Accuracy - Lowrance GlobalMap Sport Installation And Operation Instructions Manual

time against its own clock, thus it knows how long it took the signal to
reach it. Since the receiver knows the time and the speed of the signal,
the distance to the satellite can be easily calculated. Once it has the dis-
tance to three satellites, the receiver calculates and displays your posi-
tion.

ACCURACY

You may have heard tales of extraordinary accuracy from GPS receivers.
The DOD (Department of Defense) requires an accuracy of 15 meters or
less from the satellite system. However, only the military gets this preci-
sion. Civilian GPS receivers accuracy is considerably less than the
military's. There are two methods used to degrade the system's accuracy
to civilian users. The data transmitted from the satellites is encrypted us-
ing two different codes: P-code and C/A code. The military receivers use
the P-code, while civilians use the C/A code. The P-code is the more
accurate one. Theoretically, C/A code can give accurate position fixes of
up to 15 meters.
However, (as of this writing) the military isn't satisfied with C/A's potential
accuracy in the hands of the world. So, it uses Selective Availability (S/A)
to degrade the accuracy further. S/A is small, random errors intentionally
added to the system so your accuracy will typically be within 100 meters
of your true position. Of course, accuracy also depends on the angle of
the satellite above the horizon, signal-to-noise ratio, the number of satel-
lites tracked at one time (the more the better), and other factors.
Due in part to the effects of S/A, you will see your position on the mapping
display move even if you're sitting still.
For more accurate positioning, DGPS (Differential GPS) is installed in
some parts of the country. DGPS transmitters broadcast a correction sig-
nal that not only eliminates the S/A effects, but gives extremely accurate
positions. This system requires a DGPS beacon receiver that decodes
the signal from the DGPS transmitter and sends it to a DGPS capable
unit such as the GlobalMap Sport™.
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