Everything about Wide Area Augmentation System totally explained
The
Wide Area Augmentation System (
WAAS) is an
air navigation aid developed by the
Federal Aviation Administration to augment the
Global Positioning System (GPS), with the goal of improving its accuracy, integrity, and availability. Essentially, WAAS is intended to enable aircraft to rely on GPS for all phases of flight, including
precision approaches to any airport within its coverage area.
WAAS uses a network of ground-based reference stations (
Benchmark DGPSRs transmitting differential corrections (DCs)), located within spaces protected from the public inside airports in
North America and
Hawaii, to measure small variations in the GPS satellites' signals in the
western hemisphere. Measurements from the reference stations are routed to master stations, which queue the received DCs and send the correction messages to geostationary WAAS satellites in a timely manner (at least every 5 seconds or better). Those satellites broadcast the correction messages back to Earth, where WAAS-enabled GPS receiver uses the corrections while computing its position to improve accuracy. The longer any given DC has been delayed, the less benefit it'll produce.
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, the Indian Gagan, 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. 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 of a navigation system includes the ability 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.
Availability
Availability is the probability that a navigation system meets the accuracy and integrity requirements. Before the advent of WAAS, GPS could be unavailable for up to a total time of 4 days per year. The WAAS specification mandates availability as 99.999% (five nines) throughout the service area, equivalent to a downtime of just over 5 minutes per year.
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).
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 15 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 15 and Anik F1R) were launched in late 2005.
Galaxy 15
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.
As of October, 2007, the new satellites are in an operational mode, however they're not yet full replacements. While both new satellites transmit correction messages, their GPS-like signals are still being improved. both Galaxy 15'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.
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 don't change this frequently, so they're 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.
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.
Drawbacks and Limitations
For all its benefits, WAAS isn't 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.
- 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 GPS receivers, which are much more expensive than commercial units. In 2006, Garmin's least expensive receiver, the GNS 430W, had a suggested retail price of US$10,750.
- WAAS isn't capable of the accuracies required for Category II or III ILS approaches. Thus, WAAS isn't 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).
- WAAS Lateral Precision with Vertical guidance (LPV) approaches with 200-foot minimums won't be published for airports without medium intensity lighting, precision runway markings and a parallel taxiway. Smaller airports, which currently may not have these features, would have to upgrade their facilities or require pilots to use higher minimums.
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. 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.
Software Improvements
Software improvements, to be implemented by September 2008, significantly improve signal availability of vertical guidance throughout the CONUS and Alaska. Area covered by the 95% available LPV solution in Alaska improves from 62% to 86%. And in the CONUS, the 100% availability LPV-200 coverage rises from 48% to 84%, with 100% coverage of the LPV solution. Additionally, FAA status presentations indicate a third geostationary satellite could be acquired between 2009 and 2013.
[Further Information]
Get more info on 'Wide Area Augmentation System'.
|
External Link Exchanges
Do you know how hard it is to get a link from a large encyclopaedia? Well we're different and will prove it. To get a link from us just add the following HTML to your site on a relevant page:
<a href="http://wide_area_augmentation_system.totallyexplained.com">Wide Area Augmentation System Totally Explained</a>
Then simply click through this link from your web page. Our crawlers will verify your link, extract the title of your web page and instantly add a link back to it. If you like you can remove the words Totally Explained and embed the link in article text.
As long as your link remains in place, we'll keep our link to you right here. Please play fair - our crawlers are watching. Your site must be closely related to this one's topic. Any kind of spamming, dubious practises or removing the link will result in your link from us being dropped and, potentially, your whole site being banned. |
