GNSS augmentation

About: GNSS augmentation is a research topic. Over the lifetime, 2478 publications have been published within this topic receiving 28513 citations. The topic is also known as: SBAS & Satellite Based Augmentation System.

Advanced SBAS-DInSAR Technique for Controlling Large Civil Infrastructures: An Application to the Genzano di Lucania Dam.

Abstract: Monitoring surface deformation on dams is commonly carried out by in situ geodetic surveying, which is time consuming and characterized by some limitations in space coverage and frequency. More recently microwave satellite-based technologies, such as advanced-DInSAR (Differential Synthetic Aperture Radar Interferometry), have allowed the integration and improvement of the observation capabilities of ground-based methods thanks to their effectiveness in collecting displacement measurements on many non-destructive control points, corresponding to radar reflecting targets. The availability of such a large number of points of measurement, which are distributed along the whole structure and are characterized by millimetric accuracy on displacement rates, can be profitably adopted for the calibration of numerical models. These models are implemented to simulate the structural behaviour of a dam under conditions of stress thus improving the ability to maintain safety standards. In this work, after having analysed how advanced DInSAR can effectively enhance the results from traditional monitoring systems that provide comparable accuracy measurements on a limited number of points, an FEM model of the Genzano di Lucania earth dam is developed and calibrated. This work is concentrated on the advanced DInSAR technique referred to as Small BAseline Subset (SBAS) approach, benefiting from its capability to generate deformation time series at full spatial resolution and from multi-sensor SAR data, to measure the vertical consolidation displacement of the Genzano di Lucania earth dam.

IMU/Vision/Lidar integrated navigation system in GNSS denied environments

Abstract: This paper aims to develop IMU/Vision/Lidar integrated navigation system which can provide accurate relative navigation information in GNSS denied environments. The developed integrated navigation system consists of an IMU, a vision sensor and a Lidar sensor. In order to overcome limitation of low accuracy of MEMS inertial navigation solution when GNSS signal cannot be used, 2-dimensional optical flow vector from vision sensor and Lidar range information between the system and ground surface are used. Two aided sensor data are complementarily employed in the system integration. Basically, by using the optical flow vector and relative height between vision sensor and ground surface, accuracy of the horizontal position estimates is improved. At the same time, by using compensated Lidar measurement, accurate vertical position can be estimated. The overall integrated navigation filter is constructed based on Extended Kalman Filter approach. Feasibility of the navigation filter is proven via simulation results. Finally, with real hardware system, an outdoor test is carried out for analyzing performance of the proposed integrated navigation system.

Global navigation satellite system (GNSS) receivers based on satellite signal channel impulse response

Abstract: A Global Navigation Satellite System (GNSS) receiver and associated method for the reception and processing of GNSS signals. The GNSS receiver includes an antenna and an analog front-end to intercept the incoming radio-frequency signal and to convert it to an appropriate intermediate frequency for digital sampling. A baseband signal processor is organized into functionally identical channels, each dynamically assigned to a different satellite visible. The baseband signal processor processes the signal samples to generate the satellite signal channel impulse response for a number of Doppler frequency shifts. This results in a two-dimensional delay-Doppler map of satellite signal responses from which the baseband signal processor extracts the code time and carrier phase and frequency parameters as well as navigation data for timing, positioning, and environment mapping in the data processor.

Peer-to-Peer Cooperative Positioning Part I: GNSS Aided Acquisition

Abstract: To improve the performance of GNSS receivers in hostile environments, we consider a Cooperative Positioning approach, where receivers exchange data and information with their neighbors. We focus on unstructured P2P networks, without a control or fusion center. We show that a significant reduction of the acquisition time can indeed by achieved when GNSS aiding quantities like Doppler, satellite Carrierto-Noise ratio and secondary code delay are provided by some aiding peers. The approach is clearly similar to that of Assisted GNSS, but does not require a fixed infrastructure and may better take into account the local environment. Since, in the near future, multi-standard devices will be more and more inter-connected, GNSS Cooperative Positioning may soon become an alternative or a complement to fixed augmentation systems

Galileo-GPS RAIM for Vertical Guidance

Abstract: In the next ten years the number of pseudorange sources and their quality is expected to increase dramatically: The United States is going to add two new civil frequencies (L5 and L2C) in the modernized GPS, and the European Union is planning to launch Galileo, which is planned to be fully operative before 2015, also with multiple frequencies. By combining two frequencies, users will be able to remove the ionospheric delay which is currently the largest error, thus reducing nominal error bounds by more than 50%. This reduction in nominal error bounds together with the large number of satellites is not only going to increase the accuracy of the positioning, but more importantly, it is going to increase the robustness against satellite failures (or other range errors), even without augmentation (e.g., Inertial Reference Unit (IRU), baro-altimeter). Preliminary studies suggest that, using Receiver Autonomous Integrity Monitoring (RAIM), it might be possible to provide a 50m Vertical Alert Limit (VAL) worldwide, with a bound on the maximum error, even in the event of one satellite failure, one constellation failure or a multiple satellite failure. The purpose of this work is to investigate which VALs could be achieved with RAIM under conservative failure assumptions. This paper also summarizes previous work concerning RAIM algorithms and compares their results against a common standard. First, in light of the experience with the Wide Area Augmentation System (WAAS), a threat space for a dual frequency Galileo-GPS constellation is defined. This threat space is necessary in order to achieve a low VAL, as it does not suffice to assume single failures only. Second, RAIM methodologies adapted to the threat space are compared, and the most practical one was found to be a multiple hypothesis approach. Finally, the performance results of the chosen RAIM scheme with a Galileo-GPS dual frequency constellation are presented. It was found that an unaided Galileo-GPS constellation yielded Vertical Protection Level (VPL) values under 20m for the combined dual system. This optimistic conclusion indicates that it will likely be possible to provide vertical guidance to aircraft without the need for any additional augmentation when the future GPS and Galileo constellations are operational. INTRODUCTION This work aims to evaluate the performance of an unaided Galileo-GPS constellation from a vertical integrity standpoint (e.g. for aviation precision approach). A multitude of algorithms or methods were proposed for RAIM over time, both for GPS alone and more recently for combined Galileo-GPS constellations. However, the presented results were hard to compare between the different papers due to the lack of a standardized threat model and also the different assumptions made by each author. This paper seeks instead to establish a Satellite Failure Threat Space that is general enough to allow testing different algorithms and assumptions against a standard model. To accommodate the different assumptions existing in the literature, parametric studies are conducted on factors external to the integrity monitor, such as the satellite failure probability and the expected User Range Accuracy (URA). Three of the existing algorithms, called Least Squares (LS), Maximum Solution Separation and Multiple Hypothesis Solution Separation (MHSS) were implemented as part of the current study. The resulting VPL values from using these different algorithms are then compared and the origin of the inherent differences is discussed. The most practical algorithm for use with the dual constellation will be adopted. Based on the final results with this algorithm, a conclusion is drawn on the capabilities of the unaided combined constellation and direction for future work is laid out. The current work evaluates what is the maximal threat space against which it is possible to offer protection, and does not involve Fault Detection and Elimination (FDE). SATELLITE FAILURE THREAT SPACE The threat space is a consistent and complete set of assumptions about the environment in which a RAIM algorithm is applied. A standardized threat space can be regarded as a general test case against which each individual algorithm may be applied. It has to be universal enough such that it can constitute a frame in which to apply the particular set of assumptions of each particular algorithm, and it should include all considered threats. The threat space is in fact the sample space of all failure modes, including the “no failure” case or nominal conditions. A failure mode is the outcome of each of the ION NTM 2006, 18-20 January 2006, Monterey, CA 1 navigation beacons (i.e. satellites or space vehicles (SVs)) being in a “healthy” or “failed” binary state, with a certain probability. Nominal conditions contain the expected modes of behavior from the satellites, propagation medium, and user receiver with its surrounding environment. Under these conditions, the users achieve their expected level of performance. The failed state is an anomalous condition that can threaten the accuracy and integrity of the system when undetected, and the continuity and availability when it is detected. Such failures should be infrequent and short in duration. The threat model places limits on the extent and behaviors of fault modes. The threat space needs to be all-inclusive, such that all feared events are taken into account, including events introduced by the algorithm itself. Each method can be different with respect to its vulnerability to various fault modes. For the purposes of this study, dual frequency full GPS and Galileo constellations will be assumed. Therefore, ionospheric threats will not be considered (In future studies, second order TEC delays and scintillation will be investigated). The clock and ephemeris errors are assumed to be normally distributed N(0, σURA) under nominal, healthy satellite conditions. The troposphere model will match the one in the WAAS MOPS and is assumed to be bounded by the confidence level provided in [7]. Receiver noise and multipath are also bounded by the provided iono-free sigma term. Note that, like GBAS and SBAS, receiver failure and excessive multipath terms are not explicitly put into the threat space. However, RAIM offers some protection against such fault modes where no ground augmentation can. Previous literature seems to be much in agreement on a theoretical way to describe errors at the user. For that reason, the latest model in [4] was considered appropriate. The position error variance for satellite i is described there by: σi 2 = σURA 2 + σi,tropo 2 + σi,iono-free 2 + σL1L5 . Multiple independent faults will be considered in the combined constellation. This algorithm can be expanded to handle different probabilities of multiple failures, but the independent fault model was adopted in light of possible satellite clock failures as main threat sources. Additionally, separate constellation failure modes will be considered for the case where correlated faults exist across either the GPS or Galileo constellations but not both. Furthermore, a particular user can only receive information from a subset of the SVs, specifically the ones at an elevation above a predefined mask angle. Thus, it is practical to consider only the satellites in view from the location of each specific user. The a priori probability of satellite failure will be discussed and an analysis will be conducted for the relevant number of failures. The satellite failure is defined here as the behavior of a SV when its corresponding range error cannot be overbounded with a Gaussian N(0, σURA). This seems to be a more natural way to describe a failure for this algorithm. Although it is a good binary discriminator, the customary method of setting a failure threshold for pseudorange error does not help identify systematic errors when they are just below the threshold. The practicality of detecting failures according to this new definition needs nonetheless some further scrutiny in future studies. The total error budget for providing Hazardous Misleading Information (HMI) is strictly limited for the case of aviation precision approaches, such that the maximum allowable integrity risk is 10/approach. This budget needs to be divided between all the possible failure modes, and the resulting VPL will be very sensitive on the allocation of this integrity budget. Normally, in applying any RAIM algorithm, multiple failures are neglected, for modes which are less likely than a certain threshold. The reason why certain improbable failure modes need to be neglected is that the entire threat space is extremely large and impractical to compute. Therefore, it is imperative to limit the computation of the position error only to the most dangerous events from an integrity point-of-view. In the same time, one can afford to conservatively assume the worst case scenario (i.e. failure generating HMI) for the remaining threats, as they have a small enough probabilistic impact on the total error or the total integrity anyway.