- General, navigation system

GPS and RTK-GPS: A Comparison

Global Positioning System (GPS) is a global satellite navigation system. The GPS system was created by the United Statement 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 to the internally generated copy of the same signal. The receiver must delay the signal in order for the two to match up. The delay is the time for the signal to receive the signal and can be used to determine the distance from the satellite.

The accuracy of the measurement is based on the ability of the receiver’s electronics accurately compare the two signals. Generally, receivers are able to align the signals to around 1% of one 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 3 meters. However, other effects introduce errors and the accuracy of an uncorrected signal is around 15 meters.

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

RTK systems use a single base station along with a mobile unit. The base station re-broadcasts the phase of the carrier that it measured. The mobile unit compares their own phase measurements with the one received from the base station. This allows the units to calculate their relative position to millimeter accuracy. However, the 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 precision, RTK is has limited usefulness for general navigation. However, it is perfect for surveying. The base station is located at a known surveyed location. The mobile unit which is connected to the base station can then produce an accurate map by taking measurements relative to that point.

Rapid static GPS is one of the most accurate GPS techniques. A minimum of two GPS receivers are required. One receiver always remains on the control station while the other is moved progressively from one point to the next. A session is conducted for each point, but the times are significantly shorter than for static surveys. Raw GPS data is recorded continuously and the post-processed later using GPS data processing software.