High Precision GPS (RTK)
Introduction
RTK (from English Real Time Kinematic) or real-time satellite kinetic navigation, is a technique used for topography, agricultural machinery and marine navigation based on the use of phase measurements from navigators with GPS, GLONASS and/or Galileo signals "Galileo (satellite navigation)"), where a single reference station provides corrections in real time, obtaining submetric accuracy. When referring to the particular use of the GPS network, the system is also commonly called DGPS (Carrier Phase Correction).
"Normal" satellite navigation based receivers compare a pseudo-random signal that is sent from the satellite with an internal copy generated by the same signal. Since the satellite signal takes time to reach the receiver, the two signals do not "align" correctly; the satellite copy is delayed in reference to the local copy. By progressively delaying the local copy, the two signals will align correctly at some point. This delay is the time necessary for the signal to reach the receiver, and from the result of this the distance to the satellite can be calculated.
The resulting measurement accuracy is generally a function of the receiver's electronic ability to accurately compare the two signals. In general, traditional receivers can align signals within a 1% margin of error.
For example, the raw reading code (C/A) sent to the GPS system sends one bit of information every 0.98 microseconds, so a receiver is accurate to within 0.01 microseconds, or close to 3 meters in terms of distance. The military-only signal P(Y) sent by the same satellites is recorded ten times faster, so with similar techniques the receiver will have an accuracy of about 30 cm. Other effects introduce errors much larger than this, and the accuracy based on an uncorrected C/A signal is generally around 15M.
RTK follows the same general concept, but uses the satellite carrier as its signal, not the messages contained within it. The improvement possible using this signal is potentially very high if one continues to assume 1% accuracy in fixation. For example, the GPS raw data acquisition code (C/A) transmitted on L1 signal shifts phase at 1.023 megacycles (MHz), but the L1 carrier itself is 1575.42 MHz, more than a thousand times faster. This frequency corresponds to a wavelength of 19 cm for the L1 signal. In this way an error of ±1% in the phase measurement of the L1 carrier corresponds to an error of ±1.9mm in the base estimate.
The difficulty in implementing an RTK system lies in correctly aligning the signals. Navigation signals are deliberately encoded to prevent them from being easily aligned, where each carrier cycle is similar to each other. This makes it extremely difficult to know if the signals have been aligned correctly or if they are shifted in a cycle and thus introducing an error of 20 cm, or a larger multiple of 20 cm. This problem of integer ambiguity can be addressed to some degree with sophisticated statistical methods that compare measurements of C/A signals and by comparing the resulting ranges between multiple satellites. However, none of these methods can reduce this error to zero.