Global Positioning System - Obtaining GPS Data
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
Pseudo Random Code
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
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.