RTK

General information about RTK

Traditional RTK measurement that takes place with connection to a temporary reference station is usually called single-station RTK. Since the early 2000s, the technology has been further developed into so-called network RTK. In many countries today, geodetic infrastructure exists - or is being developed - with permanently established reference stations for GNSS measurement to enable network RTK technology.

RTK is currently the applied real-time method with the lowest measurement uncertainty, in the order of magnitude one to a few centimeters, provided that the receiver is initialized correctly, i.e. phase ambiguities is fixed to the correct integer (so-called fixed solution). Initialization usually takes about ten seconds up to one minute, depending on local conditions. With increasing distance between reference station and rover, initialization will be more difficult and measurement uncertainty will increase, as will rapid and large changes in the ionosphere or troposphere.

The local measurement environment and user implementation also affect the measurement uncertainty that can be achieved by RTK measurement. Advice and guidelines for RTK users can be found in HMK - GNSS-based detail survey.

Single-station RTK

The simplest form of RTK measurement takes place with the help of two RTK receivers, which is called single-station RTK. In single-station RTK, a reference receiver is set up over a point with a known position and a rover (moving receiver) is placed over the points whose position is to be determined.Using the relative positioning, the rover combines its own GNSS observations with the reference station, reduces the sources of error, and then obtains a position.This requires the reference station and rover simultaneously observes the same set of GNSS satellites, and the data link is able to transmit the reference station's position and observations in real-time to the rover.

Network RTK 

Network RTK is a further development of the single station RTK concept, with several permanently established reference stations working together to optimize the management of sources of error and give the user access to seamless RTK measurement with comparable quality over a larger coverage area. The concept means that the reference stations continuously send GNSS observations to an operations control center, which collects instantaneous information about the sources of error that arise due to the signal disturbances in the atmosphere, as well as clock and orbit errors. A special network RTK software is used to manage the whole chain: receiving incoming data flows from the stations, generating RTK corrections based on these flows and finally sending custom corrections to the users who connect to the network RTK service.

The hitherto dominant network RTK technology is called VRS or virtual reference station, which means that the control centre "simulates" a reference station near the rover. Data from the control center to the rover is transmitted in a standardized format for relative positioning, RTCM. In principle, this solution means that the same RTK equipment can be used as with single-station RTK. However, VRS technology requires two-way communication for the rover to report its approximate position to the control center, and reference station data is tailored for the simulated reference station.

Advantages and disadvantages of network RTK

Network RTK has both advantages and disadvantages compared to single station RTK.

Some advantages of network RTK: 

• The method requires the user to have only one RTK receiver. A reference station does not need to be established or quality assured by the user.
• Provides the opportunity for more even quality in a larger coverage area, unlike single station RTK where the measurement uncertainty increases significantly with the distance to the reference station.
• Measurement takes place directly in a uniform and modern reference system adapted for GNSS measurement; in Sweden SWEREF 99.

Some disadvantages of network RTK:

• The method requires working mobile internet, i.e. two-way communication, between service provider and user.
• Lack of traceability, as the user does not have access to complete information about the calculation method.

Use of network RTK in Sweden

Lantmäteriet has run SWEPOS  Network RTK service since 2004. In addition to SWEPOS Network RTK service, there are also a number of other network RTK services from Lantmäteriet's partners based on data from the same reference station network, namely the stations included in SWEPOS network (new window).

When measuring with network RTK under normal conditions a measurement uncertainty at the centimeter level is possible to achieve. The closer to a reference station the rover is located and the denser the network of reference stations is, the better the sources of error can be modeled and the measurement uncertainty in the user's position reduced. The SWEPOS network has about 35 km between the reference stations in large parts of Sweden. In the mountain areas in the north of Sweden, the distance between the reference stations can be up to 70 km in some places and in the metropolitan regions and in some areas with large infrastructure projects, the network is densified to about 10 km between the reference stations.

Estimated uncertainty values for different distances to reference stations and advice on measurement methodology for implementation of network RTK measurement can be found in HMK - GNSS-based detail surveying.

Preparations for mass market applications

The RTK technology is mainly used for professional practice with advanced GNSS receivers in geodetic detail surveying, machine control and agricultural applications and has a few thousand users in Sweden. However, the technology is developing and there are ongoing pilot projects that study the use of RTK rechnology with simpler GNSS receivers in e.g. mobile phones and for the positioning of autonomous vehicles. For such a future mass market with hundreds of thousands of users, other technologies for transmitting corrections data are required, than the virtual reference station with tailor-made corrections for each user. Lantmäteriet participates together with other parthers from the industry in a number of development projects to prepare the network RTK service for such mass market applications. Examples of projects are:

• NPAD (new window)
• Prepare Ships (new window)
• DINPAS (new window)