On Fault Detection and Exclusion in Snapshot and Recursive Positioning Algorithms for Maritime Applications

Resilient provision of Position, Navigation and Timing (PNT) data can be considered as a key element of the e-Navigation strategy developed by the International Maritime Organization (IMO). An indication of reliability has been identified as a high level user need with respect to PNT data to be supplied by electronic navigation means. The paper concentrates on the Fault Detection and Exclusion (FDE) component of the Integrity Monitoring (IM) for navigation systems based both on pure GNSS (Global Navigation Satellite Systems) as well as on hybrid GNSS/inertial measurements. Here a PNT-data processing Unit will be responsible for both the integration of data provided by all available on-board sensors as well as for the IM functionality. The IM mechanism can be seen as an instantaneous decision criterion for using or not using the system and, therefore, constitutes a key component within a process of provision of reliable navigational data in future navigation systems. The performance of the FDE functionality is demonstrated for a pure GNSS-based snapshot weighted iterative least-square (WLS) solution, a GNSS-based Extended Kalman Filter (EKF) as well as for a classical error-state tightly-coupled EKF for the hybrid GNSS/inertial system. Pure GNSS approaches are evaluated by combining true measurement data collected in port operation scenario with artificially induced measurement faults, while for the hybrid navigation system the measurement data in an open sea scenario with native GNSS measurement faults have been employed. The work confirms the general superiority of the recursive Bayesian scheme with FDE over the snapshot algorithms in terms of fault detection performance even for the case of GNSS-only navigation. Finally, the work demonstrates a clear improvement of the FDE schemes over non-FDE approaches when the FDE functionality is implemented within a hybrid integrated navigation system

Entwicklung einer Low-Cost-PNT Unit für maritime Anwendungen, basierend auf MEMS-Inertialsensoren

Although the GPS/GNSS had become the primary source for Position, Navigation and Timing (PNT) information in maritime applications, the ultimate performance of the system can strongly degrade due to space weather events, deliberate interference and overall system failures. Within the presented work the development of an affordable integrated PNT unit for future on-board integrated system is presented. The system serves the task to collect and integrate the data from individual sensors in order to deliver the PNT information with a specified performance according to the requirements of the e-Navigation initiative proposed by the International Maritime Organization (IMO). The paper discusses an ongoing activity of replacing an expensive FOG inertial measurement unit with an affordable MEMS sensor system. Preliminary results of the system performance are presented for both static and dynamic scenarios using an Unscented Kalman filter with unit quaternions for the attitude parametrization

On PNT Integrity in Snapshot And Recursive Positioning Algorithms for Maritime Applications

Resilient provision of Position Navigation and Time (PNT) data is a strategic key element of the e-Navigation strategy, developed by the International Maritime Organization (IMO). The improvement and the indication of reliability have been identified as high level user need with respect to PNT data supplied by electronics means. IMO as developed a maritime PNT system concept aiming to improve the resilience and reliability of PNT data provision during berth-to-berth navigation. The maritime PNT System comprises several structural components, where Global Navigation Satellite Systems (GNSS), have become the primary component to produce position, velocity and time information for maritime applications. For a comprehensive onboard provision of PNT data as well as to compensate the vulnerability of GNSS, further onboard sensors are needed. The PNT system is responsible for the fusion of the data provided by all the available onboard sensors and data integrity monitoring functions. A unit composed by several sensors of different classes improves the resilience of the system. DLR has developed a prototype of an onboard PNT unit and several measurement campaigns have been performed. This paper concentrates on integrity monitoring (IM) for navigation systems based on sensor fusion. IM is a mechanism that protects the user from large position and velocity errors in the presence of failures or non-scheduled events in a timely fashion. It can be seen as an instantaneous decision criterion for using or not the system and therefore constitutes a key function for the safety of navigation. The IM includes the detection and exclusion functions, they are responsible for detecting the measurements errors (faults) and exclude them from the PNT data computation algorithm. This work presents a systematic analysis of Fault Detection and Exclusion (FDE) algorithms in representative single and multi-sensor. More specifically, a pure GNSS-based snapshot weighted iterative least-square (WLS) solution is compared to a classical error-state Extended Kalman Filter (EKF) for a combined GNSS/IMU system with Euler angles for attitude parameterization. The outlier detection functionality is implemented for both pseudorange and Doppler shift observations in order to ensure the integrity of the estimated position and velocity data. The work confirms the superiority of the recursive Bayesian scheme over a snapshot algorithm in terms of the outlier detection performance. This can be explained by the recursive structure of the estimator, where the dynamical model of the system provides the additional source of information, which increases the system’s redundancy and hence improves the performance of the FDE schemes

A Method for IMU/GNSS/Doppler Velocity Log Integration in Maritime Applications

Although the GNSS/GPS had become the primary source for Positioning, Navigation and Timing (PNT) information in maritime applications, the ultimate performance of the system can strongly degrade due to space weather events, deliberate interference, shadowing, multipath and overall system failures. Within the presented work the development of an affordable integrated PNT unit for future on-board integrated systems is presented, where the GNSS information is fused both with inertial and Doppler Velocity Log (DVL) measurements. Here redundant and complementary information from different sensors serves to improve the system performance and reduce the position drift when the GNSS signals are not available. The nonlinearity of this advanced fusion problem is addressed by employing Unscented Kalman Filter (UKF) with spherical point arrangement and further detailed analysis is presented in terms of the process and measurement models implemented. The results demonstrate that position drift can be significantly reduced by incorporating DVL measurements in IMU/GNSS system and that the proposed integrated navigation algorithm is feasible and efficient for GNSS outages of prolonged duration, where pure inertial GNSS outage bridging would be either inaccurate or would require too expensive IMUs

On the Performance of Inertial/GNSS/Doppler Velocity Log Integrated Navigation Systems for Marine Applications

Although the GNSS/GPS had become the primary source for Positioning, Navigation and Timing (PNT) information in maritime applications, the ultimate performance of the system can strongly degrade due to space weather events, deliberate interference, shadowing, multipath and overall system failures. Within the presented work the development of an affordable integrated PNT unit for future on-board integrated systems is presented, where the GNSS information is fused both with inertial and Doppler Velocity Log (DVL) measurements. Here redundant and complementary information from different sensors serves to improve the system performance and reduce the position drift when the GNSS signals are not available. The nonlinearity of this advanced fusion problem is addressed by employing different forms of Sigma-Points Kalman Filter (SPKF) and further detailed analysis is presented in terms of the process and measurement models implemented. The results demonstrate that position drift can be significantly reduced by incorporating DVL measurements in IMU/GNSS system and that the proposed integrated navigation algorithm is feasible and efficient for GNSS outages of prolonged duration, where pure inertial GNSS outage bridging would be either inaccurate or would require too expensive IMUs

Long term validation of PNT-Unit processing channels onboard a ferry vessel

Resilient provision of ships own position, navigation and timing (PNT) data has been recognized as a core element for safe and efficient vessel navigation. The DLR is proposing the PNT-Unit concept, where all available navigation sensors are used in a combined way. Based on this concept a real time capable prototype of such a system has been developed. The validation of the system with was however restricted to measurement campaigns with a duration of a few hours. In order to overcome this, we have installed the PNT-Unit prototype on the Stena Line ferry vessel Mecklenburg-Vorpommern which is regularly plying between Rostock and Trelleborg. This paper gives a first overview of the validation results of GNSS based processing channels during a 10 days’ time period in March 2015. This period includes some days, where significant ionospheric perturbations have been recognized

GNSS integrity assessment of an Integrated PNT-Unit in a signal degraded inland water environment

Within the framework of the project “Precise and Integer Localisation and Navigation in Rail and Inland water Traffic” (PiLoNav) an integrated Position, Navigation and Timing (PNT) Unit for inland waterways has been developed and implemented through the combination of Global Navigation Satellite Systems (GNSS) receivers and an inertial measurement unit (IMU), where the IMU is used for short-term stabilization and to assure continuous PNT solution in GNSS signal blockage areas. The aim of this integrated navigation system is to provide precise navigation capability with defined levels of accuracy, integrity and continuity. Integrity in GNSS stands for the evaluation of the trust that can be placed in the correctness of the information supplied by a navigation system with or without augmentations. This trust can be increased by using only non-faulty GNSS signals for positioning estimation. GNSS integrity monitoring should on one hand perform self-consistency checks on redundant measurements, and on the other hand, trigger timely and valid warnings to users when it must not be used for the intended operation. Therefore, the implementation of a Receiver Autonomous Integrity Monitoring (RAIM) algorithm as a part of the PNT-Unit assures GNSS integrity. RAIM was first introduced in the aviation sector for only using reliable satellites during safety critical landing approaches. The need of RAIM in the maritime sector emerges from the requirements for maritime radio navigation systems as specified by the International Maritime Organization (IMO). Due to the limited manoeuver space on inland waterways, the requirement on positioning accuracy and integrity is very demanding. This work summarizes the difficulties of a classical snapshot-RAIM algorithm for a non- augmented GPS-based navigation system in operational degraded signal environments. Such environments include inland waterways with bridges, locks and other natural and artificial obstacles that cause GPS signal blockage and multipath. As the RAIM algorithm is based on pseudorange redundancy, the number of visible satellites is crucial for the availability of integrity information. It will be shown that the environment described above often compromises RAIM availability and performance. First results, based on several hours of collected GPS-data during a measurement campaign which took place at the river Moselle in Koblenz in August 2012, are given

Multichannel based Integrated Navigation for Scalable Maritime Applications

The capability to provide automated, onboard Position, Navigation and Timing (PNT) data compliant with the accuracy, integrity, continuity and availability requirements in accordance to the different phases of vessel navigation is considered as one core element of the International Maritime Organization (IMO) e-Navigation strategy. This capability, known as resilience of the system, is fundamental for any system supporting safety of live critical maritime applications. In order to achieve the resilience, satellite based, shipside and landside components should be fused by an “integrated PNT system”. However, the heterogeneity of the ship sensors together with the long life-cycles of vessels constitutes a practical challenge for the design of an onboard PNT module. Therefore the German Aerospace Center (DLR) promotes an open and scalable architecture for the shipborne “PNT Module”, which serves as a front-end between an integrated PNT system and shipside applications like Integrated Navigation Systems (INS), the Automatic Identification System (AIS) and the Electronic Chart Display and Information System (ECDIS). As the core component of a PNT Module, the PNT Unit processes and integrates the data of radio navigation systems and services as well as shipborne sensors (like speed log, gyro compass). The goal of the PNT Module is the provision of PNT information and associated integrity information in accordance with changing performance requirements during berth to berth navigation, taking into account the different grades of ships navigational sensors. The application of data fusion techniques within the PNT Unit improves the resilience of PNT information and enables the accuracy estimation by integrity monitoring functions. In order to fulfill these objectives, the redundancy of PNT parameters should be provided by taking redundant sensors or by applying different measuring techniques. In a first development phase of the PNT Unit, following sensors and GNSS services are taken into consideration, including three GNSS receivers, non-dedicated GNSS compass composed of distributed antennas, code- and phase based differential GNSS (DGNSS) services, Inertial Measurement Unit (IMU) and other ship sensors. To perform a robust and efficient data fusion, two fundamental architectures can be considered. The first consists in the fusion of all the available data using a centralized filter. This method yields the optimality in the mathematic aspect. However, a modeling failure of a single sensor might fail the entire system. The other approach is a multi-channel architecture, where each channel is also a filter and fuses the data of a subset of sensors. Different channels are running independently, so that a sensor failure might only affect its own channel rather than the entire system. Considering that the sensors might have individual outages or synchronization problems, the multi-channel approach provides not only a higher resilience but also the simplicity in the algorithm design. Furthermore this architecture has the advantage being adaptable for different grades of ships navigational sensors, to support the scalability of carriage requirements, and to enable a stepwise rollout into the maritime traffic system. This paper focuses on the multi-channel system architecture of the PNT Unit. The minimal task of each channel is to provide position and velocity information regarding the consistent common reference point (CCRP). The attitude information is needed in order to transfer the position from its measuring point onto CCRP. Similar to that, the angular rate information is needed for the transformation of velocity information. Based on these requirements, following channels are implemented at the first stage: Channel type 1: Single GNSS + GNSS compass. This is the basic configuration. Although a single GNSS (main GNSS antenna) allows the position and velocity estimation, multiple distributed GNSS antennas will construct the backup system in case that the main GNSS antenna does not work properly. Channel type 2: Single GNSS +IMU+GNSS compass, where the integration of single GNSS and IMU is made with a tightly-coupled architecture and the attitude information from GNSS compass is integrated in a loosely-coupled architecture. This channel can work in almost any maritime operation area. The use of IMU will, at one hand, smooth the results from single GNSS, and on the other hand, enable the error detection in the GNSS measurement domain. Considering the long antenna baseline on the ship, the attitude estimation based on GNSS compass and IMU will provide more accurate attitude estimation than the conventional GNSS/IMU integration. Channel type 3: IALA Beacon DGNSS+ IMU+ GNSS compass. This configuration can be applied if the correction data from an IALA Beacon DGNSS station are available. The DGNSS results are integrated with IMU output in a loosely-coupled architecture together with the attitude information from GNSS compass. Channel type 4: Phase based (RTK) DGNSS+ IMU+ GNSS compass. This configuration yields the highest accuracy in the sub dm regime, but of cause requires data from a RTK reference station. The RTK results are integrated with the IMU output in a loosely-coupled architecture together with the attitude information from GNSS compass. This paper is structured as follows. Firstly, an overview of the multichannel architecture is given. Secondly each of the applied channels will be described in detail and investigated regarding its advantages and disadvantages. For this purposes test data are used, collected during a measuring trip, which was carried with the vessel “Baltic Taucher II” in the Baltic Sea. Based on the data analysis from this measurement campaign, we will present the results from different channels and show how the entire system can benefit from the multichannel approach

Evaluation of a Low Cost Tactical Grade MEMS IMU for Maritime Navigation

The paper evaluates the performance of a tactical grade MicroElectroMechanical System (MEMS) Inertial Measurement Unit (IMU) with respect to possible application for maritime navigation. The evaluation is based on a measurement campaign on the ferry vessel Mecklenburg Vorpommern, where both a MEMS IMU and a reference Fiber Optical Gyroscope (FOG) based IMU were installed. The evaluation concentrates not only on the provision of a backup functionality for GNSS based heading and position determination, but also on the usage of a MEMS IMU for the integrity monitoring for GNSS faults using a tightly-coupled IMU/GNSS integration scheme