1,720,968 research outputs found
Analysis of multi-constellation GNSS PPP solutions under phase scintillations at high latitudes
Full text linkIn 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
Positioning exploiting GNSS raw measurements
No full textPosition and navigation are key advancements of smartphones technology. Nowadays, any smartphone is equipped with a GNSS chip, providing the device position and time. On one side, the improvements in electronics and communications boosted the design of smaller, cheaper and power saving GNSS chips, which are now multi-constellation and multi-frequency and outperform dedicated personal positioning devices. On the other side, the availability of accurate position and time enabled the development of services and location-based applications making the smartphone a professional positioning instrument. Starting from 2016, Android smartphones provide a set of raw GNSS measurements, inaddition to the user position, which open the way to more advanced and customizable positioning algorithms. This chapter first gives an overview on GNSS, then introduces the use of GNSS in smartphones, including the latest developments. Finally, examples of positioning performances, obtained exploiting raw GNSS measurements, are reported
An overview on Global Positioning Techniques for Harsh Environments
Full text linkAbstract This chapter presents strategies and techniques used to increase the sensitivity of global navigation satellite system (GNSS) receivers in order to make them usable in harsh environments, such as urban canyons, light indoor scenarios, deep forests, or space. It discusses the assistance that can be provided to the GNSS receiver through communication channels to ease the acquisition and tracking processes. Assisted GNSS is a consolidated standard, but other kinds of assistance and signal processing techniques can improve the ability of the receiver to process the signal at a low signal-to-noise ratio. The chapter introduces the common approaches to increase the sensitivity at the acquisition stage, discussing the impact on the accuracy of the delay and Doppler shift estimation, and the intrinsic limitation to coherent and noncoherent integration time extension. Techniques to increase robustness to low signal-to-noise ratio scenarios are presented, considering the structure of new and modernized GNSS signals
Definition and Testing of a Satellite-to-User Ranging and Communication Signal for a Martian Navigation System
No full textThe increasing interest towards scientific and exploration missions targeting Mars is coupled to the need of developing a positioning and communication infrastructure capable of offering services to end users. In this framework, Argotec is working on a constellation of small satellites, focusing in particular on the key technologies enabling such autonomous telecommunication capabilities. This study defines an improved concept for a simultaneous communication and ranging satellite-to-user signal. The novel approach is based on Unbalanced-Quadrature Phase Shift Keying (U-QPSK) modulation and one-way Pseudo-Noise (PN) ranging, adapting the existing standards to the peculiar scenario of the mission with the purpose of achieving a higher level of autonomy, while reducing the mission-related cost, weight, and bandwidth utilization. The identified solution was implemented and assessed as part of a technology demonstrator in a laboratory environment, running Digital Signal Processing (DSP) algorithms on Software Defined Radio (SDR) devices, thus emulating the behavior of the satellites and the final users, reaching a Technology Readiness Level (TRL) 4. The demonstrator validates the ability of the system to generate and transmit the navigation and communication signal modulated on the same channel using Code-Division Multiple Access (CDMA), and to correctly receive and process it to extract the ranging observables and to demodulate the communication data. This concept allows to achieve communication rates up to 64 kbps, and to determine estimates of range within few meters and of velocity within millimeters per second, thus fulfilling and exceeding the stringent requirements of future Martian missions by one order of magnitude
A machine learning approach to GNSS scintillation detection: automatic soft inspection of the events
No full textClassical approaches for the automatic detection of ionospheric scintillation events in Global Navigation Satellite System (GNSS) receivers are based on the observation of indices (e.g. S4) that are obtained by processing parameters assessed at the signal processing stages of the receiver. Such values are the result of algorithms that imply specific processing choices (such as detrending, averaging and threshold operations) which influence the final performance of the detection. To reach good levels of accuracy and generalization for the identification and classification of the physical phenomenon, these approaches may require an additional human effort to refine the detection results by means of a manual inspection of the events, which is expensive and time consuming. This paper proposes a new methodology for the detection of ionospheric scintillation events based on Machine Learning techniques applied to GNSS data. This method, based on Decision Trees algorithms, aims at overcoming the limitation of the classical approaches by identifying scintillation events “as if” done by a human operator through visual inspection. This approach is automatic, unbound from traditional scintillation indices and features improved detection, false alarm, and missed detection rates when compared to standard methods
Time-Transfer and Clock-Synchronization Technique for Microsatellites in the Lunar Region
Full text linkThe growing number of scientific and commercial missions to the Moon surface poses the need for a dedicated communication and navigation infrastructure. A precise Positioning, Navigation and Timing (PNT) service is a key technology to allow lunar assets to determine their position and velocity, to plan and execute maneuvers and to maintain time. Argotec is working on ANDROMEDA, an end-to-end Communication and Navigation service for users on the Moon surface and in Low Lunar Orbit, based on a constellation of 24 microsatellites operating in high-elliptical frozen orbits around the Moon. To support missions with Communication and Navigation capabilities, an accurate on-board frequency reference and a time-transfer technique are crucial technologies. This paper presents a trade-off analysis of different time-transfer techniques, including existing GNSS, Two-Way Pseudo-Noise ranging, Network Time Protocol, Two Way Satellite Time and Frequency Transfer and Optical links. Furthermore, an additional investigation on crucial constraints on microsatellites' design is performed with the goal of choosing the most suitable time-transfer technique and frequency reference for a microsatellite platform. A Two-Way coherent time transfer technique compatible with Consultive Committee for Space Data System (CCSDS) standards is studied and proposed. Special attention is given to synchronization accuracy, which is one of the most critical requirements for the navigation service. The choice of the technique has been supported by a trade-off analysis on the frequency reference to be adopted on board. Parameters such as Size, Weight and Power consumption (SWAP) have been taken into account in this phase, as well as costs and ground effort, paying particular attention to low-SWAP solutions. Finally, an error budget assessment is carried out considering free space propagation losses, relativistic effects, ephemeris errors, synchronization errors, and Earth's atmosphere contribution such as ionospheric and tropospheric delay
Adaptive Phase Detrending for GNSS Scintillation Detection: A Case Study Over Antarctica
Full text linkWe 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
Full text linkWe 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
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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