Wide Area RTK (WARTK)

Fundamentals
Title	Wide Area RTK (WARTK)
Author(s)	GMV
Level	Basic
Year of Publication	2011

In Real Time Kinematics (RTK), the assumption that the differential ionospheric delay between a GNSS transmitter and each of the roving or reference receivers is negligible works well for baselines up to 10-20 kilometers. A refinement of this assumption comes with the network RTK (NRTK) using a set of permanent receivers to mitigate atmospheric dependent effects over distance, increasing the allowed distance between baselines and rovers up to 50-70 kms. In the late 1990s, the Wide Area RTK concept was introduced by the Research Group of Astronomy and Geomatics (gAGE) from the Technical University of Catalonia (UPC) to address these deficiencies.

Wide-Area Real-Time Kinematics (WARTK)

During the last few years, the group gAGE/UPC has developed the so-called Wide-Area Real-Time Kinematics (WARTK) technique, which allows the extension of local services based on the real-time carrier phase ambiguity resolution to wide-area scale (i.e. baselines between the rover and reference stations greater than 100 km), for both dual-frequency (GPS) and 3-frequency systems (Galileo and modernised GPS). The Wide-Area Real-Time Kinematics (WARTK) technique for dual and 3-frequency systems are based on an optimal combination of accurate ionospheric and geodetic models in a permanent reference stations network. The main factor limiting the range extension of the RTK technique beyond a few tens of kilometres is the differential ionospheric correction between the roving and the nearest reference GNSS station. Such ionospheric correction impedes the real-time ambiguity fixing, and therefore the corresponding accurate navigation at sub-decimetre level. The ionosphere produces ambiguity biases and correlations whose mitigation becomes the main problem to sort out. Even with the aid of multi-reference-station techniques, due to the baseline limitation (<20 km), several thousands would be required to cover such a service to the whole European region, obviously unaffordable from a logistic and economic point of view.

The main techniques supporting WARTK are related to an accurate real-time computation of ionospheric corrections, combined with an optimal processing of GNSS observables (carrier phases in particular) in both 2 and 3-frequency systems. The method dramatically increases the RTK/NRTK service area, with permanent stations separated by up to 500–900 kilometers — all while requiring 100 to 1,000 times fewer receivers covering a given region. Besides the feasibility of WARTK at the positioning level, recent studies have confirmed that WARTK also assures integrity.

The target market should be a market line where the enhancement provided by the WARTK technique is needed, such as sub-decimetre accuracy, orientation and wide-area service coverage. It would be mandatory to have institutional support due to the extended permanent receiver network involved to perform such techniques. The EGNOS RIMS network would be a feasible possibility and it would diminish the initial investment for the prototype. The time-to-market should be reduced to the minimum since the current GNSS systems, already on the market, could evolve in the direction of WARTK (e.g., cheaper dual-frequency receivers). The following markets have been identified as the most suitable for the different applications that WARTK is able to provide at this stage of development: accurate navigation in deep seas, tsunami detection, instant meteorology, civil construction, precision farming, orientation, cadastral coverage, real-time wide-area mapping and auto-piloting.

WARTK Related Articles

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

• WARTK Fundamentals introduces the recently-developed WARTK technique.

• The WARTK Standards article summarizes some conventions, models and formats commonly used by WARTK.

• WARTK Systems sections provides an overview of the potential WARTK systems and applications.