WARTK based on EGNOS and Galileo: technical feasibility study
Background & Objectives
During the last few years, gAGE/UPC has developed and tested new techniques, which allow 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). These are the so-called Wide-Area Real-Time Kinematics (WARTK) technique for dual and 3-frequency systems that 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 real-time kinematics 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 WARTK technique has been invented by the Research Group of Astronomy and Geomatics (gAGE) from the Technical University of Catalonia (UPC), in collaboration with GSFC/NASA. In this project, it has been shown that a precise GNSS navigation service (with typical errors below 10 cm) is feasible based on a wide-area network of reference receivers by using the above mentioned WARTK technique.
The main working methods and approaches were:
- analysis of the ionospheric climatology in the EGNOS service area, and its impact on the expected accuracy of the ionospheric corrections and the ambiguity resolution success;
- computation of a set of true reference ionospheric values to evaluate the ionospheric models;
- analysis of tropospheric refraction characteristics for roving receivers;
- use of state-of-the-art methods to mitigate tropospheric propagation delay errors in a wide-area differential positioning service;
- assessment of the impact of tropospheric propagation delay errors (systematic and stochastic), satellite orbit and clock errors on the WARTK user with respect to ambiguity resolution and positioning performance;
- study of the potential use of EGNOS corrections in the frame of the WARTK service;
- design of the WARTK ionospheric model and network architecture;
- solve the communications aspect on the WARTK service, including communications between permanent receivers and also the links with the WARTK users;
- testing the stabilised WARTK algorithm during more than one month of continuous reference and roving GNSS receiver’s data stream;
- re-evaluation of the WARTK algorithm for 3-frequency systems in the specific geometry of the EGNOS RIMS;
- study and assessment of applications and markets for a potential WARTK-EGAL service provider.
The main goal of the WARTK-EGAL project is to show the capability of using the EGNOS reference station network for supporting wide-area sub-decimetre error level navigation over the EGNOS service area with GPS/Galileo signals. The main techniques supporting this new approach 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. This is the so-called WARTK technique, previously developed by gAGE/UPC and protected by two patents.
Work performed & results
Considering that WARTK-based applications provide the highest accuracy under the worst environmental conditions and that the required infrastructure is limited, there should be a clear place in the market for these applications. However, present GNSS applications perform quite well with the existing systems. Therefore, 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 RIMS 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. The present price of dual-frequency receivers could be a barrier for mass-market uses but hardware developers foresee lowering it in the medium term. The following markets have been identified as the most suitable for the three 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.