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. The Wide Area RTK concept was introduced in thelate 1990s to address these deficiencies by the Research Group of Astronomy and Geomatics (gAGE) from the Technical University of Catalonia (UPC)

A common assumption in real-time kinematic (RTK) techniques is that the differential ionospheric delay between a GNSS transmitter and each of the roving or reference receivers is negligible. However, increased position uncertainty — spatial decorrelation — is usually allocated to the baseline receivers as baseline distances increase. A refinement of this assumption comes with the network RTK (NRTK) using a set of permanent receivers to mitigate atmospheric dependent effects, such as the ionospheric delay, over distance.

Differential GNSS

The WARTK concept was introduced in the late 1990s to address these deficiencies. 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.

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The classical DGNSS technique technique is an enhancement to a primary GNSS system, that consists of the determination of the GNSS position for an accurately-surveyed position known as reference station. Given that the position of the reference station is accurately known, the deviation of the measured position to the actual position and more importantly the corrections to the measured pseudoranges to each of the individual satellites can be calculated. These corrections can thereby be used for the correction of the measured positions of other GNSS user receivers.

When GPS is the only constellation used for the Differential GNSS technique, the system is called DGPS. DGPS accuracy is in the order of 1 m (1 sigma) for users in the range of few tens of km from the reference station, growing at the rate of 1 m per 150 km of separation.

The DGNSS term can be refer to specific implementations using DGNSS technique. It is often used to refer specifically to systems that re-broadcast the corrections from ground-based transmitters of shorter range. For instance, the United States Coast Guard and Canadian Coast Guard run one such system in the US and Canada on the longwave radio frequencies between 285 kHz and 325 kHz. These frequencies are commonly used for marine radio, and are broadcast near major waterways and harbors. Australia runs two DGPS systems: one is mainly for marine navigation, run by Australian Maritime Safety Authority, broadcasting its signal on the longwave band; the other is used for land surveys and land navigation, and has corrections broadcast on the Commercial FM radio band.

There are other DGNSS techniques used by high-precision navigation/surveying applications, based on the use of carrier phase measurements. These are the cases of the Real Time Kinematics (RTK) and the Wide Area RTK (WARTK), where the differential GPS measurements are computed in real-time by specific GPS receivers if they receive a correction signal using a separate radio receiver.

DGNSS Related Articles

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

• DGNSS Fundamentals introduces the classical DGNSS technique, its functionalities and the objectives of a DGNSS system.

• The DGNSS Standards article summarizes the international bodies in charge of the standardization of DGNSS systems, the principle applicable documents and its current status.

• DGNSS Systems sections provides a brief overview of the current existing DGNSS systems.