GEOG 862
GPS and GNSS for Geospatial Professionals


figure of trilateration, see text description in link below
Click here to see a text description.
Depiction of trilateration - a triangle where the vertices indicating control points are labeled P1, P2, and P3. The vertex in side the triangle indicating the unknown point being positioned is labeled A.  The measured vector connecting P1 to A is called L1, the measured vector connecting P2 to A is called L2, and the measured vector connecting P3 to A is L3.

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.

GPS is often compared to triangulation, which is actually not entirely correct. More correct would be trilateration. Trilateration is based upon distances rather than the intersection of lines based on angles. Now, in a terrestrial survey as indicated in this image here, there would probably be a minimum of three control stations, and from them would emanate three intersecting distances, i.e., L1, L2, and L3.

This is very similar to what's done with GPS, except instead of the control points being on the surface of the Earth, they are orbiting the Earth. The GPS satellites are the control points orbiting about 20,000 kilometers above the Earth. There's another difference; instead of there being three lines intersecting at the unknown point, there are four. Four are needed because there are four unknowns - X, Y, Z, and time - that need to be resolved.

There are also some similarities between this image of terrestrial surveying and the GPS solution. The distances need to be paired with their correct control points in both cases. Another is that the distances are measured electronically based upon the speed of electromagnetic radiation (i.e. light) and the amount of time that the signal takes to go from the control point to the unknown point, and back in some cases. Please note that in GPS that trip is one way. We'll talk more about that. There are other similarities too, but these ideas of distances being used, several simultaneous distances, being used to find the position of an unknown point is one of the fundamental ideas behind the functioning of GPS.