Global Positioning System - Obtaining GPS Data

Triangulation

is the actual process of finding your geographic position on earth.  Following planning, the user is ready to collect data in the field.  As stated previously, the rover unit needs at least four satellites to determine position.  If elevation is known, three satellites are sufficient, but the user must define that elevation in the rover unit.  The position of the rover unit is where the vectors between the rover unit and the satellites connect.  Hence, one, two, or three satellites leave too many possibilities for the position of the rover unit.  With four satellites, the vectors can only intersect at one geographic position, that being the position of the rover unit.

 Figure 3: Pseudo Random Code

Because the satellites are moving in outer space and the rover unit is moving with the rotation of the Earth, your position is calculated by what must be two stationary points in space.  Obviously, the rover unit and the satellites are not stationary.  How is the position of the rover unit calculated via distance vectors between the rover unit and the satellites when both are moving?  The answer is pseudo random code.  The code was developed so that the rover unit could compare the received signals with those of the satellite.

Receiving and Storing Data

Remember, while collecting data, careful attention must be paid to the number of satellites in use by the rover unit at any given time.  One should also pay close attention to where the data is being stored in terms of the data dictionary within the unit.  It is easy to get confused and store line data where, for example, point data should go.

GPS Units will keep these data in files (rover files), which the user needs to be aware of and keep track of for use in differential correction after the field data has been collected.

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