The Global Positioning System (GPS) is a location system based on a constellation of satellites orbiting the earth at altitudes of approximately 11,000 miles. GPS was developed by the United States Department of Defense (DOD), for its tremendous application as a military locating utility. The DOD’s investment in NAVSTAR GPS is immense. Billions and billions of dollars have been invested in creating this technology for military uses. However, over the past decade, GPS has proven to be a useful tool in non-military mapping applications as well. Other GPS constellations have been or are being deployed by Russia (GLONASS), the European Union (Galileo), and China.
As GPS units become smaller and less expensive, there are an expanding number of applications for GPS. In transportation applications, GPS assists pilots in pinpointing their locations and avoiding collisions, and provides drivers with turn-by-turn directions on the road. Farmers can use GPS to guide equipment and control accurate distribution of fertilizers and other chemicals. Recreationally, GPS is used for providing accurate locations and as a navigation tool for hikers, hunters and boaters.
So, how do the GPS satellites help us determine our location on the earth? In a nutshell, GPS is based on satellite ranging – calculating the distances between the GPS receiver and the known positions of 4 or more satellites and then applying some good old mathematics.
The distance between a GPS satellite and a GPS receiver can be determined by measuring the amount of time it takes a radio signal (the GPS signal) to travel from the satellite to the receiver. Radio waves travel at the speed of light, which is about 186,000 miles per second. So, if the signal’s travel time is known, the distance from the satellite to the receiver can be determined (distance = speed x time).
Geometry tells us that we can determine the XYZ position of a point from knowing the distances of this point to 3 reference points. Since the clocks in the GPS receivers are not as accurate as the expensive atomic clocks in the satellites, the distances to the satellites cannot be determined precisely. Therefore, information from an additional satellite is required to determine the time variable and compute a position.
As you may already know, the positions provided by the current GPS receivers used in consumer and GIS applications are far from being precise. This is because this type of GPS receivers use code signals (C/A code)that have a relatively long wavelength.
Carrier phase GPS receivers measure the distance from the receiver to the satellites by counting the number of waves that carry the C/A Code signal. The carrier wave has a much shorter wavelength. Therefore, the carrier phase GPS receivers can determine the position much more accurately.
There are several other factors that affect the accuracy of the computed GPS position:
Ephemeris Errors (satellite deviation from predicted orbits)
Selective Availability (intentional man-made alteration)
Multipath (due to reflected GPS signals)
A technique called differential correction can be used to remove most these errors and obtain better accuracies. Differential correction requires a second GPS receiver, a base station, collecting data at a stationary position on a precisely known point (typically it is a surveyed benchmark). Because the physical location of the base station is known, a correction factor can be computed by comparing the known location with the GPS location determined by using the satellites. The differential correction process takes this correction factor and applies it to the GPS data collected by a GPS receiver in the field. Differential correction will eliminate most of the ionospheric errors and all errors due to ephemeris, clock drift, measurement noise and selective availability. However, it cannot remove errors due to multipath. There are specially designed antennas that will help keep out the “stray” signals and therefore minimize the multipath error. Carrier phase receivers typically provide 10 – 30 cm GPS position accuracy with differential correction.
Land surveying applications employ dual-frequency receivers that receive signals from the satellites on two frequencies simultaneously to determine positions with sub-centimeter accuracies. Such survey-grade GPS receivers carry a much higher price tag than the GIS-grade receivers.