Raven RPR 110, Инструкция по установке и обслуживанию

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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 capabili-ties, 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.”