GLOBAL 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 it, but not to see how it works or appreciate
what it can do for you. 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 transmits 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. It’s 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 satellite’s 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 receiver’s 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 receiver’s calculation. These errors consist of:
1.
Ionosphere and troposphere delays on the radio signal.
2.
Signal multi-path.
3.
Receiver clock biases.
4.
Orbital satellite (ephemeris) position errors.
5.
Intentional degradation of the satellite signal by the DOD (SA).
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. SA accounts for the majority of the error budget. 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
.”
9
