GPS for Land Surveyors
GPS can be compared to trilateration. Both techniques rely exclusively on the measurement of distances to fix positions. One of the differences between them, however , is that the distances, called ranges in GPS, are not measured to control points on the surface of the earth. Instead, they are measured to satellites orbiting in nearly circular orbits at a nominal altitude of about 20,183 km above the earth.
The ranges are measured with signals that are broadcast from the GPS satellites to the GPS receivers in the microwave part of the electromagnetic spectrum; this is sometimes called a passive system.
GPS is passive in the sense that only the satellites transmit signals; the users simply receive them. As a result, there is no limit to the number of GPS receivers that may simultaneously monitor the GPS signals. Just as millions of television sets may be tuned to the same channel without disrupting the broadcast, millions of GPS receivers may monitor the satellite’s signals without danger of overburdening the system.
This is a distinct advantage, but as a result, GPS signals must carry a great deal of information. A GPS receiver must be able to gather all the information it needs to determine its own position from the signals it collects
from the satellites.
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