GPS satellites send out radio
signals that a GPS receiver can detect.
A GPS receiver measures the
time needed for the radio signal to travel from
the satellite to the receiver's
position. Due to the fact that
we know how fast radio signals
travel (186,000 miles per second),
we can determine how far they
have travelled based on the amount of
time it took them to arrive.
Equations
V = D/T where V is the velocity
(186,000 miles per second),
D is the distance from the
satellite to the GPS receiver, and
T is the amount of time for
the signal to reach the receiver.
Sample Calculation: 186,000
miles/sec = D / 0.06 sec
D = 11, 160 miles
This calculation shows that
the GPS receiver is 11,160
miles from the satellite.
The distance of the receiver is measured
from at least four satellites.
This data is integral in determining
one's position on Earth.
Atomic Clocks
Atomic clocks are necessary
to implement a system in which time
must be measured accurately
in nanoseconds. Any discrepancy in
the time of either the clock
of the satellite or the receiver could result
in a serious miscalculation
of location. Atomic clocks, however,
are extremely expensive (50,000
to 100,000 dollars) and cannot be
placed on all GPS receivers.
Thus, quartz clocks are used on receivers
rather than atomic clocks.
When the distance is measured from four
satellites, four spheres can
be drawn to intersect at one point. The
spheres will not intersect
if the time and distance measurements are
incorrect. Since the
receiver measures all of its time measurements,
and therefore its distance
measurements, with the same clock, the
distances will all be proportionately
incorrect. The receiver, therefore,
can calculate exactly what
distance adjustment will cause the four
spheres to intersect at one
point.
To see the national time standard on an atomic clock visit www.time.gov/