Information about Wide Area Augmentation System

The Wide Area Augmentation System (WAAS) is an elevated-accuracy navigation system developed for civil aviation by the Federal Aviation Administration (FAA), a division of 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 North America 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. Commercial systems include StarFire and OmniSTAR.

WAAS Objectives

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.

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 extremely small probability that an error exceeding the requirements for accuracy will go undetected. Specifically, the probability is stated as 1×10^-7, and is equivalent to no more than 3 seconds of bad data per year.^[3]

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.<ref name="House_Subcommitee_0600" /> The WAAS specification mandates availability as 99.999% (five nines) throughout the service area.

Operation

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

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 October 2007 there are 38 WRS's, twenty in the contiguous United States (CONUS), seven in Alaska, one in Hawaii, one in Puerto Rico, five in Mexico, and four in Canada.^{[4] [5] [6]}

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).<ref name="FAA_WAAS_FAQ" />

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.^[7]

Space Segment

The space segment consists of multiple geosynchronous communication satellites which broadcast the correction 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. Currently, the Space segment consists of two commercial satellites, Galaxy XV and Anik F1R.

The original two WAAS satellites, named Pacific Ocean Region (POR) and Atlantic Ocean Region-West (AOR-W), were leased space on Inmarsat III satellites. These satellites ceased WAAS transmissions on July 31, 2007. With the end of the Inmarsat lease approaching, two new 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]

As of October, 2007, the new satellites are in an operational mode, however they are not yet full replacements. While both new satellites transmit correction messages, their GPS-like signals are still being improved. both Galaxy XV's and Anik F1R's ranging data are flagged as "Non Precision Approach." Both are expected to improve to "Precision Approach" during the second half of 2007.^[9]

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

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 an 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.

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.<ref name="FAA_WAAS_FAQ" /> 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 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 favor 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. 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.

Timeline

Comparison of Accuracy

A comparison of various radionavigation system accuracies

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

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.^[11]

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.^[12]
• 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.
• Aircraft conducting WAAS approaches must possess certified receivers, which are much more expensive than commercial units. Garmin's least expensive receiver, the GNS 430W, has a suggested retail price of US$10,750.^[13]
• 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).^[14]
• 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.^[15]
• 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.^[16]

The Future of WAAS

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.^[17] 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.^[18]

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. The stations are in Merida, Mexico City, Puerto Vallarta, San Jose del Cabo and Tapachula in Mexico; and Winnipeg, Iqaluit, Gander and Goose Bay in Canada). This allows for an increased number of ionospheric grid points to be calculated and transmitted.<ref name="NAS_WRS" />^[19]

Space Segment Upgrades

Both Galaxy XV (PRN #135) and Anik F1R (PRN #138) contain a L1 & L5 GPS payload. This means they will potentially be usable with the L5 modernized GPS signals when the new signals and receivers become available.

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References