1,721,021 research outputs found
Mitigation of leverage observation effects in GNSS robust positioning
No full textNowadays GNSSs are the most commonly used systems for localization; they are able to provide user absolute position with metric accuracy in benign environment, i.e. in scenarios without significant obstacles surrounding the user. On the other hand, in harsh scenarios GNSS performance are degraded, owing to the shortage of available measurements and/or to the presence of blunders among them. The blunder issue is usually addressed through RAIM techniques or robust estimation; the latter approach demonstrates often better performance, but suffers anyway the cases of multiple blunders and low redundancy. M-estimators, a particular class of robust estimators, are based on the minimization of functions of least squares residuals. A possible way to strengthen a M-estimator is to take into account for leverage observations, defined as measurements with high potential to affect estimation results. In this work, the Huber M-estimator is adapted to include information about leverage observations and is used to process GPS measurements, collected in harsh environment. The obtained results are very promising, with position errors reduction even beyond 50% with respect to classical Huber method
Performance assessment of aided Global Navigation Satellite System for land navigation
No full textLand navigation includes the methods to determine the time varying position and velocity of a moving object on Earth surface using suitable measurements; it is typically performed in signal-degraded environments where GPS signals are blocked or degraded; hence GPS-only cannot guarantee an accurate and continuous positioning. The multi-constellation approach is a possible way to fill this gap. In this work GPS/GLONASS systems are combined and single point algorithm performance is assessed for different configurations in urban scenario. GLONASS is nearly fully operational and its inclusion guarantees satellite availability improvement, but the GPS/GLONASS multi-constellation use involves the addition, as further unknown, of the intersystem time-scale offset. The considered estimation techniques are Least Squares and Kalman filter. The first method uses only the measurement model, with the drawback of solution unavailability during GNSS outages. The last uses, in addition to a measurement model, a process model allowing the estimation of the unknowns in case of GNSS outage. To improve both methods performances aiding are considered on height and intersystem time scale offset. The main purposes of this work are the performance assessment of a multi-constellation system relative to GPS-only, adopting the aforesaid estimators, and the benefit evaluation of the aiding
Treatment of acute gastrointestinal bleeding of pharmacological origin. A randomized clinical trial of octreotide i.v. vs omeprazole i.v. .
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Smartphone GNSS Performance in an Urban Scenario with RAIM Application
No full textIn an urban scenario, GNSS performance is strongly influenced by gross errors in the measurements, usually related to multipath and non-line-of-sight phenomena. The use of RAIM algorithms is a common approach to solve this issue. A significant amount of the existing GNSS receivers is currently mounted on smart devices, above all, smartphones. A typical drawback of these devices is the unavailability of raw measurements, which does not allow fully exploiting the GNSS potential; in particular, this feature limits the use of RAIM algorithms. Since 2016, for few smart devices, it has been finally possible to access GNSS raw measurements, allowing the implementation of specific algorithms and enabling new services. The Xiaomi Mi 8 is equipped with the Broadcom BCM47755 receiver, able to provide dual-frequency raw measurements from quad-constellation GPS, Glonass, Galileo, BeiDou. In this work, the performance in an urban area of the Xiaomi Mi8 GNSS was analyzed. An important issue of smartphone GNSS is related to the antenna, which is not able to protect from the multipath phenomenon; this issue has a large probability to emerge in hostile environments like urban areas. As a term of comparison, the high-sensitivity receiver NVS NV08C-CSM, connected to a patch antenna, was used. In particular, the considered receivers were placed in the same location, and their positions were estimated in single point positioning, applying a classical RAIM algorithm. An error analysis was carried out, and the obtained results demonstrated the effectiveness of RAIM when applied to Xiaomi Mi8 GNSS measurements
PANG-NAV: a tool for processing GNSS measurements in SPP, including RAIM functionality
No full textGlobal Navigation Satellite Systems (GNSSs) are theoretically able to provide accurate, three-dimensional, and continuous positioning to an unlimited number of users. An important shortcoming of GNSS is the lack of integrity, defined as the ability of a system to provide timely warnings in case of malfunction; this problem is especially felt in safety-of-life applications such as aviation. A common way to fill this gap is the use of Receiver Autonomous Integrity Monitoring (RAIM) techniques, which are able to provide integrity information by analyzing redundant measurements. A possible RAIM functionality is the ability to identify, and so discard, anomalous measurements; this functionality has made RAIM very useful also in case of severe signal degradation, such as in urban or dense vegetation areas, where blunders are common. PANG-NAV is a tool, developed by the PArthenope Navigation Group, able to process GNSS measurements (from RINEX files) in order to obtain a position solution. The core of PANG-NAV is the single point positioning (SPP) technique, including a RAIM functionality. A multi-constellation solution, with GPS and Galileo, can be provided. Both static processing and kinematic processing are possible, and in cases where ground truth is available, error analysis can be carried out
Endoscopic resection of benign very low risk gastric gastrointestinal stromal tumor. Is it enough?
No full textDiagnosing and treating sphincter of oddi dysfunction: a critical literature review and re-evaluation
No full textSphincter of Oddi dysfunction is a pathologic syndrome, without considering etiology, physiopathology, or anatomic aspects of the condition. The clinical manifestations of the syndrome may be a consequence of an "organic stenosis" of the tract or a consequence of "abnormal motility" of the sphincter. Until some years ago, the gold standard technique for studying and treating this pathologic condition was endoscopic retrograde cholangiopancreatography. Two criteria for defining patients in the Milwaukee classification are related to this procedure. The Milwaukee classification was introduced to use clinical and radiologic criteria to define patients with Sphincter of Oddi dysfunction to choose the best treatment. Subsequently, great emphasis has been placed on manometry of the sphincter performed by endoscopic cannulation. The enormous increase of cholecystectomies by means of laparoscopic technique has increased the number of patients who return to their reference-surgeon with a post-cholecystectomy pain and possible Sphincter of Oddi dysfunction. The aim of this paper is to review the literature and to evaluate an up-to-date flow chart for diagnosing and treating the syndrome by using alternative diagnostic procedures that are less invasive than endoscopic retrograde cholangiopancreatography
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