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With origin dating back to the mid-1990s, Real Time Kinematics (RTK) is a [[Differential GNSS|differential GNSS]] technique which provides high positioning performance in the vicinity of a base station. The technique is based on the use of carrier measurements and the transmission of corrections from the base station, whose location is well known, to the rover, so that the main errors that drive the stand-alone positioning cancel out. A RTK base station covers a service area spreading about 10 or 20 kilometres, and a real time communication channel is needed connecting base and rover. RTK, which achieves performances in the range of a few centimetres, is a technique commonly used in surveying applications.<ref name="RTKIAG">[http://www.wasoft.de/e/iagwg451/ International Association of Geodesy (IAG) Working Group 4.5.1: Network RTK ] </ref><ref name="RTK_WIKI">[http://en.wikipedia.org/wiki/Real_Time_Kinematic RTK in Wikipedia]</ref><ref name="RTKWPNC06">[http://www.wpnc.net/fileadmin/WPNC06/Proceedings/34_Precise_Positioning_in_Real-Time_using_Navigation_Satellites_and.pdf Remote Sensing 2009, A. Rietdorf et al., ''Precise Positioning in Real-Time using Navigation Satellites and Telecommunication'', Proceedings of the 3rd Workshop on Positioning, Navigation and Communication (WPNC’06) ]</ref>


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 centimetre-level accuracy. When referring to GPS in particular, the system is also commonly referred to as Carrier-Phase Enhancement, CPGPS.
==Introduction RTK==
Real Time Kinematics (RTK) is a [[Differential GNSS|differential GNSS]] technique originated in the mid-1990s that provides high performance positioning in the vicinity of a base station.<ref name="RTKIAG"/>


==Introduction==
From an architectural point of view, RTK consists of a base station, one or several rover users, and a communication channel with which the base broadcasts information to the users at real time.


TO DO: Resume the following and put something about RTK systems.
[[File:Summit_rtk_survey.jpg|300px|thumb|RTK Support Polar Survey]]


The classical GNSS receivers compare a pseudorandom signal being sent from the satellite with an internally generated copy of the same signal. Since the signal from the satellite takes time to reach the receiver, the two signals do not "line up" properly; the satellite's copy is delayed in relation to the local copy. By progressively delaying the local copy more and more, the two signals will eventually line up properly. That delay is the time needed for the signal to reach the receiver, and from this the distance from the satellite can be calculated.
The technique is based on the following high-level principles:
*In the neighbourhood of a clean-sky location, the main errors in the GNSS signal processing are constant, and hence they cancel out when differential processing is used. This includes the error in the satellite clock bias, the satellite orbital error, the ionospheric delay and the tropospheric delay.
*The noise of carrier measurements is much smaller than the one of the pseudo-code measurements. However, the processing of carrier measurements is subject to the so-called carrier phase ambiguity, an unknown integer number of times the carrier wave length, that needs to be fixed in order to rebuild full range measurements from carrier ones.
*The phase ambiguity can be [[RTK Fundamentals|fixed]] for dual-frequency differential measurements for two close receivers.


The accuracy of the resulting range measurement is generally a function of the ability of the receiver's electronics to accurately compare the two signals. In general receivers are able to align the signals to about 1% of one bit-width.[1] For instance, the coarse-acquisition (C/A) code sent on the GPS system sends a bit every 0.98 microsecond, so a receiver is accurate to 0.01 microsecond, or about 3 metres in terms of distance. 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. Other effects introduce errors much greater than this, and accuracy based on an uncorrected C/A signal is generally about 15 m.
The base station broadcasts its well-known location together with the code and carrier measurements at frequencies L1 and L2 for all in-view satellites. With this information, the rover equipment is able to fix the phase ambiguities and determine its location relative to the base with high precision. By adding up the location of the base, the rover is positioned in a global coordinate framework.


RTK follows the same general concept, but uses the satellite's carrier as its signal, not the messages contained within. 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 ±1.9mm error in baseline estimation.[1]
The RTK technique can be used for distances of up to 10 or 20 kilometres,<ref name="RTKIAG"/><ref name="RTKWPNC06"/> yielding accuracies of a few centimetres in the rover position. RTK is extensively used in surveying applications.


The difficulty in making an RTK system is properly aligning the signals. The navigation signals are deliberately encoded in order to allow them to be aligned easily, whereas 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. 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.
The main limitations of RTK are as follows:
*Limited range with respect to the base location.
*The need of a communication channel for real time applications.
*Some convergence time is needed to fix the phase ambiguities. This time depends on the processing algorithm and the distance between base and rover, and ranges from a few seconds to a few minutes.
*In order to avoid re-initialization of the processing, the rover has to track the GNSS signals continuously. This makes the RTK not suitable for urban applications.


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. There are several ways to transmit a correction signal from base station to mobile 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.
Recently, different approaches have been followed to improve the limitation regarding the range of the base station, namely Network RTK<ref name="RTKIAG"/><ref name="RTCM_V3">[http://www.rtcm.org/ RTCM STANDARD 10403.1, FOR DIFFERENTIAL GNSS (GLOBAL NAVIGATION SATELLITE SYSTEMS) SERVICES – VERSION 3, RTCM 10403.1, RTCM Paper 177-2006-SC104-STD, 2006]</ref> and [[Wide Area RTK (WARTK)|Wide Area Real Time Kinematics (WARTK)]]. Network RTK is based on the provision of corrections from a network of base stations in such a way that the phase measurements are provided with consistent ambiguities; this has the advantage that the rover can switch from one base station to another without the need of re-initializing the ambiguity fixing filters. WARTK is further described in dedicated [[Wide Area RTK (WARTK)|articles]].


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 centimetre ± 2 parts-per-million (ppm) horizontally and 2 centimetres ± 2 ppm vertically.
==RTK Related Articles==
 
Although these parameters limit the usefulness of the RTK technique in terms of general navigation, it is perfectly suited to roles like surveying. In this case, the base station is located at a known surveyed location, often a benchmark, and the mobile units can then produce a highly accurate map by taking fixes relative to that point. 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.
The following articles include further information about different important topics related to RTK:
 
* [[RTK Fundamentals|RTK Fundamentals]].  
==RTK Related Articles==
* [[RTK Standards|RTK Standards]].
* [[RTK Systems|RTK Systems]].


The following articles include further information about different important topics related to a RTK. [[Work in Progress:RTK Fundamentals|RTK Fundamentals]] introduces the main general architecture of a RTK. The [[Work in Progress:RTK Standards|RTK Standards]] section summarizes the standardization of RTK systems, the principle applicable documents and its current status. Finally, [[Work in Progress:RTK Systems|RTK Systems]] sections provides a brief overview of the current existing RTK.


==Notes==
==Notes==

Latest revision as of 17:14, 18 September 2014


FundamentalsFundamentals
Title Real Time Kinematics
Edited by GMV
Level Basic
Year of Publication 2011
Logo GMV.png

With origin dating back to the mid-1990s, Real Time Kinematics (RTK) is a differential GNSS technique which provides high positioning performance in the vicinity of a base station. The technique is based on the use of carrier measurements and the transmission of corrections from the base station, whose location is well known, to the rover, so that the main errors that drive the stand-alone positioning cancel out. A RTK base station covers a service area spreading about 10 or 20 kilometres, and a real time communication channel is needed connecting base and rover. RTK, which achieves performances in the range of a few centimetres, is a technique commonly used in surveying applications.[1][2][3]

Introduction RTK

Real Time Kinematics (RTK) is a differential GNSS technique originated in the mid-1990s that provides high performance positioning in the vicinity of a base station.[1]

From an architectural point of view, RTK consists of a base station, one or several rover users, and a communication channel with which the base broadcasts information to the users at real time.

RTK Support Polar Survey

The technique is based on the following high-level principles:

  • In the neighbourhood of a clean-sky location, the main errors in the GNSS signal processing are constant, and hence they cancel out when differential processing is used. This includes the error in the satellite clock bias, the satellite orbital error, the ionospheric delay and the tropospheric delay.
  • The noise of carrier measurements is much smaller than the one of the pseudo-code measurements. However, the processing of carrier measurements is subject to the so-called carrier phase ambiguity, an unknown integer number of times the carrier wave length, that needs to be fixed in order to rebuild full range measurements from carrier ones.
  • The phase ambiguity can be fixed for dual-frequency differential measurements for two close receivers.

The base station broadcasts its well-known location together with the code and carrier measurements at frequencies L1 and L2 for all in-view satellites. With this information, the rover equipment is able to fix the phase ambiguities and determine its location relative to the base with high precision. By adding up the location of the base, the rover is positioned in a global coordinate framework.

The RTK technique can be used for distances of up to 10 or 20 kilometres,[1][3] yielding accuracies of a few centimetres in the rover position. RTK is extensively used in surveying applications.

The main limitations of RTK are as follows:

  • Limited range with respect to the base location.
  • The need of a communication channel for real time applications.
  • Some convergence time is needed to fix the phase ambiguities. This time depends on the processing algorithm and the distance between base and rover, and ranges from a few seconds to a few minutes.
  • In order to avoid re-initialization of the processing, the rover has to track the GNSS signals continuously. This makes the RTK not suitable for urban applications.

Recently, different approaches have been followed to improve the limitation regarding the range of the base station, namely Network RTK[1][4] and Wide Area Real Time Kinematics (WARTK). Network RTK is based on the provision of corrections from a network of base stations in such a way that the phase measurements are provided with consistent ambiguities; this has the advantage that the rover can switch from one base station to another without the need of re-initializing the ambiguity fixing filters. WARTK is further described in dedicated articles.

RTK Related Articles

The following articles include further information about different important topics related to RTK:


Notes


References

  1. ^ a b c d International Association of Geodesy (IAG) Working Group 4.5.1: Network RTK
  2. ^ RTK in Wikipedia
  3. ^ a b Remote Sensing 2009, A. Rietdorf et al., Precise Positioning in Real-Time using Navigation Satellites and Telecommunication, Proceedings of the 3rd Workshop on Positioning, Navigation and Communication (WPNC’06)
  4. ^ RTCM STANDARD 10403.1, FOR DIFFERENTIAL GNSS (GLOBAL NAVIGATION SATELLITE SYSTEMS) SERVICES – VERSION 3, RTCM 10403.1, RTCM Paper 177-2006-SC104-STD, 2006
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