Primary Sources: Wikipedia: GPS and Wikipedia: GNSS


The Global Positioning System, usually called GPS (the US military refers to it as NAVSTAR GPS - Navigation Signal Timing and Ranging Global Positioning System), is a fully operational satellite navigation system as of 2006. It is one of two fully operational Global Navigation Satellite Systems (GNSS), the other being the Russian GLONASS, although others are under development. A constellation of more than two dozen GPS satellites allow accurate determination of location (longitude, latitude, and altitude) in real time, day or night, anywhere on Earth.

A GPS receiver compares time signal transmissions from four or more satellites to calculate the precise time and its current position (latitude, longitude, elevation), using trilateration. The receiver computes the distance to each of the four satellites from the difference between local time and the time the satellite signals were sent (this distance is called a pseudorange). It then decodes the satellite's locations from their radio signals and an internal database.

The receiver should be located at the intersection of four spheres, one around each satellite, with a radius equal to the time delay between the satellite and the receiver multiplied by the speed of the radio signals. Because the receiver does not have a very precise clock it cannot compute the time delays. The receiver does not need a precise clock, but does need a clock with good short-term stability so it can measure with high precision the differences between the times when the various messages were received and hence use multilateration to accurately locate itself. This yields 3 hyperboloids of revolution of two sheets, whose intersection point gives the precise location of the receiver. This is why at least four satellites are needed: fewer than 4 satellites yield 2 hyperboloids, whose intersection is a curve; it is impossible to know where the receiver is located along the curve without supplemental information, such as elevation. If elevation information is already known, only signals from three satellites are needed (the point is then defined as the intersection of two hyperboloids and an ellipsoid representing the Earth at this altitude).

For more information on the technologies commonly employed in CyArk's Digital Preservation projects, see the Related Articles below.



References:
• Alber, C., R.H. Ware, C. Rocken and F.S. Solheim. "GPS Surveying with 1 mm Precision Using Corrections for Atmospheric Slant Path Delay." Geophys. Res. Lett., 24. 1997: 1859-1862
• Rizos, C. 1996. Principles and Practice of GPS Surveying. Monograph 17, School of Geomatic Engineering, the University of New South Wales, Sydney, Australia, 555pp.
• Boehler, W. and G. Heinz. "Documentation, Surveying, Photogrammetry." XVII CIPA International Symposium. Recife, Brazil, Proceedings (1999).
• Wikipedia: GPS
• Wikipedia: GNSS