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Real Time Kinematics
| Fundamentals | |
|---|---|
| Title | Real Time Kinematics |
| Author(s) | See Credits section |
| Level | Basic |
| Year of Publication | 2011 |
Real Time Kinematics (RTK) satellite navigation is a DGNSS technique that uses the carrier phase measurements of GNSS signals. RTK is commonly used in land and hydrographic survey. The positioning accuracy obtained is of the order of centimeter-level. When only GPS signals are used, the RTK system is named Carrier-Phase Enhancement, CPGPS.[1]
No->Real Time Kinematic (RTK) satellite navigation is a DGNSS technique used in land survey and in hydrographic survey based on the use of carrier phase measurements of the GPS, GLONASS and/or Galileo signals where a single reference station provides the real-time corrections, providing up to centimeter-level accuracy. When referring to GPS in particular, the system is also commonly referred to as Carrier-Phase Enhancement, CPGPS.[1]
Introduction RTK
The RTK technique follows the same general principle as classical DGNSS, but instead of using corrections to C/A code pseudoranges, it uses the carrier phase as its signal.[1]
The RTK technique consists on a rover user that applies real-time corrections provided by a base station. In the classical DGNSS Technique, there are also 2 receivers, one at a known location (base station) and one at an unknown position, that see the same GNSS satellites in common. By fixing the location of the base station, the other location may be found either by computing corrections to the position of the unknown receiver or by computing corrections to the pseudoranges. In the classical DGNSS technology, only corrections to C/A code pseudoranges are being transmitted, which brings rover positional errors down to values about 1 m for distances between rover and base station of a few tens of kilometers.[1]
The military-only P(Y) signal sent by the same satellites is clocked ten times as fast, so with similar techniques the receiver will be accurate to about 30 cm. Therefore, in RTK system using the satellite's carrier as its signal, the improvement possible using this signal is potentially very high if one continues to assume a 1% accuracy in locking. For instance, the GPS coarse-acquisition (C/A) code broadcast in the L1 signal changes phase at 1.023 MHz, but the L1 carrier itself is 1575.42 MHz, over a thousand times as fast. This frequency corresponds to a wavelength of 19 cm for the L1 signal. Thus a ±1% error in L1 carrier phase measurement corresponds to a ±1.9 mm error in baseline estimation.[1]
The difficulty of the use of carrier phase data comes at a cost in terms of overall system complexity because the measurements are ambiguous (i.e. every cycle of the carrier is similar to every other). This makes it extremely difficult to know if you have properly aligned the signals or if they are "off by one" and are thus introducing an error of 20 cm, or a larger multiple of 20 cm. Solving this problem requires that ambiguity resolution (AR) algorithms must be incorporated as an integral part of the data processing. This integer ambiguity problem can be addressed to some degree with sophisticated statistical methods that compare the measurements from the C/A signals and by comparing the resulting ranges between multiple satellites. However, none of these methods can reduce this error to zero.[1]
In practice, 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 measured, and the mobile units compare their own phase measurements with the ones received from the base station. This allows the units to calculate their relative position to millimeters, although their absolute position is accurate only to the same accuracy as the position of the base station. The typical nominal accuracy for these dual-frequency systems is 1 centimeter ± 2 parts-per-million (ppm) horizontally and 2 centimeters ± 2 ppm vertically.
Although these parameters limit the usefulness of the RTK technique in terms of general navigation, it is perfectly suited to roles like surveying. RTK has also found uses in autodrive/autopilot systems, precision farming and similar roles. The Virtual Reference Station (VRS) method extends the use of RTK to a whole area of a reference station network. Operational reliability and the accuracies to be achieved depend on the density and capabilities of the reference station network.[2][1]
RTK Related Articles
The following articles include further information about different important topics related to RTK:
Credits
Edited by GMV. Most of the information in this article includes text taken from Wikipedia with minor adaptation,[1] provided under Creative Commons Attribution-ShareAlike License.
Notes