Real-Time GNSS satellite SISRE and its integrity for LEO satellite POD

The real-time Global Navigation Satellite System (GNSS) precise orbital and clock products are essential prerequisites for the Positioning, Navigation, and Timing (PNT) services and have been assessed in various studies. Compared to the precision of the orbital and clock products, their combined effect expressed in Signal-In-Space Ranging Error (SISRE) is of higher concern for positioning users. As a special user of the GNSS, the Low Earth Orbit (LEO) satellites need high-precision real-time GNSS products for their Precise Orbit Determination (POD) and clock determination in real time, which enables the future LEO-augmented GNSS PNT service. This study performs a comprehensive analysis of the real-time GNSS products from five different analysis centers, including analysis of their continuity, accuracy of their orbits, precision of their clocks, and their SISREs for LEO satellites at different altitudes. Using the tested products, the LEO POD was also performed to verify the correlation between the quality of the GNSS products and the accuracy of the LEO POD. Furthermore, to assess the integrity of real-time GNSS products, the overbounding standard deviations and mean values of the combined clock and orbital errors were computed and compared for different institutions. It was found that the GPS and Galileo SISRE range from a few centimeters to around 8 cm, while the SISRE of the Beidou Satellite Navigation System (BDS) Medium Earth Orbit (MEO) is a bit worse, i.e., around 1-2 dm. It has been demonstrated that there exists a positive correlation between the SISRE and user altitude, which implies a higher bias introduced to LEO satellites than ground users. The overbounding standard deviations and mean values of the GPS and Galileo products are all within 1 dm, whereas for BDS they are about 1-2 dm. Among the tested products, the smallest SISRE and overbounding values were delivered by the National Centre for Space Studies (CNES) in France for GPS and Galileo, while the GNSS Research Center of Wuhan University (WHU) provided the best accuracy and integrity for the BDS MEO products

Integrity Monitoring of PPP-RTK Positioning; Part I: GNSS-Based IM Procedure

Nowadays, integrity monitoring (IM) is required for diverse safety-related applications using intelligent transport systems (ITS). To ensure high availability for road transport users for in-lane positioning, a sub-meter horizontal protection level (HPL) is expected, which normally requires a much higher horizontal positioning precision of, e.g., a few centimeters. Precise point positioning-real-time kinematic (PPP-RTK) is a positioning method that could achieve high accuracy without long convergence time and strong dependency on nearby infrastructure. As the first part of a series of papers, this contribution proposes an IM strategy for multi-constellation PPP-RTK positioning based on global navigation satellite system (GNSS) signals. It analytically studies the form of the variance-covariance (V-C) matrix of ionosphere interpolation errors for both accuracy and integrity purposes, which considers the processing noise, the ionosphere activities and the network scale. In addition, this contribution analyzes the impacts of diverse factors on the size and convergence of the HPLs, including the user multipath environment, the ionosphere activity, the network scale and the horizontal probability of misleading information (PMI). It is found that the user multipath environment generally has the largest influence on the size of the converged HPLs, while the ionosphere interpolation and the multipath environments have joint impacts on the convergence of the HPL. Making use of 1 Hz data of Global Positioning System (GPS)/Galileo/Beidou Navigation Satellite System (BDS) signals on L1 and L5 frequencies, for small-to mid-scaled networks, under nominal multipath environments and for a horizontal PMI down to 2 × 10−6, the ambiguity-float HPLs can converge to 1.5 m within or around 50 epochs under quiet to medium ionosphere activities. Under nominal multipath conditions for small-to mid-scaled networks, with the partial ambiguity resolution enabled, the HPLs can converge to 0.3 m within 10 epochs even under active ionosphere activities

A review of system integration and current integrity monitoring methods for positioning in intelligent transport systems

Applications of intelligent transportation systems are continuously increasing. Since positioning is a key component in these systems, it is essential to ensure its reliability and robustness, and monitor its integrity so that the required levels of positioning accuracy, integrity, continuity and availability can be maintained. In challenging environments, such as urban areas, a single navigation system is often difficult to fulfil the positioning requirements. Therefore, integrating different navigation sensors becomes intrinsic, which may include the global navigation satellite systems, the inertial navigation systems, the odometers and the light detection and ranging sensors. To bind the positioning errors within a pre-defined integrity risk, the integrity monitoring is an essential step in the positioning service, which needs to be fulfilled for integrated vehicular navigation systems used in intelligent transportation systems. Developing such innovative integrity monitoring techniques requires knowledge of many relevant aspects including the structure, positioning methodology and different errors affecting the positioning solution of the individual and integrated systems. Moreover, knowledge is needed for the current mitigation techniques of these errors, for possible fault detection and exclusion algorithms and for computation of protection levels. This paper provides an overview and discussion of these aspects with a focus on intelligent transportation systems

Integrity analysis for GPS-based navigation of UAVs in urban environment

The increasing use of Unmanned Aerial Vehicles (UAVs) in safety-critical missions in both civilian and military areas demands accurate and reliable navigation, where one of the key sources of navigation information is presented by Global Navigation Satellite Systems (GNSS). In challenging conditions, for example, in urban areas, the accuracy of GNSS-based navigation may degrade significantly due to user-satellite geometry and obscuration issues without being noticed by the user. Therefore, considering the essentially dynamic rate of change in this type of environment, integrity monitoring is of critical importance for understanding the level of trust we have in positioning and timing data. In this paper, the dilution of precision (DOP) coefficients under nominal and challenging conditions were investigated for the purpose of integrity monitoring in urban environments. By analyzing positioning information in a simulated urban environment using a software-based GNSS receiver, the integrity monitoring approach based on joint consideration of GNSS observables and environmental parameters has been proposed. It was shown that DOP coefficients, when considered together with a number of visible satellites and cut-off elevations specific to the urban environment carry valuable integrity information that is difficult to get using existing integrity monitoring approaches. This has allowed generating indirect integrity measures based on cut-off elevation and satellite visibility that can be used for UAV path planning and guidance in urban environments

Modeling of Barometric Altimeter Measurements to support Geodetic Altitude Navigation

Vertical Navigation is of great importance for safe aircraft navigation and guidance, which have been for decades based on standard pressure altitude to support the determination of aircrafts flight levels. This altitude is obtained from airborne pressure measurements performed by barometers and is referenced to the International Standard Aatmosphere Mean Sea Level isobar surface. Standard pressure altitude deviates from true geodetic altitude, that is the one used by GNSS and referenced to an Earth's reference ellipsoid, up to several hundreds of meters for aircrafts flying at typical civil aviation cruise altitudes. Accurate and reliable geodetic altitude navigation is necessary and critical for airport vicinities operations and for new applications like Urban Air Mobility or Alternative Positioning Navigation applications. Although Inertial Navigation Systems and Global Navigation Satellite Systems are able to provide geodetic altitude estimation, both kinds of navigation systems show normally poorer performances in vertical navigation than in the horizontal one. First, this thesis investigates the accuracy in the computation of geodetic altitude from a corrected pressure altitude computed with barometric pressure and external weather data. This computation method is herein shown to remarkably reduce the deviation of the standard pressure altitude from the true geodetic altitude. Secondly, this work derives two robust error models to support the use of barometric pressure measurements for safe geodetic altitude navigation. The first overbounding model is suitable for the use in snapshot (i.e., single-epoch) algorithms. The second dynamic overbounding model is suitable to be included in sequential estimators and in those applications where the time correlation of the pressure measurements must be properly taken into account. The evaluation of the accuracy obtained in computing geodetic altitude from the corrected pressure altitude as well as the analysis of the residual error models is obtained by the use of data gathered during more than 20 flight hours performed with the Dassault Falcon 20-E5 aircraft within a DLR flight tests campaign