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The Global Positioning System (GPS)

By

The Global Positioning System (GPS)
Photo © NOAA/NASA GOES Project

The global positioning system, or GPS as it is commonly known, is a vital component to modern air navigation, and an invaluable component to the FAA's NextGen program.

GPS data allows pilots to obtain precise three-dimensional or four-dimensional location data. The GPS system uses triangulation to determine an aircraft's exact location, as well as speed, track, distance to or from checkpoints, and time.

History of GPS:
The United States military first used GPS as a navigation tool in the 1970s. In the 1980s, the U.S. government made GPS available to the general public, free of charge, with one catch: A special mode, called Selective Availability, would be enabled to purposefully reduce the accuracy of GPS for public users, reserving only the most accurate version of GPS for the military.

In 2000, under the Clinton administration, selective availability was turned off, and the same accuracy that the military had benefited from was made available to the general public.

GPS Components:
The GPS system has three components: The space segment, control segment and user segments.

The space component consists of about 31 GPS satellites. The United States Air Force operates these 31 satellites, plus three to four decommissioned satellites that can be reactivated if needed. At any given moment, a minimum of twenty-four satellites are operational in a specially designed orbit, ensuring that at least four satellites are in view at the same time from almost any point on earth. The complete coverage that satellites offer makes the GPS system the most reliable navigation system in modern aviation.

The control segment is made up of a series of ground stations used to interpret and relay satellite signals to various receivers. Ground stations include a master control station, an alternate master control station, 12 ground antennas and 16 monitoring stations.

The user segment of the GPS system involves various receivers from all different types of industries. National security, agriculture, space, surveying and mapping are all examples of end users in the GPS system. In aviation, the user is typically the pilot, who views GPS data on display in the cockpit of the aircraft.

How it Works:
GPS satellites orbit about 12,000 miles above us, and complete one orbit every 12 hours. They are solar powered, fly in medium Earth orbit and transmit radio signals to receivers on the ground.

Ground stations use the signals to track and monitor satellites, and these stations provide the master control station (MCS) with data. The MCS then provides precise position data to the satellites.

The receiver in an aircraft receives time data from the satellites' atomic clocks. It compares the time it takes for the signal to go from the satellite to the receiver, and calculates distance based on that very accurate and specific time. GPS receivers use triangulation -- date from at three satellites --to determine a precise two-dimensional location. With at least four satellites in view and operational, three-dimensional location data can be obtained.

GPS Errors:
Ionosphere interference: the signal from the satellites actually slows down as it passes through the Earth's atmosphere. GPS technology accounts for this error by taking an average time, which means the error still exists but is limited.

  • Clock error: The clock on the GPS receiver might not be as accurate as the atomic clock on the GPS satellite, creating a very slight accuracy problem.
  • Orbital error: Orbit calculations can be inaccurate, causing ambiguity in determining the satellite's exact location.
  • Position error: GPS signals can bounce off of buildings, terrain, and even electrical interference can occur. GPS signals are only available when the receiver can "see" the satellite, meaning the data will be missing or inaccurate among tall buildings, dense terrain and underground.

Practical Use of GPS:

GPS is widely used in aviation today. Almost every aircraft built today come with a GPS unit installed as standard equipment. General aviation, business aviation and commercial aviation have all found valuable uses for GPS.

From basic navigation and position data to airspeed, tracking and airport locations, GPS is a precious tool for aviators.

Installed GPS units can be approved for use in IMC and for other IFR flights. Instrument pilots find GPS to be extremely helpful in maintaining situational awareness and flying instrument approach procedures. Handheld units, while not approved for IFR use, can be a helpful back-up for instrument failures, as well as a valuable tool for maintaining situational awareness in any situation.

Pilots flying VFR also use GPS as a navigation tool and a back-up to traditional pilotage and dead reckoning techniques.

All pilots can appreciate GPS data in emergency situations, as the database will allow them to search for the nearest airport, calculate time en route, fuel on board, time of sunset and sunrise, and much, much more.

Most recently, the FAA has enabled WAAS GPS procedures for approaches, introducing a new precision approach to pilots in the form of a Localizer Performance with Vertical Guidance (LPV) approach. This is a precision approach that will enable the national airspace system to become much more efficient and assist in meeting the needs of the national airspace system in the future.

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