Wide Area Augmentation System

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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 system augments the Global Positioning System (or GPS) to provide the additional accuracy, integrity, and availability necessary to enable users to rely on GPS for all phases of flight, from en route through GLS approach for all qualified airports within the WAAS coverage area.^[1] 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 locations, as ground-based systems are quite expensive.

The worst-case accuracy is within 7.6 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 and measure the GPS signal. Measurements from the reference stations are routed to two master stations which generate and send the correction messages to geostationary satellites. Those satellites broadcast the correction messages back to Earth, where WAAS-enabled GPS receivers apply the corrections to their computed GPS position.

The International Civil Aviation Organization (ICAO) calls this type of system a Satellite Based Augmentation System (SBAS). Europe and Asia are developing their own SBASs 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.

Contents 1 WAAS Objectives 1.1 Accuracy 1.2 Integrity 1.3 Availability 2 Operation 2.1 Ground Segment 2.2 Space Segment 2.3 User Segment 3 History and Development 3.1 Timeline 4 Comparison of Accuracy 5 Benefits 6 Drawbacks and Limitations 7 The Future of WAAS 7.1 Improvement to Aviation Operations 7.2 Ground Segment Improvements 7.3 Space Segment Upgrades 8 See also 9 References 10 External links

[edit] WAAS Objectives

[edit] Accuracy

The WAAS specification requires it to provide a position accuracy of 7.6 meters or better (for both lateral and vertical measurements), at least 95% of the time. Actual performance measurements of system at specific locations have shown it typically provides better than 1.0 meters laterally and 1.5 meters vertically throughout most of the contiguous United States and large parts of Canada and Alaska.^[2] With these results, WAAS is capable of achieving the required Category I precision approach accuracy of 16 m laterally and 4.0 m vertically.

[edit] Integrity

Integrity is the ability of a navigation system to provide timely warnings when its signal is providing misleading data that could potentially create hazards. The WAAS specification requires the system detect errors in the GPS or WAAS network and notify users within 5.2 seconds. Certifying that WAAS is safe for IFR flight requires proving there is only an extreme small probability that an error exceeding the requirements for accuracy will go undetected. Specifically, the probability is stated as 1×10^-7, and is equivelant to no more than 3 seconds of bad data per year.^[3]

[edit] Availability

Availability is the probability that a navigation system meets the accuracy and integrity requirements. Without the WAAS improvement, GPS could be unavailable for up to a total time of 4 days per year.^[3] Where-as the WAAS specification mandates availability as 99.999% (five nines) throughout the service area.

[edit] Operation

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

[edit] Ground Segment

The Ground Segment is composed of multiple Wide-area Reference Stations (WRS). These precisely surveyed ground stations monitor and collect information on the GPS signals, and then send their data to the two Wide-area Master Stations (WMS) using a terrestrial communications network. The reference stations also monitor the signal from the WAAS geostationary satellites, providing integrity information regarding them as well. As of November 2006 there are 29 WRS's, 19 in the contiguous United States (CONUS), six in Alaska, one in Hawaii, one in Puerto Rico.^{[4] [5]}

Using the data from the WRS sites, the WMSs generate two different sets of corrections: fast and slow. The fast corrections are for errors which are changing rapidly and primarily concern the GPS satellites' instantaneous positions and clock errors. These corrections are considered user position independent, which means they can be applied instantly by any receiver in the WAAS broadcast footprint. The slow corrections include long-term ephemeric and clock error estimates, as well as ionospheric delay information. WAAS supplies ionospheric delay corrections for a number of points (organized in a grid pattern) across the WAAS service area (See the User Segment, below, to understand how these corrections are used).^[1]

Once these corrections are generated, the WMSs then send them to the two pairs of Ground Uplink Stations (GUS) which transmit them to the satellites in the Space segment for broadcast to the User segment.^[6]

[edit] Space Segment

The Space segment consists of multiple geosynchronous communication satellites which broadcast the messages generated by the Wide-area Master Stations for reception by the User segment. The satellites also broadcast the same type of range information as normal GPS satellites, effectively increasing the number of satellites available for a position fix. As of November 2006, the Space segment consists of three operational satellites named Pacific Ocean Region (POR), Atlantic Ocean Region-West (AOR-W), and Galaxy XV. A fourth satellite, named Anik F1R, has been launched and is undergoing testing.

The first two satellites (POR and AOR-W) are each leased space on Inmarsat III satellites and are the original WAAS satellites. Each leased for a multi-year perioid, these satellites will cease WAAS transmissions at their expiration, which was scheduled for July 2007,^[7] although FAA presentations to the aerospace industry indicate that AOR-W's lease has been extended to September 2008.

To enable continued WAAS transmissions, the latter two satellites (Galaxy XV and Anik F1R) were launched in late 2005. Galaxy XV is a PanAmSat, and Anik F1R is a Telesat. As with the previous satellites, these are leased services under the FAA's Geostationary Satellite Communications Control Segment contract with Lockheed Martin for WAAS geostationary satellite leased services, who is contracted to provide up to three satellites through the year 2016.^[8]

Satellite Name & Details	SVN / PRN	Location
Pacific Ocean Region (POR)	ID #47 / PRN #134	178°E
Atlantic Ocean Region-West	ID #35 / PRN #122	142°W
Galaxy XV	ID #48 / PRN #135	133°W
Anik F1R	ID #51 / PRN #138	107°W

[edit] User Segment

The User segment is the GPS and WAAS receiver, which uses the information broadcast from each GPS satellite to determine its location and the current time, and receives the WAAS corrections from the Space segment. The two types of correction messages received (fast and slow) are used in different ways.

The GPS receiver can immediately apply the fast type of correction data, which includes the corrected satellite position and clock data, and determines its current location using normal GPS calculations. Once an approximate position fix is obtained the receiver begins to use the slow corrections to improve its accuracy. Among the slow correction data is the ionospheric delay. As the GPS signal travels from the satellite to the receiver, it passes through the ionosphere. The receiver calculates the location where the signal pierced the ionosphere and, if it has received a ionospheric delay value for that location, corrects for the error the ionosphere created.

While the slow 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.^{[citation needed]}

[edit] History and Development

The WAAS was jointly developed by the United States Department of Transportation (DOT) and the Federal Aviation Administration (FAA), beginning in 1994, to provide performance comparable to category 1 instrument landing system (ILS) for all aircraft possessing the appropriately certified equipment.^[1] Without WAAS, ionospheric disturbances, clock drift, and satellite orbit errors create too much error and uncertainty in the GPS signal to meet the requirements for a precision approach (See GPS sources of error). A precision approach includes altitude information and 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.

Prior to the WAAS, the U.S. National Airspace System (NAS) did not have the ability to provide lateral and vertical navigation for precision approaches for all users at all locations. The traditional system for precision approaches is the instrument landing system (ILS), 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.

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 one or two boxes 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.

During the development of MLS, 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, this source of error can be significantly reduced.

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.^{[citation needed]} 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.

[edit] Timeline

[edit] Comparison of Accuracy

A comparison of various radionavigation system accuracies

System	95% Accuracy (Lateral / Vertical)	Details
LORAN-C Specification	460 meters / 460 meters	The specificed absolute accuracy of the LORAN-C system.
Distance Measuring Equipment (DME) Specification	185 meters / 185 meters	DME is a radionavigation aid that can calculate the distance of an aircraft
GPS Specification	100 meters / 150 meters	The specified accuracy of the GPS system with the Selective Availability (SA) option turned on. SA was employed by the U.S. Government until May 1, 2000.
LORAN-C Measured Repeatability	50 meters / 50 meters	The U.S. Coast Guard reports "return to position" accuracies of 50 meters in time difference mode.
eLORAN Repeatability	10 meters / 10 meters[citation needed]	Modern LORAN-C receivers, which use all the available signals simuletaniously and H-field antennas.
Differential GPS (DGPS)	10 meters / 10 meters	This is the Differential GPS (DGPS) worst-case 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.
Wide Area Augmentation System (WAAS) Specification	7.6 meters / 7.6 meters	The worst-case accuracy that the WAAS must provide to be used in precision approaches.
GPS Measured	2.5 meters / 4.7 meters	The actual measured accuracy of the system (excluding receiver errors), with SA turned off, based on the NSTB's findings.
WAAS Measured	0.9 meters / 1.3 meters	The actual measured accuracy of the system (excluding receiver errors), based on the NSTB's findings.
Local Area Augmentation System (LAAS) Specification	1.0 meter / 1.0 meter [citation needed]	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.

[edit] Benefits

WAAS addresses 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.

The above benefits create not only convience, but also have the potential to generate significant cost savings. The cost to provide the WAAS signal, serving all 5,400 public use airports, is just under US$50 million per year. Where-as the current ground based systems like the traditional Instrument Landing System (ILS), installed at only 600 airports, cost US$82 million in annual maintenance. Without ground navigation hardware to purchase, the total cost of installing a WAAS approach is less than 10% of an ILS and limited only to the cost of the procedure (approximately US$20,000 per approach; the same as developing a new ILS/MLS approach).^[9]

Further savings can come from the nighttime closure of airport towers with a low volume of traffic. The FAA is reviewing 48 towers for such a potential reduction of services, which it estimates will save around US$100,000 per year at each tower, for a total annual savings of nearly US$5 million.^[10]

[edit] Drawbacks and Limitations

For all its benefits, WAAS is not without drawbacks and critical limitations.

• The broadcasting satellites are geostationary, which causes them to be less than 10° above the horizon for locations north of 71.4° latitude. This means aircraft in areas of Alaska or northern Canada may have difficulty maintaining a lock on the WAAS signal.^[11]
• To calculate an ionospheric grid point's delay, that point must be located between a satellite and a reference station. The low number of satellites and ground stations limit the number of points which can be calculated. This ultimately limits the operational area and accuracy due to undersampling.^{[citation needed]}
• Aircraft conducting WAAS approaches must possess certified receivers, which are much more expensive than commerical units. Garmin's least expensive receiver, the GNS 430W, has a suggested retail price of US$10,750.^[12]
• WAAS is not capable of the accuracies required for Category II or III ILS approaches. Thus, WAAS is not a sole-solution and either existing ILS equipment must be maintained or it must be replaced by new systems, such as the Local Area Augmentation System (LAAS).^[13]
• WAAS LPV approaches with 200 foot minimums can not be used at airports without medium intensity lighting, runway markings and a parallel taxiway. Smaller airports may not have these, and therefore require pilots to use higher minimums or pay to upgrade the airport.^[14]
• The 2004 baseline estimates the final program cost to the US Federal government as over US$3.3 billion when delivered in 2013; more than 3.7 times the original budget and 12 years behind schedule.^[15]

[edit] The Future of WAAS

[edit] Improvement to Aviation Operations

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.^[16] 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. With these projections, the FAA announced on March 24, 2006 that the first procedures that allow operations down to 200 feet will be published in 2007.^[17]

[edit] Ground Segment Improvements

Future improvements to WAAS include the integration of nine additional international Wide-area Reference Stations, bringing the total number of stations to 38. 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. When these new stations go online, they will allow for an increased number of ionospheric grid points to be calculated and transmitted.^[4][18]

[edit] Space Segment Upgrades

Galaxy XV (PRN #135) is expected to be transmitting GPS ranges in mid 2007, after 6 to 9 months of testing.^[19] Anik F1R (PRN #138) is expected to be integrated with full capacity (WAAS messages and GPS range) in spring of 2007.^[20]

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

"GPS modernization is compatible with and complements the WAAS. The L5 signal will be available on additional WAAS GEOs launched in September and October 2005. When both L1 and L5 are available, avionics will use a combination of signals to provide the most accurate service possible, thereby increasing availability of the service. These avionics will use ionospheric corrections broadcast by WAAS, or self-generated onboard dual frequency corrections, depending on which one is more accurate."^[21]

[edit] See also

• Satellite Based Augmentation System (SBAS)
• CDGPS Canadian Differential GPS
• Local Area Augmentation System (LAAS)
• Joint Precision Approach and Landing System (JPALS)
• 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)

Satellite navigation systems

Transit | GPS | GLONASS | Galileo | Beidou

Related topics: EGNOS | WAAS | LAAS

[edit] References

• U.S. Department Of Transportation & Federal Aviation Administration, Specification for the Wide Area Augmentation System (WAAS)

1. ^ ^{a b c} Federal Aviation Administration (FAA), FAQ for WAAS
2. ^ National Satellite Test Bed (NSTB), WAAS PAN Report (July 2006). Retrieved November 22nd, 2006.
3. ^ ^{a b} US House of Representatives Committee on Transportation's Subcommittee on Aviation Hearing on Cost Overruns & Delays in the FAA's Wide Area Augmentation System (WAAS) & Related Radio Spectrum Issues, June 29, 2006
4. ^ ^{a b} Federal Aviation Administration (FAA), National Airspace System Architecture Wide-Area Reference Station Accessed 07 November 2006
5. ^ Federal Aviation Administration (FAA), National Airspace System Architecture, WAAS Master Station. Accessed 29 November 2006
6. ^ Federal Aviation Administration (FAA), National Airspace System Architecture, Ground Uplink Stations
7. ^ Federal Aviation Administration (FAA), Presentation on Sept 12, 2005
8. ^ Federal Aviation Administration (FAA) Announcement March 2005
9. ^ Federal Aviation Administration (FAA), NAS FAQ. Accessed June 12, 2006.
10. ^ Testimony to US House Aviation Subcommitte by Phil Boyer, dated May 4, 2005
11. ^ 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.
12. ^ Garmin International Press Release dated November 9, 2006.
13. ^ Federal Aviation Administration. WAAS FAQ. Accessed June 12, 2006.
14. ^ Aircraft Owners and Pilots Association, AOPA welcomes improved WAAS minimums. March 7, 2006. Accessed June 14, 2006.
15. ^ Testimony to US House Aviation Subcommittee by US DOT Inspector General Kenneth Mead, pages 18 and 21, dated April 14, 2005.
16. ^ Federal Aviation Administration. WAAS 200ft Minimum Related Questions and Answers. Accessed June 12, 2006.
17. ^ Federal Aviation Administration (FAA), Press Release FAA Announces Major Milestone for Wide Area Augmentation System (WAAS). March 24, 2006.
18. ^ Federal Aviation Administration (FAA), National Airspace System Architecture Wide Area Augmentation System
19. ^ National Satellite Test Bed (NSTB), WAAS Status Announcement Accessed Nov 2nd 2006
20. ^ Federal Aviation Administration (FAA), WAAS Current News
21. ^ Federal Aviation Administration (FAA), GPS Modernization Page. Accessed 29 November 2006.

[edit] External links

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