S. Schon

Leibniz Universität Hannover, Hanover, Lower Saxony, Germany

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Publications (16)0 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Multipath propagation has a strong impact on the accuracy of satellite-based positioning. In this work, multipath effects are investigated by means of a measurement campaign and ray tracing propagation modelling. A GNSS receiver placed in a scenario with a single ground reflector was used for this purpose. From the deterministic channel modelling, the impact of multipath propagation is analysed by means of the oscillations introduced in the carrier-to-noise density ratio.
    Antennas and Propagation (EuCAP), 2013 7th European Conference on; 01/2013
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    ABSTRACT: Multipath propagation between satellite and receiver constitutes an important impairment in satellite navigation systems. To address this problem, an extensive investigation on the propagation channel is required. In this paper, the satellite-to-earth channel is modelled deterministically by using the ray tracing method in combination with an accurate 3-dimensional description of the environment in which the receiver is located. Moreover, this work focuses on the influence of diffraction propagation phenomena based on the Uniform Theory of Diffraction (UTD). For a typical GNSS reference station environment we show different diffracted rays and power delay profiles. We find a good agreement between the measurements and simulation results.
    Antennas and Propagation (EUCAP), 2012 6th European Conference on; 01/2012
  • T. Kersten, S. Schon
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    ABSTRACT: The Institut für Erdmessung (IfE) is an official IGS calibration institution, calibrating phase center variations (PCV) for receiver antennae routinely in the field, using the actual GNSS satellite signals in space. Current research activities focus on the antenna code phase calibration with the Hannover Concept of absolute antenna calibration. The receiving antenna as a part of a reception chain can introduce systematic effects, currently known as Group Delay Variations (GDV), i.e. azimuth and elevation dependent code-phase delays. This error introduces additional range variations along the line-of-sight for every satellite depending on the corresponding incident angle in the antennas body frame. Depending on the antenna design, suitable for specific applications, GDV can degrade the accuracy of code based applications, such as precise landing approaches as well as for time and frequency transfer. The paper can be subdivided into two major parts: In the first part, we focus on the current investigations on receiver antenna GDV calibration. Beside the theoretical background of a concept to determine GDV for different GPS antennae based on the Hannover Concept of absolute antenna calibration, the obtained GDV from several antennae with different characteristics will be presented and critically discussed. The second part focuses on the consequent analysis of the impact of the determined GDV on position and navigation applications. The contribution of GDV on the observation and position domain can be shown by using a special experimental set-up. In addition, GDV for a real C/A based autonomous navigation approach are investigated and critically discussed.
    Satellite Navigation Technologies and European Workshop on GNSS Signals and Signal Processing, (NAVITEC), 2012 6th ESA Workshop on; 01/2012
  • T. Kersten, S. Schon, U. Weinbach
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    ABSTRACT: Group Delay Variations (GDVs) are azimuth and elevation dependent code delays that can limit the accuracy of the GNSS code observables. This contribution focuses on the GDV determination and discusses several solutions with respect to repeatability and separability. On-site tests at a laboratory network as well as simulations of several time links are discussed analyzing the stochastic processes apparently introduced by GDV. The stability of the P3 links are not effected. However, offsets up to 0.6 ns can occur. In a detailed study, GDVs are applied to an inter-continental Precise Point Positioning (PPP) time transfer link. This analysis shows that GDV are not an issue for the stability of the PPP links, since small weights reduce the impact of the P3 GDV. It can be shown that the stochastic process, induced by GDV in a PPP analysis, is similar to a random walk noise, well below the L3 carrier phase observation noise of σφ = 6 mm. Offsets for the receiver clock estimates of up to 0.4 ns are reported for the link WTZS (Wettzell) and Boulder (NIST).
    European Frequency and Time Forum (EFTF), 2012; 01/2012
  • U. Weinbach, S. Schon
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    ABSTRACT: Kinematic orbit positions of Low Earth Orbiting satellites (LEOs) derived from GPS observations are frequently used for single satellite gravity field recovery. Unfortunately, the precision of the kinematic coordinates is compromised by the estimation of the receiver clock synchronization offset in addition to the three kinematic coordinates for every observation epoch. In this paper the potential of receiver clock modeling to improve the precision of the kinematic orbit determination is investigated. The formation flying twin satellites of the Gravity Recovery And Climate Experiment (GRACE) mission are considered as a case study. A unique feature of these satellites is the combination of a dual frequency GPS receiver with an Ultra Stable Oscillator (USO), that provides the required frequency stability for the proposed clock modeling approach. Based on a piece-wise linear clock parametrization with 60 s intervals, a significant reduction of the high-frequency radial orbit differences with respect to a reduced-dynamic orbit is shown.
    European Frequency and Time Forum (EFTF), 2012; 01/2012
  • Franziska Kube, Steffen Schon, Thomas Feuerle
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    ABSTRACT: The civil aviation of the future should be ecological, economical, and safe. The citizens in metropolises have on the one hand the need to travel efficiently between airports near the city. On the other hand they, do not want to suffer from noise and emission of airplanes starting and landing at airports close to cities. Consequently, the future airplane should harmonize these needs. In order to fulfill these requirements new concepts for departure and approach procedures are mandatory. Curved approaches are one solution since critical regions can be omitted. These complex maneuvers can only be carried out with the help of satellite navigation systems. One major challenge of curved approaches is the changing obstruction of satellite signals due to shadowing effects by the aircraft itself during curved approaches. Existing GPS landing systems suffer therefore degraded performance (continuity, availability, integrity and accuracy) during curved approaches, especially with higher roll angles and at high and low latitudes.
    01/2012;
  • F. Kube, S. Schon, T. Feuerle
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    ABSTRACT: The civil aviation of the future should be ecological, economical, and safe. The citizens in metropolises have on the one hand the requirement to travel efficiently between airports near the city. On the other hand they do not want to suffer from noise and emission of aircraft starting and landing at airports close to cities. Consequently, the future aircraft should harmonize these needs. In order to fulfil these requirements, new concepts for departure and approach procedures are mandatory. Curved approaches are one solution since critical regions can be omitted. These complex manoeuvres can only be carried out with the help of satellite navigation systems. One major challenge of curved approaches is the changing obstruction of satellite signals due to shadowing effects by the aircraft itself during curved approaches. Existing GPS landing systems suffer therefore degraded performance in terms of continuity, availability, integrity, and accuracy during curved approaches, especially with higher roll angles and at high and low latitudes. To avoid this problem and to meet the requirements of integrity, accuracy, continuity, and availability for future GNSS landing systems, the concept of the so called "virtual receiver" has been developed in the framework of the research project "Bürgernahes Flugzeug" (Metropolitan Aircraft). The main idea is to avoid obstructions by combining GNSS observations of few antennas, optimally installed on the aircraft and computing one common aircraft position solution. This paper presents the concept of the virtual receiver. The benefits in terms of precision, reliability and accuracy are evaluated based on the analysis of real data from a test flight with the research aircraft D-IBUF, a Dornier 128-6 from the Institut of Flight Guidance (TU Braunschweig). It will be shown, that virtual receiver outperforms a single antenna solution concerning the precision (DOP-values are reduced by 30%), reliability (MDBs are reduced by 10 % on- average) and vertical accuracy (the 95 % error bound is reduced by 14%).
    Satellite Navigation Technologies and European Workshop on GNSS Signals and Signal Processing, (NAVITEC), 2012 6th ESA Workshop on; 01/2012
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    ABSTRACT: For many water engineering tasks, precise information about the water flow characteristics is required to model discharge and sediment transport. In many cases, the Global Positioning System (GPS) is used to measure floater trajectories and to derive flow velocities indirectly from floater positions. In this paper, we describe a new GPS-based measurement system that avoids the use of expensive equipment and which uses a direct velocity determination approach. Despite its low costs, the new floater system achieves position accuracies of several meters and accuracies of about 5 cm/s for velocities. We describe both hardware and software details of the new system. Results from a first measurement campaign show the system's capabilities for hydrometric applications.
    01/2012;
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    ABSTRACT: Multipath propagation has a major contribution in the degradation of the achieved positioning accuracy. In high precision applications, where mm accuracy is crucial, multipath caused errors dominate the total error budget. In this investigation, in a first step, we use the results of a ray-tracing approach in order to characterize the influence of multiple-multipath propagation. The characteristics of the propagation channel, which are estimated by the ray-tracing tool, are used as input for the investigation of the impact of multipath propagation on GNSS observables. In a second step, a closer look on GNSS signal amplitudes will be presented. The model presented in this paper is adopted from wireless network simulations and the signal amplitude of each signal component involved is estimated for each epoch separately based on the geometry and the receiving antenna pattern. The model is validated with a simple geometry controlled experiment.
    Satellite Navigation Technologies and European Workshop on GNSS Signals and Signal Processing, (NAVITEC), 2012 6th ESA Workshop on; 01/2012
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    ABSTRACT: Satellite positioning accuracy is degraded by multipath propagation between satellite and the receiver. In this paper we give an overview of a new approach to model and correct this impairment, and present some preliminary results. High accuracy D laser scanning is utilized to model the environment. This gives an accurate basis for the characterization of the satellite to earth radio channel by using ray tracing. The information extracted from the ray tracing simulations is used to design correction algorithms that will be implemented in GNSS software defined radio (SDR) receivers.
    01/2011;
  • U. Weinbach, S. Schon
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    ABSTRACT: We have given a brief overview of the conventional troposphere modeling strategy for GNSS analysis. Subsequently we have shown that a ZWD error is mapped into the estimated receiver clock with a factor of 1.4 to 2.4 depending on the elevation cut-off. Furthermore, we have demonstrated that by including low elevation data into the PPP solution, the peak variations in the receiver clock can be reduced and the ZWD estimates become more stable.
    EFTF-2010 24th European Frequency and Time Forum; 01/2010
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    ABSTRACT: Precise point positioning (PPP) software combines dual frequency code and phase GPS observation data with precise information about the satellite clocks and orbits provided by the International GNSS Service (IGS). It is also a very interesting method for time metrology. Error sources like tropospherical path delay and site displacements have to be modeled or estimated, while the error induced by the first order ionosphere terms can be canceled due to the usage of both GPS frequencies. In case of long baselines the atmospherical effects dominate. Here the physical correction models become important. In this work we compare several precise point positioning (PPP) software packages and study not only the clock solution but also other estimated parameters like positions and zenith tropospheric zenith path delays in order to find the software that is most suitable for time transfer. Especially the repeatability of the position could be a good indicator for quality. We use Dicom GTR50 time and frequency transfer receivers connected to local time scale realizations on baselines with different lengths.
    Frequency Control Symposium, 2009 Joint with the 22nd European Frequency and Time forum. IEEE International; 05/2009
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    ABSTRACT: In this paper we show results of a test campaign, designed to investigate the stability of GPS receiver clocks locked to an external frequency source. Two types of geodetic receivers and one timing receiver have been compared. The receivers were operated in pairs in zero and short baseline setups with common reference frequency. Care was taken to minimize environmental effects. The observation data were processed using both a simple carrier phase single difference algorithm and the Bernese GPS Software 5.0 in Precise Point Positioning (PPP) mode. Some shortcomings of previous studies in terms of receiver equipment and the experimental setup of the short baseline could be avoided. It is shown that using geodetic receivers and the ionosphere-free linear combination of the carrier phase observations, frequency comparisons at a level of sigma<sub>y</sub>(tau)=2ldr10<sup>-16</sup> for averaging times of one day are feasible on a zero baseline, i.e. if all environmental effects are eliminated.
    Frequency Control Symposium, 2009 Joint with the 22nd European Frequency and Time forum. IEEE International; 05/2009
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    ABSTRACT: In this paper, we present an analysis of the influence of attenuation and multipath propagation on the GPS signal strength and the accuracy of the observed pseudoranges in an indoor scenario. Our investigations include experimental data from a u-blox high-sensitivity GPS receiver as well as simulation data derived from a ray tracing based deterministic GPS channel model. Our primary goal is to get a deeper understanding of the complex interactions of the GPS signals on their paths to indoor locations. In the first part we will verify that the indoor propagation of the satellite signals leads to characteristic and repeatable patterns in the observation data. Thanks to the sidereal repeatability of the GPS satellite constellation, this verification can be carried out using measurements performed on two successive days. In the second part we will present the ray tracing based deterministic simulation model used to perform the GPS outdoor-to-indoor channel simulations, show the results and compare the measured data and the results obtained with the ray tracer.
    Positioning, Navigation and Communication, 2009. WPNC 2009. 6th Workshop on; 04/2009
  • S. Schon, O. Bielenberg
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    ABSTRACT: High-sensitivity GPS receivers allow to track very weak GPS signals (-180 dBW or even below) that are due to strong signal attenuation by e.g. foliage or constructive materials. Consequently, they outperform classical geodetic receivers of more than 30 dB and enable the signal tracking and positioning even indoors. This increase of GPS availability opens up new applications such as seamless outdoor-indoor positioning and navigation. However the positioning accuracy is often poor. In this paper, we use an exemplary data set from our GPS indoor test network to analyse the capability of high sensitivity GPS receivers for precise indoor positioning. A general study of the performance parameters (e.g. DOP factors, C/No-values) gives a first impression of the improved availability. The detailed analysis of the observed-computed values of the pseudo-ranges quantify the indoor delay which can reach up to 100 m. Considering the positioning solution, it is shown that a C/N<sub>0</sub>- based signal weighting can improve the accuracy of about 70 % for the mean value over 40 min observation time and up to 40% for the scatter of the coordinate time series.
    Positioning, Navigation and Communication, 2008. WPNC 2008. 5th Workshop on; 04/2008
  • S. Schon, F. Dilssner
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    ABSTRACT: In geodesy, GNSS-based methods are widely used for positioning and navigation. Since the precision (< some mm) and accuracy (< 1 cm) requirements are very high, sophisticated correction models and analysis strategies were developed, e.g. like the network RTK concept realised in different active permanent reference station networks. However, multipath and signal diffraction as well as the incomplete stochastic model for GNSS observations limits still the attainable accuracy and precision, especially for real time applications. In this paper, we focus on multipath and diffraction mitigation. The state of the art procedures for mitigation are revisited as well as the challenges for new strategies are addressed and explained using examples from the Satellite Positioning Service of the German National Survey (SAPOS) and from typical geodetic monitoring networks. Finally the importance of a complete stochastic model for GPS observations for statistically tests and meaningful interpretations of the results is underlined.
    Positioning, Navigation and Communication, 2007. WPNC '07. 4th Workshop on; 04/2007