Search

The Ambiguity Dilution of Precision (ADOP) is a well-known scalar measure that can be used to infer the strength of the Global Navigation Satellite System (GNSS) model of the carrier phase ambiguities involved in precise relative GNSS positioning. Odijk and Teunissen (2008a) derived closed-form expressions for single-baseline GNSS models that allow verifying the factors affecting the ambiguity resolution as well as the probability of its correct resolution as integer values. However, this weighted-ionosphere ADOP closed form assumes that the standard deviation of the ionosphere delay is dependent on a function associated with the baseline length. This means that a baseline of the same length at different locations around the world and in different moments in time would have the same standard deviation, which in practice is not true. This becomes even worse in regions like Brazil, where the ionospheric anomalies are more intense and frequent, especially in periods of high solar activity. In this work, a new method for calculating the ionospheric delay standard deviation that considers the ionospheric scintillation S4 index is proposed to improve the closed-form ADOP performance. Experiments with a baseline of 280 m located in Presidente Prudente, Brazil (magnetic latitude of around −13°), were carried out. The results showed that in periods of both weak and strong ionospheric scintillation, the introduction of S4 into the ionospheric delay standard deviation was beneficial for ADOP. The average improvement of the ADOP closed form was of around 72% in the analyzed period of weak scintillation and of 35% in the period of strong scintillation. In addition, the results showed that in periods of strong scintillation, relative positioning accuracy is around 100 times worse, up to 30 m, when compared to periods of weak ionospheric scintillation. [ABSTRACT FROM AUTHOR]

Mapping widespread dyke swarms shed light on the tectonic processes that culminate in continental fragmentation, especially in the early phases of the crustal extension and magma emplacement. Airborne magnetic data are an effective geophysical tool to clarify the extent of dyke swarms in continental-scale areas due to the expressive magnetic contrast between magmatic bodies and host rocks. However, many geological features display similar magnetic patterns, making the qualitative interpretation of magnetic anomaly maps quite subjective and ambiguous. To improve and optimize the predictive mapping of dyke swarms, this research presents a multivariate analysis of airborne geophysical surveys, applying a Self-Organizing Maps (SOM) approach using two magnetic and three gamma-spectrometric variables. This semiautomatic technique has allowed an integrated spatial analysis and the classification of the lithological units based on their magnetic and gamma-spectrometric signatures. We applied the SOM method to investigate a set of mafic dyke swarms that intruded in the Neoproterozoic Borborema Province (BP), Parnaíba Basin (PB) and the São Francisco Craton (SFC) in NE Brazil. These dykes are part of a large magmatic event associated with the continental breakup, which formed the Equatorial Atlantic Ocean in the Early Cretaceous, denominated EQUAMP. First, the SOM hyperparameters were defined by running the algorithm in a representative area in the central part of the BP. This training test area worked as a SOM template through which all data from the area were processed. The SOM analysis identified seven different populations, according to responses found in the five geophysical input variables. Two of these populations were associated with the mafic dykes, reducing the ambiguity of the magnetic anomaly interpretation. These populations represent high SOM quantization error, which means that these groups are the most anomalous data, evidenced in airborne magnetic data. These results were checked during fieldwork, revealing that dyke swarms occur more widely than were previously known, throughout the BP intruding the SFC, and showing some occurrences embedded in the Paleozoic sedimentary infill of the eastern border of the PB. • An unsupervised data mining approach for mapping mafic dykes. • Prediction of dyke swarms using self-organizing maps. • Integrated spatial analysis and classification of airborne geophysical data. • Optimization of geophysical and geological mapping. [ABSTRACT FROM AUTHOR]

To perform network RTK positioning, a user must receive and apply the information generated from a network. Corrections are generated for the user to consider the atmospheric delays (ionosphere and troposphere) between ther location and the network stations. In order to generate reliable corrections and send them to users, one must resolve the ambiguities in the processing step of the network stations. Ambiguities are the number of integer cycles of the carrier phase in the first data collection epoch and are introduced as parameters to be estimated in the processing. Thus, the ambiguity resolution process becomes a prerequisite for obtaining high accuracy in network RTK positioning. This process can be divided into two steps: estimation and validation. The estimation step aims to obtain the integer values of ambiguities and in the validation step it is inferred if the estimated integer value may or may not be accepted. In this work, the LAMBDA (Least squares AMBiguity Decorrelation Adjustment) method was used to obtain the integer ambiguity estimate and for the validation the Ratio and FF-RT (Fixed Failure Ratio Test) tests were used. The objective of this work was to investigate the performance of the ambiguities validation tests for long baselines. The results showed that the FF-RT test presented the highest fixed ambiguity percentages when compared to the Ratio test in all analyzed periods. Thus, for long baselines positioning, in periods of either weak or strong ionospheric activity, the use of the FF-RT test will cause a greater number of fixed ambiguities to be used in the process of generating the corrections for the user. • Evaluation of ionospheric activity effects on GNSS ambiguity resolution. • Data considering the last solar maximum (2012–2014). • Evaluation at the south crest of the equatorial anomaly in Brazil, one of the most affect countries. • The use of fixed failure rate for the critical value in ratio test improves the number of fixed ambiguities when compared to the commonly used critical value. [ABSTRACT FROM AUTHOR]