Wide Area Augmentation System
From Free net encyclopedia
←Older revision | Newer revision→
Image:Waas.png The Wide Area Augmentation System (WAAS) is a system that improves the precision and accuracy of global positioning system (GPS) signals. It uses a combination of specialized satellites and ground-based stations to send correction signals to GPS receivers, as well as providing integrity information for each satellite's signal, equivalent or better than receiver autonomous integrity monitoring (or RAIM) thereby improving the accuracy of the GPS signal by approximately 5 times. The European Geostationary Navigation Overlay System (EGNOS) is the European parallel to this United States system. In Asia, it's the Japanese Multi-Functional Satellite Augmentation System (MSAS). The International Civil Aviation Organization (ICAO) calls this type of system a Satellite Based Augmentation System (SBAS).
Contents |
Approaches
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 aircraft. 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, and provides lower minimum weather requirements than non-precision approaches. It provides course guidance, distance from the runway, and elevation information.
The only precision approach currently used by civilian aviation today is the ILS (Instrument Landing System), which is a combination of a localizer, a glidepath, marker beacons and Distance Measuring Equipment, or DME. The localizer gives course guidance, lining the aircraft up with the runway; the glidepath tells the pilot how far he/she is above or below the ILS glideslope; and the DME tells the pilot how far away he/she is from the DME equipment on the ground, usually co-located with the airfield. In short, the ILS gives three-dimensional information to the pilot, although in a manner specifically suited for an approach.
The Microwave Landing System (MLS), an effort that began in the 1980s, was intended to be a higher-precision replacement for the ILS, but was rapidly overshadowed by the far less-expensive (per aircraft) WAAS technology, and the FAA has since discontinued MLS capability throughout the United States.
By providing sufficiently accurate information, WAAS enables pilots to make precision approaches based on GPS signals and an on-board database without relying on an instrument landing system or other ground-based signals. It also enhances GPS capability for en-route navigation. When a pilot files for "area navigation," or "RNAV" using GPS capability, he/she can bypass existing aviation navigation systems such as VHF omnidirectional range (VOR), TACAN and Non-directional beacon (NDB), and proceed directly from the departure point on his/her instrument departure termination point to the initial approach point for the approach to his/her destination. This direct routing saves 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.
In order to actually fly a Lateral Precision with Vertical Guidance (or LPV) approach, the pilot's WAAS-enabled GPS needs to be Class 3 or 4 TSO-C146 compliant. Currently, the only civilian GPS unit available that complies is Garmin's GNS 480 GPS/WAAS unit.
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 as being "better than three meters 95% of the time."
The following table lists the accuracy of the historical GPS systems:
100 meters: Original GPS system accuracy. 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. [1]
15 meters: This is the best non-SA accuracy. It's considered the "normal" accuracy for the GPS system. 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 and 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.
Operation
WAAS begins with approximately twenty-five ground stations positioned throughout the United States which compare the GPS signal with known (surveyed) coordinates. These ground stations send their findings to a WAAS Master Station (WMS) using a land-based communications network. The WMSs then broadcast a correction signal to the two WAAS satellites covering the U.S., which in turn broadcast that correction signal on a per-satellite basis to each WAAS-enabled GPS receiver. The WAAS-enabled GPS receiver adds the correction factor to the GPS signals to derive a corrected GPS signal which is far more accurate than the original GPS signal.
The Future of WAAS
On July 10, 2003, the WAAS signal was activated for general aviation, covering 95% of the United States, and portions of Alaska offering 350ft minimums. The FAA indicates that LPV capability will be enabled for WAAS in September 2003, enabling 250-ft minimums.
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 XV 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. Satellites will be positioned in slots at 54W, 107W, 133W and 178E.
In 2006, WAAS is projected to be available nearly all the time (over 99%), and its coverage will encompass the full continental U.S. and most of Alaska.
See also
External links
de:WAAS nl:Wide Area Augmentation System no:Wide Area Augmentation System pl:WAAS sv:WAAS