GPS and RTK-GPS: a comparison

The Global Positioning System (GPS) is a global satellite navigation system. The GPS system was created by the United States Department of Defense. It was established in 1973 to produce a better navigation system that would replace previous navigation systems.

Normal satellite receivers compare a signal sent from the satellite with the internally generated copy of the same signal. The receiver must delay the signal for the two to coincide. The delay is the time it takes for the signal to receive the signal and can be used to determine the distance from the satellite.

Measurement accuracy relies on the ability of the receiver electronics to accurately compare the two signals. In general, receivers can align signals to about 1% of a bit. This translates to a receiver being accurate to within 0.01 microseconds, since the GPS system sends a bit every 0.98 microseconds. In terms of distance, this is accurate to within 3 meters. However, other effects introduce errors and the accuracy of an incorrect signal is about 15 meters.

Real Time Kinematics (RTK) is based on the use of GPS signal carrier phase measurements, where a single reference station provides real time corrections. This allows an accuracy of up to centimeters. RTK can also be used with the Russian GLONASS, the Chinese Compass or the European Union Galileo. Carrier-Phase Enhancement or CPGPS is another common name for RTK GPS.

RTK systems use a single base station in conjunction with a mobile unit. The base station retransmits the carrier phase it measured. The mobile unit compares its own phase measurements with those received from the base station. This allows the units to calculate their relative position with millimeter precision. However, absolute accuracy is only as accurate as the position of the base station. Typically, this allows for accuracies of 1 centimeter horizontally and 2 centimeters vertically.

Since a base station connection is required for accuracy, RTK has limited utility for general navigation. However, it is perfect for surveying. The base station is located at a known surveyed location. The mobile unit that is connected to the base station can then produce an accurate map by taking measurements relative to that point.

Fast Static GPS is one of the most accurate GPS techniques. A minimum of two GPS receivers are required. One receiver always remains at the control station while the other gradually moves from one point to the next. One session is done for each point, but the times are significantly shorter than for static lifts. Raw GPS data is continuously recorded and then post-processed using GPS data processing software.

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