segment.

The original two geostationary satellites providing WAAS signals, POR (Pacific Ocean Region - ID #47 / PRN #134) and AOR-W (Atlantic Ocean Region-West - ID #35 / PRN #122), are each leased spectrum/bandwidth on Inmarsat III satellites. The lease on POR expires in September of 2007 and AOR-W's lease will expire in September of 2008.

In March 2005, the FAA finalized the Geostationary Satellite Communications Control Segment contract with Lockheed Martin for WAAS geostationary satellite leased services through 2016. To support the contract, two additional satellites were launched in the fall of 2005; PanAmSat Galaxy XV^[3] located at 133°W and Telesat Anik F1R located at 107°W. Compared to the previous system's orbital positions, this will enhance coverage of North America for all but the northwest part of Alaska.

Galaxy XV (ID #48 / PRN #135) begin to transmit normal, non-test mode WAAS corrections on November 9th, 2006.^[4] Anik F1R (ID #51 / PRN #138) is undergoing testing.^[5] During testing, the satellite may transmit correction messages but these messages are set to Type 0, not for navigation use and the correction data is not guaranteed to be correct.

Receiver Segment

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."^[6]

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."^[7]

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.^[8]

15 meters: This is considered the "normal" accuracy for the GPS system, when SA is turned off. 2001 FRS states this as ≤ 13 m horizontally and ≤ 22 m vertically.

≤ 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

WAAS has tremendous benefits, not only to the aviation community, but also to non-precision navigation on North American waterways, highways, and even geocachers. Even so, there are a few drawbacks. Because the satellites are geostationary, they are low on the horizon for locations at high latitudes. The system has performed well in the past, but aircraft in areas of Alaska or northern Canada may have difficulty maintaining a lock on the WAAS signal.^[9] Accuracy of the signal relies on the presence of ground stations to calculate and correct various errors such as ionospheric delay. Although computer algorithms are used to calculate corrections for other areas, the correction will decrease in accuracy with distance from a reference station and increasing the number of stations will increase system cost. Also, development of a WAAS approach procedure costs the FAA at least $20,000 per approach, and aircraft conducting these approaches must possess certified receivers.^[10] As of 2006 there are a limited number of aircraft receiver models certified for WAAS LPV approaches. WAAS is not capable of the accuracies required for Category II or III ILS approaches. Thus, either existing ILS equipment must be maintained, or replaced by new systems such as the Local Area Augmentation System (LAAS).^[11] Nevertheless, WAAS LPV approaches with 200 foot minimums can not be used at smaller airports without additional airport modifications such as approach lighting, specific runway markings and a parallel taxiway, requiring pilots to use higher minimums.^[12]

The Future of WAAS

Aviation Operations

The FAA announced on March 24, 2006, that the first procedures that allow operations down to 200 feet will be published in 2007, equivalent to the capability of ILS Category I.^[13]

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.^[14]

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.

Ground Segment

Future improvements to WAAS include the integration of nine additional international Wide-area Reference Stations. Five have been installed, but are not yet fully incorporated (Merida, Mexico City, and Puerto Vallarta all in Mexico; and Gander and Goose Bay both in Canada). These should be incorporated roughly in the summer of 2007. The last four (San Jose del Cabo and Tapachula in Mexico, and Iqualuit and Winnipeg in Canada) should be operational in the fall of 2007. This will bring the total number of reference stations to 38 and expand Localizer Performance with Vertical Guidance (LPV) coverage.^{[1][15] [16]}

Space Segment

Galaxy XV (PRN #135) is expected to be transmitting GPS range information in mid 2007, after 6 to 9 months of testing.^[17] Anik F1R (PRN #138) is expected to be integrated with full capacity (WAAS and GPS pseudorange) in spring of 2007.^[18]

Both satellites contain an L1 / L5 GPS payload. This means they will potentially be usable with the L5 modernized GPS signals when receivers become available.[3]

Timeline

(Source ^[19])

See also

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

Satellite navigation systems

Transit | GPS | GLONASS | Galileo | Beidou

Related topics: EGNOS | WAAS | LAAS

References

1. ^ ^{a b} FAA's National Airspace System Architecture Wide-Area Reference Station Accessed 07 Nov 2006
2. ^ Federal Aviation Administration. WAAS Benefits. Accessed June 12, 2006.
3. ^ PanAmSat Details on Galaxy 15 Accessed June 14, 2006.
4. ^ NSTB WAAS Announcement [1] Accessed 10 November, 2006.
5. ^ Federal Aviation Administration. WAAS News June 2006
6. ^ gpsinformation.net. Differential GPS. Accessed June 12, 2006.
7. ^ Bolstad, Paul, GIS Fundamentals: A first text on geographic information systems. Eider Press. June 2003. p.139. ISBN 0-9717647-1-9
8. ^ National Space-Based PNT Executive Committee. Frequently Asked Questions About SA Termination. June 12, 2006.
9. ^ Department of Aeronautics and Astronautics, Stanford University. WAAS Performance in the 2001 Alaska Flight Trials of the High Speed Loran Data Channel. Accessed June 12, 2006.
10. ^ Federal Aviation Administration. NAS FAQ. Accessed June 12, 2006.
11. ^ Federal Aviation Administration. WAAS FAQ. Accessed June 12, 2006.
12. ^ Aircraft Owners and Pilots Association. AOPA welcomes improved WAAS minima. March 7, 2006. Accessed June 14, 2006.
13. ^ Federal Aviation Administration. FAA Announces Major Milestone for Wide Area Augmentation System (WAAS). March 24, 2006.
14. ^ Federal Aviation Administration. WAAS 200’ Minimum Related Questions and Answers. Accessed June 12, 2006.
15. ^ FAA's National Airspace System Architecture Wide Area Augmentation System
16. ^ Federal Aviation Administration. SatNav News. April 2006. Accessed June 13, 2006.
17. ^ NSTB WAAS Announcement [2] Accessed Nov 2nd 2006
18. ^ Federal Aviation Administration. WAAS Current News
19. ^ Federal Aviation Administration. WAAS Current news. Accessed June 12, 2006.

External links

• FAA WAAS
• Garmin WAAS
• 2005 Federal Radionavigation Plan (FRP)
• WAAS Coverage in Canada

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