CALIBRA WP100 - Problem characterisation

Primary work within WP100 is the characterisation of the problems affecting scientific and commercial applications in Brazil, which rely on carrier based high accuracy GNSS positioning, such as RTK (Real Time Kinematics) and PPP (Precise Point Positioning). Main objective and work of WP100 is therefore to qualify and quantify how residual errors, which remains in both the undifferenced and double differenced GNSS observables, are driven by ionospheric related phenomena. Based on first analysis of correlation between GNSS positioning and scintillation occurrence, WP300 was able to implemented and test several different schemes, with main aim to obtain a 'cleaner' observable.

As a starting point in WP100, first a detailed study of the Brazilian GNSS market was performed with main aim to capture the current industrial context and to provide an indication on how the commercialisation of project results may be pursued. The market segments are presented in Figure 2. Another study which was also performed was the identification of already existing data inventory, currently being used within all WP, and deployment of new receivers. Figure 3 is presenting the CIGALA/CALIBRA stations in Brazil in both their geographical location, considering the status on Jan/2015; up to date data availability status can be found at http://is-cigala-calibra.fct.unesp.br. UNESP also performed preliminary analysis of RTK and PPP related approaches which provided some insights on the correlation between GNSS positioning errors and scintillation effects. Experiments involved both PPP and RTK solutions, where in both cases results showed the vulnerabilities of GNSS carrier phase based techniques to ionosphere disturbances, which in turn degrade the positioning (PPP or RTK). Based on the results and because the main objective of WP100 is problem characterisation, in terms of correlation between ionospheric effects and positioning performance, we have chosen amplitude and phase scintillation indices as a primary estimators for a general characterisation of ionospheric effects. Suitable parameters that can be used for the characterisation of the problem when their performance is correlated with the ionospheric parameters were identified as follows. Since the necessity to resolve double differenced carrier phase ambiguities as integers is a key computational component in high accuracy relative GNSS positioning, the first characterisation parameter identified was the success rate of ambiguity fixing, or ambiguity resolution (AR). The second selected parameter, which can be severely degraded under strong scintillation is positioning accuracy. Because the main aim of WP100 is to obtain and present possible correlation between ionospheric estimators and characterisation parameters, we have created a characterisation metric, which is being used in current analysis. The preliminary results showed that the correlation between scintillation indices and positioning errors at the rover station in an RTK setup is overall reasonably visible, even though not significantly high. Nevertheless, the results showed that the development of characterisation metric offer the scope for the incorporation in larger data set and additional information. Therefore, current work within WP100 is strongly connected to systematic analysis of larger data sets and additional information, in terms of characterisation parameters.


Figure 2: GNSS Market Segments in Brazil


Figure 3: stations of the CIGALA/CALIBRA Network on January/2015.