Performance of Receiver Autonomous Integrity Monitoring (RAIM) for Maritime Operations

The use of GNSS in the context of maritime applications has evolved during the past. The International Maritime Organization (IMO) has defined and published requirements for those applications. Comparing the requirements on the one hand specified by ICAO and on the other hand by IMO, significant differences get obvious. A major focus is on the evaluation of the performance of the integrity algorithms. Also concept drivers are discussed

GNSS Reliability Testing in Signal-Degraded Scenario

Multiconstellation satellite navigation is critical in signal-degraded environments where signals are strongly corrupted. In this case, the use of a single GNSS system does not guarantee an accurate and continuous positioning. A possible approach to solve this problem is the use of multiconstellation receivers that provide additional measurements and allows robust reliability testing; in this work, a GPS/GLONASS combination is considered. In urban scenario, a modification of the classical RAIM technique is necessary taking into account frequent multiple blunders. The FDE schemes analysed are the "Observation Subset Testing," "Forward-Backward Method," and "Danish Method"; they are obtained by combining different basic statistical tests. The considered FDE methods are modified to optimize their behaviour in urban scenario. Specifically a preliminary check is implemented to screen out bad geometries. Moreover, a large blunder could cause multiple test failures; hence, a separability index is implemented to avoid the incorrect exclusion of blunder-free measurements. Testing the RAIM algorithms of GPS/GLONASS combination to verify the benefits relative to GPS only case is a main target of this work too. The performance of these methods is compared in terms of RMS and maximum error for the horizontal and vertical components of position and velocity

Processing and integrity of DC/DF GBAS for CAT II/III operations

In Civil Aviation domain, to cope with the increasing traffic demand, research activities are pointed toward the optimization of the airspace capacity. Researches are thus ongoing on all Civil Aviation areas: Communication, Navigation, Surveillance (CNS) and Air Traffic Management (ATM). Focusing on the navigation aspect, the goals are expected to be met by improving performances of the existing services through the developments of new NAVigation AIDS (NAVAIDS) and the definition of new procedures based on these new systems. The Global Navigation Satellite System (GNSS) is recognized as a key technology in providing accurate navigation services with a worldwide coverage. The GNSS concept was defined by the International Civil Aviation Organization (ICAO). A symbol of its importance, in civil aviation, can be observed in the avionics of new civil aviation aircraft since a majority of them are now equipped with GNSS receivers. The GNSS concept includes the provision of an integrity monitoring function by an augmentation system in addition to the core constellations. This is needed to meet all the required performance metrics of accuracy, integrity, continuity and availability which cannot be met by the stand-alone constellations such as GPS. Three augmentation systems have been developed within civil aviation: the GBAS (Ground Based Augmentation System), the SBAS (Satellite Based Augmentation System) and the ABAS (Aircraft Based Augmentation System). GBAS, in particular, is currently standardized to provide precision approach navigation services down to Category I (CAT I) using GPS or Glonass constellations and L1 band signals. This service is known as GBAS Approach Service Type-C (GAST-C). In order to extend this concept down to CAT II/III service, research activities is ongoing to define the new service called a GAST-D. Among other challenges, the monitoring of the ionospheric threat is the area where the integrity requirement is not met. Thanks to the deployment of new constellations, Galileo and Beidou, and the modernization process of the existing ones, GPS and Glonass, the future of GNSS is envisaged to be Multi-Constellation (MC) and Multi-frequency (MF). In Europe, research activities have been focused on a Dual-Constellation (DC) GNSS and DC GBAS services based on GPS and Galileo constellations. Moreover, to overcome the problems experienced by Single-Frequency (SF) GBAS due to ionosphere anomalies, the use of two frequencies (Dual Frequency, DF) has been selected as a mean to improve ionosphere anomalies detection and to mitigate ionosphere residual errors. Advantages in using a DC/DF GBAS (GAST-F) system are, however, not only related to the integrity monitoring performance improvement. Benefits, brought by DC and DF, are also related to •the robustness of the entire system against unintentional interference thanks to the use of measurements in two protected frequency bands, •the robustness against a constellation failure, •the accuracy improvement by using new signals with improved performance, and more satellites. However, the use of new signals and a new constellation, does not bring only benefits. It also raises a series of challenges that have to be solved to fully benefit from the new concept. In this thesis, some challenges, related to DC/DF GBAS, have been investigated. One of them, rising from the use of new GNSS signals, is to determine the impact of error sources that are uncorrelated between the ground station and the aircraft and that induce an error on the estimated position. Using two frequencies, there is the possibility to form measurement combinations like Divergence-free (D-free) and Ionosphere-free (I-free) for which the errors impact has to be analyzed. In this thesis, the impact of the uncorrelated errors (noise and multipath as main sources) on ground measurements is analyzed. The aim is to compare the derived performances with the curve proposed in (RTCA,Inc DO-253C, 2008) for th

Encoding AIS Binary Messages in XML Format for Providing Hydrographic-related Information

A specification is proposed to enable hydrographic and maritime safety agencies to encode AIS messages using Extensible Markup Language (XML). It specifies the order, length, and type of fields contained in ITU-R.M.1371-1. A XML schema validates the message definitions, and a XSLT style sheet produces reference documentation in \u27html\u27 format. AIS binary messages in XML are an effective means to communicate dynamic and real-time port/waterway information. For example, tidal information can be continuously broadcast to maritime users and applied to a tide-aware ENC. The XML format aligns with the type of data encapsulation planned for the IHO Geospatial Standard for Digital Hydrographic Data (S-100)

On the use of a signal quality index applying at tracking stage level to assist the RAIM system of a GNSS receiver

In this work, a novel signal processing method is proposed to assist the Receiver Autonomous Integrity Monitoring (RAIM) module used in a receiver of Global Navigation Satellite Systems (GNSS) to improve the integrity of the estimated position. The proposed technique represents an evolution of the Multipath Distance Detector (MPDD), thanks to the introduction of a Signal Quality Index (SQI), which is both a metric able to evaluate the goodness of the signal, and a parameter used to improve the performance of the RAIM modules. Simulation results show the effectiveness of the proposed method

An Integrity Framework for Image-Based Navigation Systems

This work first examines fundamental differences between measurement models established for GPS and those of proposed image-based navigation systems. In contrast to single value per satellite GPS pseudorange measurements, image measurements are inherently angle-based and represent pixel coordinate pairs for each mapped target. Thus, in the image-based case, special consideration must be given to the units of the transformations between the states and measurements, and also to the fact that multiple rows of the observation matrix relate to particular error states. An algorithm is developed to instantiate a framework for image-based integrity analogous to that of GPS RAIM. The algorithm is applied cases where the navigation system is estimating position only and then extended to cases where both position and attitude estimation is required. Detailed analysis demonstrates the impact of angular error on a single pixel pair measurement and comparisons from both estimation scenario results show that, from an integrity perspective, there is significant benefit in having known attitude information. Additional work demonstrates the impact of pixel pair measurement relative geometries on system integrity, showing potential improvement in image-based integrity through screening and adding measurements, when available, to the navigation system solution

High accuracy tightly-coupled integrity monitoring algorithm for map-matching

A map-matching algorithm employs data from Global Positioning System (GPS), a Geographic Information System (GIS)-based road map and other sensors to first identify the correct link on which a vehicle travels and then to determine the physical location of the vehicle on the link. Due to uncertainties associated with the raw measurements from GPS/other sensors, the road map and the related methods, it is essential to monitor the integrity of map-matching results, especially for safety and mission-critical intelligent transport systems such as positioning and navigation of autonomous and semi-autonomous vehicles. Current integrity methods for map-matching are inadequate and unreliable as they fail to satisfy the integrity requirement due mainly to incorrect treatment of all the related uncertainties simultaneously. The aim of this paper is therefore to develop a new tightly-coupled integrity monitoring method for map-matching by properly treating the uncertainties from all sources concurrently. In this method, the raw measurements from GPS, low-cost Dead-Reckoning (DR) sensors and Digital Elevation Model (DEM) are first integrated using an extended Kalman Filter to continuously obtain better position fixes. A weight-based topological map-matching process is then developed to map-match position fixes onto the road map. The accuracy of the map-matching process is enhanced by employing a range of network features such as grade separation, traffic flow directions and the geometry of road link. The Receiver Autonomous Integrity Monitoring (RAIM) technique, which has been successfully applied to monitor the integrity of aircraft navigation, is modified and enhanced so as to apply it to monitor the quality of map-matching. In the enhanced RAIM method, two modifications are made: (1) a variable false alarm rate (as opposed to a constant false alarm rate) is considered to improve the fault detection performance in selecting the links, especially near junctions. (2) a sigma inflation for a non-Gaussian distribution of measurement noises is applied for the purpose of satisfying the integrity risk requirement. The implementation and validation of the enhanced RAIM method is accomplished by utilising the required navigation performance parameters (in terms of accuracy, integrity and availability) of safety and mission-critical intelligent transport systems. The required data were collected from Nottingham and central London. In terms of map-matching, the results suggest that the developed map-matching method is capable of identifying at least 97.7% of the links correctly in the case of frequent GPS outages. In terms of integrity, the enhanced RAIM method provides better the fault detection performance relative to the traditional RAIM

Mass-Market Receiver for Static Positioning: Tests and Statistical Analyses

Nowadays, there are several low cost GPS receivers able to provide both pseudorange and carrier phase measurements in the L1band, that allow to have good realtime performances in outdoor condition. The present paper describes a set of dedicated tests in order to evaluate the positioning accuracy in static conditions. The quality of the pseudorange and the carrier phase measurements let hope for interesting results. The use of such kind of receiver could be extended to a large number of professional applications, like engineering fields: survey, georeferencing, monitoring, cadastral mapping and cadastral road. In this work, the receivers performance is verified considering a single frequency solution trying to fix the phase ambiguity, when possible. Different solutions are defined: code, float and fix solutions. In order to solve the phase ambiguities different methods are considered. Each test performed is statistically analyzed, highlighting the effects of different factors on precision and accurac

GNSS Integrity Monitoring assisted by Signal Processing techniques in Harsh Environments

The Global Navigation Satellite Systems (GNSS) applications are growing and more pervasive in the modern society. The presence of multi-constellation GNSS receivers able to use signals coming from different systems like the american Global Positioning System (GPS), the european Galileo, the Chinese Beidou and the russian GLONASS, permits to have more accuracy in position solution. All the receivers provide always more reliable solution but it is important to monitor the possible presence of problems in the position computation. These problems could be caused by the presence of impairments given by unintentional sources like multipath generated by the environment or intentional sources like spoofing attacks. In this thesis we focus on design algorithms at signal processing level used to assist Integrity operations in terms of Fault Detection and Exclusion (FDE). These are standalone algorithms all implemented in a software receiver without using external information. The first step was the creation of a detector for correlation distortion due to the multipath with his limitations. Once the detection is performed a quality index for the signal is computed and a decision about the exclusion of a specific Satellite Vehicle (SV) is taken. The exclusion could be not feasible so an alternative approach could be the inflation of the variance of the error models used in the position computation. The quality signal can be even used for spoofinng applications and a novel mitigation technique is developed and presented. In addition, the mitigation of the multipath can be reached at pseudoranges level by using new method to compute the position solution. The main contributions of this thesis are: the development of a multipath, or more in general, impairments detector at signal processing level; the creation of an index to measure the quality of a signal based on the detector’s output; the description of a novel signal processing method for detection and mitigation of spoofing effects, based on the use of linear regression algorithms; An alternative method to compute the Position Velocity and Time (PVT) solution by using different well known algorithms in order to mitigate the effects of the multipath on the position domain