Eagle Electronics II - Manual (Page 7)

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more precise system is coding. In other words, the data
coming from the satellites is encrypted. Civilian GPS
receivers use “C/A Code.” Its accuracy is intentionally
worse than the military’s “P Code.” In this manner,
civilian users worldwide can benefit from excellent po-
sition fixes. Meanwhile, the military keeps the most
accurate system away from potential enemies. Theo-
retically, C/A code can give accurate position fixes up to
15 meters. This is more than adequate for most people.
However (as of this writing), the military isn’t satisfied
with C/A’s potential accuracy in the hands of the world.
So, it’s degrading it further with “Selective Availability” or
SA. This is small, random errors intentionally added to
the system so your accuracy will typically be within 100
meters. Of course, accuracy also depends on the angle
of the satellite above the horizon, signal-to-noise ratio,
the number of satellites tracked at one time (the more
As the receiver locks on to each satellite, it calculates
the distance from the satellite by measuring the length
of time it takes the radio signal to reach it. Each satellite
has an extremely accurate clock that tells the receiver
when the radio transmission started. The receiver com-
pares that time against its own clock, thus it knows how
long it took the radio signal (travelling at the speed of
light!) to reach it. If you know time and speed, then you
can calculate distance. Once you have this from three
satellites, then the receiver can determine your posi-
tion.
ACCURACY
You may have heard tales of extraordinary accuracy
from GPS receivers. The DOD requires accuracy of 10
to 15 meters from the satellite system. However, only
the military gets this precision. The way the military
keeps us and other unauthorized people from using the