In Short
The GPS satellites are equipped with atomic clocks and so give very accurate
time readings. The satellites transmit the time and their exact position to
control centres on the ground. GPS receivers get this information after a short
delay, and using the length of delay can calculate how far away each satellite
is and, therefore, where it is. If your GPS has mapping software, it can then
display your position on a map. The more satellites your GPS can see, the more
accurate its reading will be. It requires at least three satellites to find
your location. When your GPS has information from more than three satellites it
can calculate not only your longtitude and latitude, but your altitude, speed,
bearing, track, trip distance, distance to destination, and even sunrise and
sunset times.
The accuracy of a position determined with GPS depends on the type of receiver
but most hand-held GPS units have 10-20 metre accuracy. In Depth
GPS satellites transmit information in high frequency radio waves to control
centres on the ground. Civil GPS receivers access the C/A-code
(coarse/acquisition code) transmitted on the L1 frequency (1575.42 MHz).
Military receivers, in addition, use the encrypted P-code (precise or precision
code) which is transmitted on both L1 and the L2 frequency (1227.60 MHz). Each
satellite transmits a unique sequence of zeros (0) and ones (1) and GPS
Receivers use the CDMA (Code Division Multi Access) method to identify it. The
signals are precisely timed by the satellite’s atomic clock. The GPS system then uses the satellite’s unique ID to calculate its precise
location. The receiver determines how long it takes the signals to travel from
the satellite to the receiver, with the knowledge that radio waves travel at
the speed of light. Determining a position from measurements of distances is
known as trilateration (not triangulation, which involves the measurement of
angles). The actual trilateration is similar to basic geometry. The information
from the first satellite tells the GPS receiver that it is on the circumfrance
of a circle, since the distance it is away from the satellite applies to every
other point on that circle (the radius of which is the distance from the
satellite). The second satellite’s information will be an circle which
intersects the first at two points. The third satellite will confirm which of
those two points the receiver is on. A GPS receiver needs at least three satellites to calculate a 2D position
(latitude and longitude) and to track movement. With four or more satellites it
can calculate altitude as well, producing a 3D reading. The additional channels
on your reciever will track any other visible satellites creating greater
accuracy, and allowing reliable, continuous navigation, even in adverse weather
or terrain. |
