Technology

Topics:

• Surveying
• Remote Sensing
• Panoramic Photography
• Laser Scanning
• Kite Photography
• HDR Photography
• HDD Survey
• GPS Surveying
• Digital Video

GPS Surveying

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 the only fully operational satellite navigation system as of 2006. 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).

Although the GPS is the only current satellite navigation system, others have existed or are in developmental stages: the former Soviet (and now Russian) GLONASS system is still in orbit but suffers from disrepair and has essentially been rendered useless as of 2001; there are plans to revitalize the Russian satellite network. The European Union agreed in 2002 to introduce its own European counterpart to GPS, called Galileo, with satellites being placed in orbit between 2006 and 2008. The Chinese have also begun to launch satellites to form a system called the Beidou Navigation System.

Source: Wikipedia: GPS and Wikipedia: Satellite Navigation System

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).