LINES GP-4- 1542137 - Caterpillar

The Global Positioning System (GPS)
GPS is a system of satellites that orbit the earth twice a day at very high altitudes. The orbit radius is approximately 26600 km (16528 miles). GPS was set up by the government of the United States. GPS provides precise timing and positioning anywhere on earth, 24 hours per day with no charge to users.In very general terms, a GPS receiver computes a position based on radio signals received from several different satellites. The satellites have highly reliable clocks, so the timing of these satellite signals is known very accurately. The GPS receiver calculates the relative distance to each of the satellites. This calculation is based on the travel time of the signal and the speed of light (speed of the signal). The receiver then uses these distances in order to calculate the receiver's location on earth.As a broadcast only radio system, GPS supports an unlimited number of users. The broadcast frequencies penetrate clouds, rain, and snow. GPS can also accurately guide operations in fog or dust as well as at night. There are different levels of precision and accuracy available from GPS. These levels depend on the GPS receivers and the methods that you use.The four basic levels of position accuracy (1 sigma) that are available in real time (that is, instantaneously) from GPS are as follows:
Autonomous, 10 m (30 ft) to 15 m (50 ft)
DGPS, 0.3 m (1.0 ft) to 1.0 m (3.0 ft)
RTK (Float), 0.2 m (0.7 ft) to 1.0 m (3.0 ft)
RTK (Fixed), .02 m (.07 ft) to .03 m (.10 ft)Illustration 1 shows the GPS accuracy levels.
Illustration 1 g01015622
GPS accuracy levels (1) Autonomous (2) DGPS (3) RTK Fixed (4) RTK FloatThe method that is used for machine grade control in construction is the same as that used by surveyors for stakeout in construction. The method is called Real-Time Kinematic (RTK), GPS.In order to produce RTK positions two GPS receivers are required. One receiver is known as the GPS base station and is fixed in one place. The other GPS receiver is known as the rover. This receiver can be stationary or mobile.The base station communicates to the rover ('s) through a wireless data link using a data radio. Illustration 2 shows this setup.
Illustration 2 g01173793
GPS and the AccuGradeTM System (1) GPS satellites (2) GPS base station and data radio (3) Repeater radio (optional) (4) Machine equipped with the AccuGrade SystemBoth GPS receivers make observations (measurements) of the GPS signals at the same time. The base station broadcasts the observed information together with the location and other information across the data radio link to the rover. The rover then combines the data from the base station together with its own data in order to compute a very accurate position relative to the base station.A single base station can support an unlimited number of rovers, provided the rovers are within about 20 km (12.4 miles) of the base station. Normally, the range of the data radio link is the more limiting factor. You