Analysis of multi-constellation GNSS PPP solutions under phase scintillations at high latitudes

In the past few years, the rapid evolution of multi-constellation navigation satellite systems boosted the development of many scientific and engineering applications. More than 100 satellites will be available in a few years, when all the four global constellations (GPS, GLONASS, Galileo, and Beidou) will be fully deployed. This high number of visible satellites has improved the performance of precise point positioning (PPP) techniques both in terms of accuracy and of session length, especially easing the modeling of ionospheric biases. However, in the presence of severe environmental and atmospheric conditions, the performance of PPP considerably deteriorates. It is the case of high-latitude scenarios, where the satellites coverage is limited, the satellites geometry is poor and ionospheric scintillation are frequent. This paper analyzes the quality of PPP solutions in terms of accuracy and convergence time, for a GNSS station in Antarctica. Single and multi-constellation results are compared, proving the benefits of the availability of a higher number of satellites as well as the improved robustness to the presence of moderate and strong phase scintillations. The use of PPP multi-constellation at high latitudes is indeed essential to guarantee high accuracy, and to obtain a low convergence time, of the order of tens of minutes

A mass-market Galileo receiver: Its algorithms and performance

The two main GNSS receiver market segments, professional high-precision receivers and mass market/consumer receivers, have very different structure, objectives, features, architecture, and cost. The code-delay estimation is performed in the software receiver by a parallel correlation unit, giving as output a multi-correlation with certain chip spacing. This approach presents some advantages, mostly the fact that the number of correlation values that can be provided is thousands of times greater, compared to a standard receiver channel. Use of multiple correlators increases multipath-rejection capabilities, essential features in mass-market receivers, especially for positioning in urban scenarios. The TTFF was estimated with about 50 tests, in hot, warm, and cold start, first using both GPS and Galileo satellites, and then using only one constellation. In the second case only the 2D fix is considered, since, according to the scenario described, at maximum three satellites are in view

Adaptive Phase Detrending for GNSS Scintillation Detection: A Case Study Over Antarctica

We aim at contributing to the reliability of the phase scintillation index on Global Navigation Satellite System (GNSS) signals at high-latitude. To the scope, we leverage on a recently introduced detrending scheme based on the signal decomposition provided by the fast iterative filtering (FIF) technique. This detrending scheme has been demonstrated to enable a fine-tuning of the cutoff frequency for phase detrending used in the phase scintillation index definition. In a single case study based on Galileo data taken by a GNSS ionospheric scintillation monitor receiver (ISMR) in Concordia Station (Antarctica), we investigate how to step ahead of the cutoff frequency optimization. We show how the FIF-based detrending allows deriving adaptive cutoff frequencies, whose value changes minute-by-minute. They are found to range between 0.4 and 1.2 Hz. This allows better accounting for diffractive effects in phase scintillation index calculation and provides a GNSS-based estimation of the relative velocity between satellite and ionospheric irregularities

Disentangling ionospheric refraction and diffraction effects in GNSS raw phase through fast iterative filtering technique

We contribute to the debate on the identification of phase scintillation induced by the ionosphere on the global navigation satellite system (GNSS) by introducing a phase detrending method able to provide realistic values of the phase scintillation index at high latitude. It is based on the fast iterative filtering signal decomposition technique, which is a recently developed fast implementation of the well-established adaptive local iterative filtering algorithm. FIF has been conceived to decompose nonstationary signals efficiently and provide a discrete set of oscillating functions, each of them having its frequency. It overcomes most of the problems that arise when using traditional time–frequency analysis techniques and relies on a consolidated mathematical basis since its a priori convergence and stability have been proved. By relying on the capability of FIF to efficiently identify the frequencies embedded in the GNSS raw phase, we define a method based on the FIF-derived spectral features to identify the proper cutoff frequency for phase detrending. To test such a method, we analyze the data acquired from GPS and Galileo signals over Antarctica during the September 2017 storm by the ionospheric scintillation monitor receiver (ISMR) located in Concordia Station (75.10° S, 123.33° E). Different cases of diffraction and refraction effects are provided, showing the capability of the method in deriving a more accurate determination of the σϕ index. We found values of cutoff frequency in the range of 0.73–0.83 Hz, providing further evidence of the inadequacy of the choice of 0.1 Hz, which is often used when dealing with ionospheric scintillation monitoring at high latitudes

Analysis of the ionospheric scintillations during 20-21 January 2016 from SANAE by means of the DemoGRAPE scintillation receivers

This paper presents ionospheric scintillation data recorded at SANAE in Antarctica during a moderate geomagnetic storm on 20-21 January 2016 which gives evidence of the advantages of the new generation of instrumentation for monitoring ionospheric scintillation. The data was collected as part of the DemoGRAPE project aimed at the demonstration of cutting edge technology for the empirical assessment of the ionospheric delay and ionospheric scintillations in the polar regions which affect the accuracy of satellite navigation

Formation of ionospheric irregularities over Southeast Asia during the 2015 St. Patrickˈs Day storm

We investigate the geospace response to the 2015 St. Patrickˈs Day storm leveraging on instruments spread over Southeast Asia (SEA), covering a wide longitudinal sector of the low-latitude ionosphere. A regional characterization of the storm is provided, identifying the peculiarities of ionospheric irregularity formation. The novelties of this work are the characterization in a broad longitudinal range and the methodology relying on the integration of data acquired by Global Navigation Satellite System (GNSS) receivers, magnetometers, ionosondes, and Swarm satellites. This work is a legacy of the project EquatoRial Ionosphere Characterization in Asia (ERICA). ERICA aimed to capture the features of both crests of the equatorial ionospheric anomaly (EIA) and trough (EIT) by means of a dedicated measurement campaign. The campaign lasted from March to October 2015 and was able to observe the ionospheric variability causing effects on radio systems, GNSS in particular. The multiinstrumental and multiparametric observations of the region enabled an in-depth investigation of the response to the largest geomagnetic storm of the current solar cycle in a region scarcely reported in literature. Our work discusses the comparison between northern and southern crests of the EIA in the SEA region. The observations recorded positive and negative ionospheric storms, spread F conditions, scintillation enhancement and inhibition, and total electron content variability. The ancillary information on the local magnetic field highlights the variety of ionospheric perturbations during the different storm phases. The combined use of ionospheric bottomside, topside, and integrated information points out how the storm affects the F layer altitude and the consequent enhancement/suppression of scintillations.Published12211–122331A. Geomagnetismo e Paleomagnetismo2A. Fisica dell'alta atmosfera1IT. Reti di monitoraggio e Osservazioni5IT. Osservazioni satellitariJCR Journalope

Galileo/GPS Mass Market Receivers: Tracking Algorithms Analysis and Performance

The scope of the work is the development and demonstration of the main GNSS algorithms currently used in GNSS mass-market receivers, with a particular interest toward Galileo signals. An exhaustive survey on existing commercial receivers signal processing techniques has been carried out and the most promising state-of-the-art algorithms for GPS signals have been analyzed and extended to E1B and E1C Galileo signals. The selected techniques have then been implemented in a software receiver, able to process raw GPS and Galileo samples. First the accuracy of the techniques is analyzed, both in terms of code delay and Doppler frequency estimates accuracy, proving the functionality of the multicorrelator processing unit. Second, some tests on robustness sensitivity have been carried out with a simulated LMS channel, proving the benefits of open loop strategies. Finally some tests on power consumption, representing one of the key drivers for the mobile consumer devices design, are carried out. The analysis of the results provides an early assessment on the suitability of these techniques, improving the attractiveness of Galileo to the massmarket community

Doppler Frequency Estimation in GNSS Receivers Based on Double FFT

This work presents an innovative Doppler frequency estimation technique, particularly suited for GNSS receivers operating in vehicular scenarios. Mass-market and commercial navigation devices are more and more exploited for in-car navigation and for vehicular applications based on positioning. However, the low computational burden affordable by such devices requires the implementation of low complexity algorithms, allowing real-time and on-demand processing. This is the case for instance of open-loop architectures and of MLE-based techniques, which estimate the frequency component of the GNSS signal through a discrete Fourier transform. A state-of-the-art of such methods is first carried out, outlining their benefits, regarding robustness and stability, and their limitations, mainly concerning the accuracy. Successively an innovative refinement technique is introduced, based on the computation of a frequency correction term. Further enhancements are then proposed to solve particular issues, as the estimation of the sign of the correction term and the impact of the initial frequency error. In particular, zero-forcing and a double FFT – which represent the main contribution of this work – are proposed to increase the accuracy without increasing the computational load. A complete analytical derivation and theoretical description is provided, along with a detailed performance assessment. Finally a performance comparison with existing techniques and with the Cramer-Rao lower bound for frequency estimation is given, confirming the excellent behavior of the proposed algorithm for the signal conditions and strengths typical of a vehicular scenario and in the presence of frequent interruptions

Doppler Frequency Estimation in GNSS Receivers Based on Double FFT

This work presents an innovative Doppler frequency estimation technique, particularly suited for GNSS receivers operating in vehicular scenarios. Mass-market and commercial navigation devices are more and more exploited for in-car navigation and for vehicular applications based on positioning. However, the low computational burden affordable by such devices requires the implementation of low complexity algorithms, allowing real-time and on-demand processing. This is the case for instance of open-loop architectures and of MLE-based techniques, which estimate the frequency component of the GNSS signal through a discrete Fourier transform. A state-of-the-art of such methods is first carried out, outlining their benefits, regarding robustness and stability, and their limitations, mainly concerning the accuracy. Successively an innovative refinement technique is introduced, based on the computation of a frequency correction term. Further enhancements are then proposed to solve particular issues, as the estimation of the sign of the correction term and the impact of the initial frequency error. In particular, zero-forcing and a double FFT – which represent the main contribution of this work – are proposed to increase the accuracy without increasing the computational load. A complete analytical derivation and theoretical description is provided, along with a detailed performance assessment. Finally a performance comparison with existing techniques and with the Cramer-Rao lower bound for frequency estimation is given, confirming the excellent behavior of the proposed algorithm for the signal conditions and strengths typical of a vehicular scenario and in the presence of frequent interruptions

Exploiting standardized metadata for GNSS SDR remote processing: A case study

The proliferation of ad-hoc, highly customized, GNSS Software-defined radio (SDR) receivers and raw data collection systems is opening new opportunities for research focused on the analysis of new approaches in GNSS data processing. Especially when looking at the post-processing of GNSS signals for monitoring purposes, the availability of big-sized raw digital samples allows for the application of different algorithms and processing strategies on the same data-set, thus enabling valuable comparisons and proper tuning of the algorithms themselves. A side effect is that the huge quantity of GNSS data collections, grabbed in disparate scenarios and environmental conditions and distributed all around the world, is making hard to share this data between different research labs, thus limiting the possibility of conducting extensive analysis of relevant ionospheric phenomena affecting the quality to the GNSS signal. In this paper, the authors propose a novel methodology to deal with the processing and the management of this huge amount of data, and guarantee interoperability of different data collection and data processing systems leveraging on the adoption of a new emerging and open standard for GNSS metadata which is now under standardization by the ION GNSS SDR Metadata Standardization Working Group