Basic GPS Concept - Fundamentals of Global Positioning System Receivers: A Software Approach - page 8

3. It should have worldwide coverage. Thus, in order to cover the polar regions
the satellites must be in inclined orbits.
4. The transmitted signals should tolerate, to some degree, intentional and
unintentional interference. For example, the harmonics from some narrow-
band signals should not disturb its operation. Intentional jamming of GPS
signals is a serious concern for military applications.
5. It cannot require that every GPS receiver utilize a highly accurate clock
such as those based on atomic standards.
6. When the receiver is first turned on, it should take minutes rather than
hours to find the user position.
7. The size of the receiving antenna should be small. The signal attenuation
through space should be kept reasonably small.
These requirements combining with the availability of the frequency band
allocation determines the carrier frequency of the GPS to be in the L band
(1 - 2 GHz) of the microwave range.
2.3 BASIC GPS CONCEPT
The position of a certain point in space can be found from distance measured
from this point to some known positions in space. Let us use some examples to
illustrate this point. In Figure 2.1, the user position is on the x -axis; this is a one-
dimensional case. If the satellite position S 1 and the distance to the satellite x 1
are both known, the user position can be at two places, either to the left or right
of S 1 . In order to determine the user position, the distance to another satellite
with known position must be measured. In this figure, the positions of S 2 and x 2
uniquely determine the user position U .
Figure 2.2 shows a two-dimensional case. In order to determine the user posi-
tion, three satellites and three distances are required. The trace of a point with
constant distance to a fixed point is a circle in the two-dimensional case. Two
satellites and two distances give two possible solutions because two circles inter-
sect at two points. A third circle is needed to uniquely determine the user position.
For similar reasons one might decide that in a three-dimensional case four
satellites and four distances are needed. The equal-distance trace to a fixed point
is a sphere in a three-dimensional case. Two spheres intersect to make a circle.
This circle intersects another sphere to produce two points. In order to determine
which point is the user position, one more satellite is needed.
FIGURE 2.1 One-dimensional user position.