Global Positioning System (GPS)

Coordinates of objects, people, or places represent fundamental information required for any GIS. Since achieving initial operational capability in December 1993, the Global Positioning System (GPS) has allowed users to determine position easily, quickly, and cheaply. With the global market predicted to top $20 billion by 2008, GPS has rapidly become the primary technique for location determination for GIS-related applications.

GPS is a military system run by the U.S. Department of Defense, which maintains a constellation of 24 satellites (plus several spares) at an altitude of about 20,000 km. The satellites transmit signals on L-band frequencies onto which timing codes are modulated. A receiver with knowledge of these codes can measure the time taken for a signal to arrive from any particular satellite and, hence, by multiplying the time taken by the speed of light, compute its range to that satellite. Information about the position of the satellites is also modulated onto the transmitted signals. Position is computed by combining satellite positions and the measured satellite-receiver ranges in a range resection solution.

A minimum of four satellites must be visible for a receiver to achieve full 3D positioning (the fourth satellite is required to account for the time offset between the receiver and the satellites). The L-band signals of GPS are greatly weakened by obstructions such as buildings and forest canopies, and the operation of standard, off-the-shelf receivers can become severely restricted if direct line of sight from satellites to receiver is not available. In clear operating conditions, the Department of Defense quotes the accuracy of the GPS Standard Positioning Service (SPS) as being 13 m in the horizontal (95% confidence) and 22 m in the vertical (95% confidence). GPS positions tend to be less accurate in the vertical because of the geometrical distribution of the satellites in the sky above a receiver.

Although GPS costs U.S. taxpayers around $400 million per year to maintain, including the replacement of aging satellites, GPS SPS is freely available for civilian applications. Because it is a passive system, a user needs only a receiver capable of receiving and decoding the GPS signals to be able to determine location in real time at any time and any place in the world. GPS receivers vary greatly in cost and capability. Less-expensive receivers typically receive and decode only a single L-band signal (known as the L1 C/A code). More advanced receivers can also receive a second GPS signal (L2) and record the incoming phase of both signals, resulting in substantially more precise measurements. Such units are used for surveying and geodetic applications where centimeter-level accuracy is required.

Two aspects of GPS receivers are of importance to GIS users. First, some receivers can record and link attribute data to GPS location data. The data may then be transferred to a personal computer (PC) or other device. The standard format for transferring this type of data is the NMEA 0183 protocol (and the newer NMEA 2000), which is freely available from the U.S. National Marine Electronics Association. Second, many of the errors inherent in GPS range observations, mainly signal propagation errors caused by the atmosphere, errors in the position of the satellite, and errors in the timing of the satellite on-board clocks, may be reduced by the input of differential corrections into a receiver. Such corrections are provided commercially by differential service providers who use control networks of GPS receivers to monitor the errors associated with the system. Models for these errors are transmitted to the user via a communication link (e.g., FM radio) in the RTCM SC-104 format. Receivers with the capacity to receive differential corrections can typically improve on the accuracy of the SPS by a factor of 10 or better.

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