Real-time kinematic positioning
Centimetre-level GNSS correction using carrier-wave phase measurements.
Real-time kinematic positioning (RTK) is a surveying technique that corrects common errors in satellite navigation (GNSS) systems. It works by measuring the phase of the signal's carrier wave alongside the signal's information content, and relies on a single reference station or an interpolated virtual station to deliver corrections in real time, achieving up to centimeter-level accuracy. When applied specifically to GPS, it is often called carrier-phase enhancement, or CPGPS.
The method calculates the distance between a satellite and a receiver by timing how long the signal takes to travel. The receiver aligns a pseudorandom binary sequence from the satellite with its own internally generated sequence; because the satellite signal is delayed, the receiver delays its sequence until they match. The accuracy of this range measurement depends on the receiver's ability to process signals cleanly, as well as on errors from ionospheric and tropospheric delays, multipath interference, and satellite clock and ephemeris inaccuracies.
RTK follows the same general idea but uses the satellite's carrier wave instead of the information encoded in the signal. It employs a fixed base station and a rover to reduce the rover's position error. The base station sends correction data to the rover. The range to a satellite is found by multiplying the carrier wavelength by the number of whole carrier cycles between the satellite and the rover, then adding the phase difference. Determining the number of cycles is difficult because signals can shift by one or more cycles, causing an error equal to the cycle-count error times the wavelength—19 cm for the L1 signal. Solving this integer ambiguity problem yields centimeter precision. The error can be reduced by comparing measurements from C/A signals and ranges between multiple satellites. If a nearby base station measures the bias for the rover, the rover can skip the more complex calculation.
The potential improvement is high if one assumes 1% locking accuracy. For GPS, the C/A code on the L1 signal changes phase at 1.023 MHz, while the L1 carrier itself is 1575.42 MHz, changing phase over a thousand times more often. A ±1% error in L1 carrier-phase measurement thus gives a ±1.9 mm error in baseline estimation.
RTK systems use one base-station receiver and several mobile units.
Quick Facts
- Accuracy horizontal
- 8 mm / 0.315 in + 1 ppm (single base station); 8 mm / 0.315 in + 0.5 ppm (network RTK)
- Accuracy vertical
- 15 mm / 0.591 in + 1 ppm (single base station); 15 mm / 0.591 in + 0.5 ppm (network RTK)
- Typical range
- up to about 20 km (12.4 mi) from the base station
Facts from the source article.
Lore & Background
RTK follows the same general concept as standard satellite navigation but uses the satellite signal's carrier wave as its signal, ignoring the information contained within. The range to a satellite is calculated by multiplying the carrier wavelength with the number of whole carrier cycles between the satellite and the rover and adding the phase difference. Determining the number of cycles is non-trivial, since signals may be shifted in phase by one or more cycles, resulting in an error equal to the error in the estimated number of cycles times the wavelength (19 cm for the L1 signal). Solving this integer ambiguity search problem yields centimeter precision. The error can be reduced with sophisticated statistical methods that compare the measurements from the C/A signals and by comparing the resulting ranges between multiple satellites. If a nearby base station measures the bias for the rover, the rover can forgo the more complex calculation.
RTK systems use a single base-station receiver and a number of mobile units. The base station re-broadcasts the phase of the carrier that it observes, and the mobile units compare their own phase measurements with the one received from the base station. The most popular way to achieve real-time, low-cost signal transmission is to use a radio modem, typically in the UHF Band. In most countries, certain frequencies are allocated specifically for RTK purposes. Most land-survey equipment has a built-in UHF-band radio modem as a standard option. RTK provides accuracy enhancements up to about 20 km from the base station.
Network RTK extends the use of RTK to a larger area containing a network of reference stations. A continuously operating reference station (CORS) continuously broadcasts corrections, usually over an Internet connection. A CORS network is a network of CORS; accuracy is increased because more than one station helps ensure correct positioning and guards against a false initialization of a single base station. A virtual reference station (VRS) is a simulated reference station, most commonly from combining multiple nearby stations in the same network to estimate what the corrections would be near at the user's position, reducing the apparent baseline length. PPP-RTK, also known as SSR-RTK, combines globally applicable corrections from PPP-AR with location-dependent information (ionosphere and tropospheric corrections) from a base station.
Reader's Guide
RTK is perfectly suited to roles like surveying, where the base station is located at a known surveyed location, often a benchmark, and the mobile units can produce a highly accurate map by taking fixes relative to that point. It has also found uses in autodrive/autopilot systems, precision farming, machine control systems and similar roles. Although the range limitation of about 20 km from the base station restricts its usefulness for general navigation, the technique provides relative accuracy to within millimeters. For RTK with a single base station, accuracy of 8 mm + 1 ppm horizontal and 15 mm + 1 ppm vertical relative to the base station can be achieved. With network RTK, accuracy improves to 8 mm + 0.5 ppm horizontal and 15 mm + 0.5 ppm vertical relative to the nearest station. PPP-RTK/SSR-RTK obtains position fixes as quickly as RTK when near the base station, and when further away it acts as a faster and more accurate version of PPP. The technique has applications in land surveying, hydrographic surveying, and in unmanned aerial vehicle navigation.
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