PISTON 3162292 - Caterpillar

Illustration 1 g06151189
GPS and the Cat® Grade Control System
(1) GPS Satellites
(2) Machine equipped with the Cat® Grade Control System
(3) GPS Base Station and Data Radio
(4) Repeater radio (optional)The method that is used for machine grade control in construction is the same used by surveyors for stakeout in construction. The method is called Real-Time Kinematic (RTK), GPS. Two GPS receivers are required to produce RTK positions. One GPS receiver is known as the GPS base station. The GPS base station has a fixed position. The other GPS receiver is known as the rover. This receiver is mounted on the machine.The base station communicates to the rover through a wireless data link using a data radio. Illustration 1 shows this setup.Both GNSS receivers make observations (measurements) of the GNSS 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 the machine data to compute an accurate position relative to the base station.Note: Poor placement of the reference station can adversely affect the position accuracy of the rover.A single base station can support an unlimited number of rovers, provided the rovers are within about 20 km (12.4 mi) of the base station. Normally, the range of the data radio link is the more limiting factor. The operator can use radio repeaters to extend the radio coverage.Low accuracy corrected GPS (SBAS)
If low accuracy GNSS positions are adequate for the required site work, the operator can use Satellite-Based Augmentation System (SBAS) error corrections. SBAS corrected systems do not require additional site infrastructure.A Satellite-Based Augmentation System (SBAS) supports wide-area or regional augmentation by using additional satellite-broadcast messages. Such systems are commonly composed of multiple ground stations that take measurements of one or more of the GNSS satellites, the satellite signals, or environmental factors that may impact the signal received by users. These measurements are used to send information messages to one or more satellites for broadcast to the end users to augment and improve accuracy, reliability, and availability of GNSS positional information.The following SBAS networks are currently available:
Table 1
WAAS Wide Area Augmentation System (United States)
EGNOS European Geostationary Navigation Overlay Service (Europe)
MSAS Multi-Functional Satellite Augmentation System (Japan) GNSS receivers use a GNSS receiver configuration file (.cfg) to convert from WGS84 coordinates to the site coordinate system. High accuracy (RTK) GNSS systems use a configuration file generated from the site calibration. If the position of the base station entered for site calibration is not accurate, the site coordinate systems will shift away from the local coordinate system. When this faulty receiver configuration file is used by a GNSS RTK receiver, the shift will not be obvious. The site relative positions points will still be accurate and the accuracy of RTK positioning is unaffected. However, if the same receiver configuration file is used by a GNSS receiver that uses SBAS corrections, the shift will become obvious which will result