To determine an object’s location, the GPS system must receive a radio signal from at least three satellites. Since each satellite emits a unique signal, the receiver can then match the signal to the satellite and its orbital position. Distance from the receiver is then calculated (for each satellite), and from that data, the receiver accurately calculates its geographic position. But just how accurate is GPS?
GPS accuracy is affected by a number of factors, including satellite positions, noise in the radio signal, atmospheric conditions, and natural barriers to the signal. Noise can create an error between 1 to 10 meters and results from static or interference from something near the receiver or something on the same frequency. Clouds and other atmospheric phenomena, and objects such a mountains or buildings between the satellite and the receiver can also produce error, sometimes up to 30 meters. The most accurate determination of position occurs when the satellite and receiver have a clear view of each other and no other objects interfere.
Obviously, mountains and clouds can not be controlled or moved, nor can interference and blockage from buildings always be prevented. These factors then, will affect GPS accuracy. To overcome or get around these factors, other technology, AGPS, DGPS, and WAAS, has been developed to aid in determining an accurate location.
AGPS (Assisted Global Positioning System) is a system that assists conventional GPS when reception of the radio signal from the satellite is poor or non-existent (line of sight is blocked). To aid the GPS, the AGPS gains information via a wireless network, such as the GPS receivers on cell towers, to relay the satellite information to the receiver. With this assistance, the GPS doesn’t have to calculate the satellite’s orbit, which shortens initialization time, and increases battery life.
To further increase accuracy, DGPS (Differential Global Positioning System) technology was developed. Like the AGPS, the DGPS uses a fixed GPS location (such as a cell tower) to send information to the GPS receiver. DGPS, however, looks at both the satellite and the fixed location adjusts for any difference between the two, and then sends that information to the receiver. DGPS is particularly helpful when atmospheric conditions interfere with reception.
The most recent innovation in GPS technology is the WAAS (Wide Area Augmentation System) developed by the FAA and DOD to augment GPS for air navigation. Utilizing a network of ground-based stations (WRS or Wide-area Reference Stations) which are protected from the public, WAAS transmits corrections to geosynchronous communications satellites, which then transmit the corrections to the user. WAAS was designed to allow aircraft to rely on GPS for all phases of flights, including precision, or “instrument only” landings. Specifications for WAAS require accuracy of 7 meters or better both vertically and laterally, 95% of the time. In practice, WAAS achieved a lateral accuracy of 1 meter and of 1.5 meters vertically when over the contiguous United States.
Many GPS manufacturers market their products as the more accurate, or having greater sensitivity than their competitors, but the bottom line is that GPS accuracy depends on the GPS technology in use.
WAAS stands for Wide Area Augmentation System. Normally, unaided GPS is good for an accuracy of about 10 meters. WAAS provides an accuracy of 1.5 meters. WAAS was jointly developed by the Federal Aviation Administration (FAA) and the Department of Transportation (DOT) to provide precision approach capability for aircraft. The system was put into operation on July 10, 2003.
WAAS is similar to DGPS only it covers a much greater area – the entire continental U.S. and some of Alaska. While the system was developed for aviation, other areas benefit, too. It is used for mapping and surveying as well as providing greater accuracy for handheld and auto navigation receivers. For mapping and surveying purposes, accuracy is often less than one meter.
How it works
There are a few things that cause GPS not to be perfectly accurate. The charged particles of the ionosphere, and water vapor of the troposphere slow the signal slightly. Multipath, ephemeris errors, and the atomic clocks on the satellites themselves can also contribute to the inaccuracies.
Throughout the continental U.S. are 25 “reference stations.” These are GPS receivers that are placed at points that have been very accurately surveyed. These reference stations receive the same GPS signals as the moving receivers. The difference is instead of using the signal’s travel time to calculate position, the reference stations use their known position to calculate timing. Because the reference stations know exactly where they are, they can figure out what the travel time of the signals should be. They then compare the calculated times with the actual times. The difference is an “error correction” factor.
The error information is sent to two master stations (one on the U.S. east coast, one on the west coast) which calculate correction algorithms and assess the integrity of the system. A correction message is uplinked to the two WAAS satellites via a ground uplink system.
The two satellites are in geostationary orbits. These satellites then transmit the correction information back down to the GPS user on the GPS frequency. The GPS receiver then decodes this information and applies it to its calculated position to significantly improve the accuracy.
The benefits of WAAS to civil aviation will be considerable. WAAS improves the efficiency of aviation operations due to:
Greater runway capacity
Reduced aircraft separation which allows increased capacity in a given airspace
More direct enroute flight paths
New precision approach services
Reduced and simplified equipment on board aircraft
Substantial cost savings due to the elimination of maintaining older navigation aids
Europe has an equivalent of WAAS called EGNOS (European Geostationary Navigation Overlay Service). . Japan is also developing a similar system called MSAS (Multi Functional Transport Satellites MTSAT] Satellite – Based Augmentation System).