WARTK Fundamentals

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

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

WARTK Technique

In RTK technique the differential ionospheric refraction on the signals typically limits the real-time ambiguity resolution (and the corresponding navigation with sub-decimeter errors) to baselines of few tens of km from the nearest reference site. The main techniques supporting this new approach, 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 GNSS systems. The navigation can be performed with few centimeters of error at distances of hundreds of kilometers from the nearest reference station.

Indeed, the Ionosphere produces ambiguity estimation biases and correlations whose mitigation is the main problem of several techniques such as LAMBDA method, which takes into account these correlations in order to get realiable ambiguities for short baselines. With this RTK technique several thousands of reference receivers would be needed to provide service to Europe. To solve this limitation, WARTK provides to the users with a very accurate ionospheric refraction estimate to be removed from the user navigation filter equations. This was fulfilled by developing a very precise technique to compute ionospheric corrections in real-time using a 3-D voxel model of the ionosphere, estimated by means of a Kalman filter, and using exclusively GNSS data gathered from fixed receivers separated several hundreds of kilometers. In this way, just few dozens of fixed reference GNSS receivers are enough to ensure a sub-decimeter positioning service at continental scale, over Europe for example.

'Figure 1:' WARTK Technique (image cortesy of gAGE/UPC)

The ionospheric model running in the WARTK central processing facitility (CPF) (see Figure 1), precisely captures the real-time, linear and larger scale electron conten variations. The model tomographically maps the ionospheric state as measured by a network o permanent GNSS receivers, each separated up to many hundreds of kilometers. A second component of the model, needed to provide precise ionospheric corrections to the users, characterizes and mitigates ionospheric waves, known as medium scale traveling ionospheric disturbances (MSTID), which are frequent non-linear phenomenon affecting GNSS users at mid latitude. This precise real-time ionopsheric model is well integrated in the geodetic filter of the CPF, and ensuers successful ambiguity fixing among the permanent GNSS stations and especially between the nearest permanent site.

Then, then WARTK user navigation employs multi-frequency carrier phase data, combined with the accurate corrections provided by the CPF, most importantly, ionospheric delay. Once the ionospheric corrections are applied by the user, cycle ambiguities can be fixed either by using a three-carrier ambiguity resolution (TCAR) approach or the well-know LAMBDA method. A layout of this approach for a WARTK user receiver is shown in Figure 2.

Figure 2: WARTK algorithm layouth for the user receiver (image cortesy of gAGE/UPC)