Positioning System (GPS)
GPS is a satellite-based global navigation
system created and operated by the United States Department of Defense
(DOD). Originally intended solely to enhance military defense capabilities,
GPS capabilities have expanded to provide highly accurate position
and timing information for many civilian applications.
An in-depth study of GPS is required to
fully understand how it works, but simply stated: Twenty four satellites
in six orbital paths circle the earth twice each day at an inclination
angle of approximately 55 degrees to the equator. This constellation
of satellites continuously transmit coded positional and timing information
at high frequencies in the 1500 Megahertz range. GPS receivers with
antennas located in a position to clearly view the satellites, pick
up these signals and use the coded information to calculate a position
in an earth coordinate system.
GPS is the navigation system of choice
for today and many years to come. While GPS is clearly the most accurate
worldwide all-weather navigation system yet developed, it still can
exhibit significant errors. GPS receivers determine position by calculating
the time it takes for the radio signals transmitted from each satellite
to reach earth. Itís that old "Distance = Rate x Time" equation.
Radio waves travel at the speed of light (Rate). Time is determined
using an ingenious code matching technique within the GPS receiver.
With time determined, and the fact that the satelliteís position is
reported in each coded navigation message, by using a little trigonometry
the receiver can determine its location on earth.
Position accuracy depends on the receiverís
ability to accurately calculate the time it takes for each satellite
signal to travel to earth. This is where the problem lies. There are
primarily five sources of errors which can affect the receiverís calculation.
These errors consist of (1) ionosphere and troposphere delays on the
radio signal, (2) signal multi-path, (3) receiver clock biases, (4)
orbital errors, also known as ephemeris errors of the satellite's
exact location, and (5) the intentional degradation of the satellite
signal by the DOD. This intentional degradation of the signal is known
as "Selective Availability (SA)" and is intended to prevent
adversaries from exploiting highly accurate GPS signals and using
them against the United States or its allies. However, on May 1, 2000,
U.S. President Bill Clinton ordered Selective
Availability (SA) turned off at midnight (Coordinated Universal
Time). Now, civilian GPS users around the world will no longer experience
the up to 100 meter (approximate 300 feet) random errors that SA added
to keep GPS a more powerful tool for the military. Today,
GPS units are accurate to within 20 meters (approximately 60 feet);
although in good conditions, units should display an error of less
than 10 meters.
The combination of these errors in conjunction with poor satellite
geometry can limit GPS accuracy to 100 meters 95% of the time and
up to 300 meters 5% of the time. Fortunately,
many of these errors can be reduced or eliminated through a technique
known as "Differential."