Showing papers by "Patrick Henkel published in 2018"

Snow Water Equivalent of Dry Snow Derived From GNSS Carrier Phases

Abstract: Snow water equivalent (SWE) is a key variable for various hydrological applications. It is defined as the depth of water that would result upon complete melting of a mass of snow. However, until now, continuous measurements of the SWE are either scarce, expensive, labor-intense, or lack temporal or spatial resolution especially in mountainous and remote regions. We derive the SWE for dry-snow conditions using carrier phase measurements from the Global Navigation Satellite System (GNSS) receivers. Two static GNSS receivers are used, whereby one antenna is placed below the snow and the other antenna is placed above the snow. The carrier phase measurements of both receivers are combined in double differences (DDs) to eliminate clock offsets and phase biases and to mitigate atmospheric errors. Each DD carrier phase measurement depends on the relative position between both antennas, an integer ambiguity due to the periodic nature of the carrier phase signal, and the SWE projected into the direction of incidence. The relative positions of the antennas are determined under snow-free conditions with millimeter accuracy using real-time kinematic positioning. Subsequently, the SWE and carrier phase integer ambiguities are jointly estimated with an integer least-squares estimator. We tested our method at an Alpine test site in Switzerland during the dry-snow season 2015–2016. The SWE derived solely by the GNSS shows very high correlation with conventionally measured snow pillow (root mean square error: 11 mm) and manual snow pit data. This method can be applied to dense low-cost GNSS receiver networks to improve the spatial and temporal information on snow.

Estimation of satellite position, clock and phase bias corrections

Abstract: Precise point positioning with integer ambiguity resolution requires precise knowledge of satellite position, clock and phase bias corrections. In this paper, a method for the estimation of these parameters with a global network of reference stations is presented. The method processes uncombined and undifferenced measurements of an arbitrary number of frequencies such that the obtained satellite position, clock and bias corrections can be used for any type of differenced and/or combined measurements. We perform a clustering of reference stations. The clustering enables a common satellite visibility within each cluster and an efficient fixing of the double difference ambiguities within each cluster. Additionally, the double difference ambiguities between the reference stations of different clusters are fixed. We use an integer decorrelation for ambiguity fixing in dense global networks. The performance of the proposed method is analysed with both simulated Galileo measurements on E1 and E5a and real GPS measurements of the IGS network. We defined 16 clusters and obtained satellite position, clock and phase bias corrections with a precision of better than 2 cm.

Reliable Multi-GNSS Real-Time Kinematic Positioning

Abstract: Surveying, agriculture and the navigation of autonomous robots, cars, ships and aerial systems require Global Navigation Satellite Systems (GNSS) for precise positioning. In this paper, we describe a Real-Time Kinematic (RTK) positioning method, that uses both GPS and Galileo measurements with a common reference satellite, estimates a pseudorange multipath error for each satellite to prevent a mapping of multipath errors into the position, and performs a hypothesis testing for ambiguity fixing. The proposed method is tested on the new Multi-Sensor, Multi-GNSS RTK positioning module of ANavS, which includes up to 3 GNSS receivers, an inertial sensor, a barometer and a processor for RTK positioning on a single board. The measurement results show a repeatable millimeter-level positioning accuracy.

Estimation of Snow Parameters with GPS and Galileo

Abstract: The Snow Water Equivalent (SWE) is a key parameter for hydrological applications. In this paper, we describe a novel snow monitoring station that determines SWE solely from differential carrier phase measurements of both GPS and Galileo satellites. The SWE estimates are compared with several traditional sensors to demonstrate the achievable performance. The measurement results also show that just 4 Galileo satellites reduce the combined fixing time of RTK positioning by up to 3 minutes.

Precise Positioning in Alpine Areas with Troposphere and Multipath Estimation

Abstract: Real-Time Kinematic (RTK) positioning with Global Navigation Satellite System (GNSS) signals is widely used, e.g. for surveying, agriculture, and potentially for autonomous vehicles and Unmanned Air Vehicles (UAVs) in the future. In this paper, an extended RTK positioning for significant height differences between two low-cost GNSS receivers with patch antennas is presented. In this case, the classical model for double difference measurements needs to be extended, i.e. a differential tropospheric zenith delay and pseudorange multipath errors need to be estimated besides the baseline and carrier phase integer ambiguities. The increased number of unknowns results in an ill-conditioned system of observation equations and a substantial correlation between the state estimates. We introduce prior information on the differential tropospheric zenith delay based on the differential air pressure, and exploit the temporal correlation of pseudorange multipath errors. Moreover, a criterion for the integer ambiguity candidate vector selection is provided, which is robust over phase multipath and unavoidable errors in the float ambiguity covariance matrix. The proposed method is validated with two low-cost GNSS modules that were placed at the Zugspitzplatt (2601 m a.s.l.) and Eibsee (1018 m a.s.l.). The obtained estimates for the baseline and differential tropospheric zenith delay show a high repeatability over several independent ambiguity fixings. The residuals of the fixed carrier phase measurements are within two centimeters for almost all satellites, which confirms both the measurement model and the ambiguity fixing.

Precise Positioning in Alpine Areas With Troposphere and Multipath Estimation

Abstract: Real-time kinematic (RTK) positioning with Global Navigation Satellite System (GNSS) signals is widely used, e.g., for surveying, agriculture, and potentially for autonomous vehicles and unmanned air vehicles in the future. In this paper, an extended RTK positioning for significant height differences between two low-cost GNSS receivers with patch antennas is presented. In this case, the classical model for double difference measurements needs to be extended, i.e., a differential tropospheric zenith delay and pseudorange multipath errors need to be estimated besides the baseline and carrier phase integer ambiguities. The increased number of unknowns results in an ill-conditioned system of observation equations and a substantial correlation between the state estimates. We introduce prior information on the differential tropospheric zenith delay based on the differential air pressure and exploit the temporal correlation of pseudorange multipath errors. Moreover, a criterion for the integer ambiguity candidate vector selection is provided, which is robust over phase multipath and unavoidable errors in the float ambiguity covariance matrix. The proposed method is validated with two low-cost GNSS modules that were placed at the Zugspitzplatt (2601 m a.s.l.) and Eibsee (1018 m a.s.l.). The obtained estimates for the baseline and differential tropospheric zenith delay show a high repeatability over several independent ambiguity fixings. The residuals of the fixed carrier phase measurements are within 2 cm for almost all satellites, which confirms both the measurement model and the ambiguity fixing.

Integration of Barometric Height into RTK Positioning for Fast Ambiguity Refixing

Abstract: The horizontal positioning accuracy of Global Navigation Satellite System receivers is in general two times higher than the vertical positioning accuracy. The integration of barometric height information improves in particular the vertical positioning accuracy. In this paper, we integrate differential air pressure measurements into Real-Time Kinematic (RTK) positioning with a Kalman filter. W e show that t he d ifferential a ir pressure measurements enable a faster convergence of the float RTK solution and a more reliable ambiguity fixing. $T$ hereby, the proposed method is especially attractive for improving the RTK performance after temporary GNSS outages.

The Wireless Seat Belt Requirements, Experiments, and Solutions for Pedestrian Safety

Abstract: The World Health organization in its latest report on road safety states that 22% of all road traffic deaths comprise pedestrians, approximately 275,000 pedestrians worldwide. Based on an overview of typical accident scenarios, we argue how an ideal pedestrian safety system should look like. Then we investigate how much our approach of a pedestrian safety system, which we call “Wireless Seat Belt” (WSB), can satisfy the requirements of such an ideal system in terms of position accuracy. Next, we experimentally show what can be done with current smartphones in terms of these requirements. We show that current smartphone accuracy is not sufficient for reliable collision avoidance. To address this problem, we introduce and evaluate two approaches using context information to enable “off-the-shelf” smartphones to reach the required accuracy requirements.

Cascaded Real-Time Kinematic Positioning with Multi-Frequency Linear Combinations

Abstract: Real-Time Kinematic (RTK) positioning with Global Navigation Satellite Systems (GNSS) enables centimeter-level positioning accuracies. However, the small wavelength of the GNSS carrier signals often prevents a reliable ambiguity resolution. This paper proposes a cascaded RTK positioning with multifrequency linear combinations. The combinations are characterized by a large wavelength, which enables a robust ambiguity resolution even in the presence of uncorrected geometric and ionospheric biases at the price of a slightly increased noise level.