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  • 1
    Language: English
    In: GPS Solutions, 2015, Vol.19(2), pp.277-286
    Description: Precise global navigation satellite system (GNSS) positioning requires an accurate mapping function (MF) to model the tropospheric delay. To date, the most accurate MF is the Vienna mapping function 1 (VMF1). It utilizes data from a numerical weather model and therefore captures the short-term variability of the atmosphere. Still, the VMF1, or any other MF that is based on the VMF1 concept, is a parameterized mapping approach, and this means that it is tuned for specific elevation angles, station heights, and orbital altitudes. In this study, we analyze the systematic errors caused by such tuning on a global scale. We find that, in particular, the parameterization of the station height dependency is a major concern regarding the application in complex terrain or airborne applications. At this time, we do not provide an improved parameterized mapping approach to mitigate the systematic errors but instead we propose a (ultra-) rapid direct mapping approach, the so-called Potsdam mapping factors (PMFs). Since for any station–satellite link the ratio of the tropospheric delay in the slant and zenith direction is computed directly, the PMFs effectively eliminate the systematic errors.
    Keywords: Precise positioning ; Numerical weather model ; Mapping functions ; Rapid direct mapping
    ISSN: 1080-5370
    E-ISSN: 1521-1886
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  • 2
    Language: English
    In: GPS Solutions, 2017, Vol.21(3), pp.873-885
    Description: We develop an ionospheric mapping function (MF) for the global navigation satellite system (GNSS) which is based on the electron density field derived from the international reference ionosphere (IRI). The station specific MF utilizes a look-up table which contains a set of ray-traced ionospheric phase advances and code delays. Hence, unlike the simple MFs that are currently in use, the developed MF depends on the time, location, elevation and azimuth angle. Ray-bending is taken into account, which implies that the MF depends on the carrier frequency as well. The frequency dependency of the MF can be readily used to examine higher-order ionospheric effects due to ray-bending. We compare the proposed MF with the so-called single-layer model MF and find significant differences in particular around the equatorial anomaly. In so far as the proposed MF is based on a realistic electron density field (IRI), our comparison shows the potential error of the single-layer model MF in practice. We conclude that the developed MF concept might be valuable in the GNSS total electron content estimation. The frequency dependency of the MF can be used to mitigate higher-order ionospheric effects.
    Keywords: Mapping functions ; International reference ionosphere ; Higher-order ionospheric effects
    ISSN: 1080-5370
    E-ISSN: 1521-1886
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  • 3
    In: Geophysical Research Letters, 28 May 2015, Vol.42(10), pp.4173-4181
    Description: The developing multi‐Global Navigation Satellite Systems (GNSS) constellations have the potential to provide accurate high‐resolution tropospheric gradients. Such data, closely linked to strong humidity gradients accompanying severe weather phenomena, are considered a new important data source for meteorological studies, e.g., nowcasting of severe rainfall events. Here we describe the development of a multi‐GNSS processing system for the precise retrieval of high‐resolution tropospheric gradients. The retrieved products were validated by using independent water vapor radiometer (WVR) observations and numerical weather model (NWM) data. The multi‐GNSS high‐resolution gradients agree well with those, derived from NWM and WVR, especially for the fast‐changing peaks which were mostly associated with synoptic fronts. Compared to GPS‐only gradients, the correlations with the validation data are significantly improved up to 20–35%. The new data product has significant potential to improve numerical weather prediction and to advance meteorological studies. Retrieving high‐resolution tropospheric gradients from multi‐GNSS observations Gradients agree well with weather models and other meteorological observations Compared to GPS only, the correlation is improved significantly up to 20–35%
    Keywords: Gnss Meteorology ; High‐Resolution Tropospheric Gradient ; Multiconstellation Gnss ; Numerical Weather Model ; Water Vapor Radiometer
    ISSN: 0094-8276
    E-ISSN: 1944-8007
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  • 4
    Language: English
    In: Atmospheric Measurement Techniques, Jan 3, 2017, Vol.10(1), p.15
    Description: A 1-year data set of ground-based GPS signal observations aiming at geometric elevation angles below +2#xB0; is analysed. Within the "GLESER" measurement campaign about#xC2;#xA0;2600 validated setting events were recorded by the "OpenGPS" open-loop tracking receiver at an observation site located at#xC2;#xA0;52.3808#xB0;#xE2;#x80;#xAF;N, 13.0642#xB0;#xE2;#x80;#xAF;E between January and December#xC2;#xA0;2014. The measurements confirm the feasibility of open-loop signal tracking down to geometric elevation angles of#xC2;#xA0;minus;1 to#xC2;#xA0;minus;1.5#xB0; extending the corresponding closed-loop tracking range by up to#xC2;#xA0;1#xB0;. The study is based on the premise that observations of low-elevation events by a ground-based receiver may serve as test cases for space-based radio occultation measurements, even if the latter proceed at a significantly faster temporal scale. The results support the conclusion that the open-loop Doppler model has negligible influence on the derived carrier frequency profile for strong signal-to-noise density ratios above about 30#xE2;#x80;#xAF;dB#xE2;#x80;#xAF;Hz. At lower signal levels, however, the OpenGPS receiver's dual-channel design, which tracks the same signal using two Doppler models differing by 10#xE2;#x80;#xAF;Hz, uncovers a notable bias. The repeat patterns of the GPS orbit traces in terms of azimuth angle reveal characteristic signatures in both signal amplitude and Doppler frequency with respect to the topography close to the observation site. Mean vertical refractivity gradients, extracted from ECMWF meteorological fields, correlate weakly to moderately with observed signal amplitude fluctuations at geometric elevation angles between +1 and +2#xB0;. Results from multiple phase screen simulations support the interpretation that these fluctuations are at least partly produced by atmospheric multipath; at negative elevation angles diffraction at the ground surface seems to contribute.
    Keywords: Global Positioning System
    ISSN: 1867-1381
    E-ISSN: 18678548
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  • 5
    Language: English
    In: Atmospheric Measurement Techniques Discussions, 04/18/2016, pp.1-29
    ISSN: Atmospheric Measurement Techniques Discussions
    E-ISSN: 1867-8610
    Source: CrossRef
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  • 6
    Language: English
    In: Advances in Space Research, Jan 15, 2014, Vol.53(2), p.272(8)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.asr.2013.11.013 Byline: Florian Zus, Ludwig Grunwaldt, Stefan Heise, Grzegorz Michalak, Torsten Schmidt, Jens Wickert Abstract: On 21 June 2010 the TerraSAR-X satellite was joined by the TanDEM-X satellite. A Global Positioning System (GPS) radio occultation (RO) experiment using the twin satellites has been carried out to estimate the precision of GPS atmospheric soundings. For the Day Of Year (DOY) 330-336, 2011, we analyze phase and amplitude data recorded by GPS receivers separated by a few hundred meters in a low earth orbit and derive collocated atmospheric refractivity profiles. In the altitude range 10-20km the standard deviation between TerraSAR-X and TanDEM-X refractivity does not exceed 0.15%. The standard deviation is rapidly increasing for lower and higher altitudes; close to the surface and at an altitude of 30km the standard deviation reaches 0.8% and 0.5%, respectively. Systematic deviations between TerraSAR-X and TanDEM-X refractivity in the considered altitude range (0-30km) are negligible. The results confirm the anticipated high precision of the GPS RO technique. However, the difference in the retrieved refractivity in the lower troposphere for different Open Loop (OL) signal tracking parameters, altered onboard TanDEM-X for DOY 49-55, 2012, calls for an in depth analysis. At the moment we can not exclude that a potential bias in the OL Doppler model introduces a bias in our retrieved refractivity at altitudes 〈8 km. Article History: Received 8 January 2013; Revised 15 October 2013; Accepted 1 November 2013 Article Note: (footnote) [star] This template can be used for all publications in Advances in Space Research.
    Keywords: Troposphere -- Analysis
    ISSN: 0273-1177
    Source: Cengage Learning, Inc.
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  • 7
    In: Radio Science, May 2015, Vol.50(5), pp.393-405
    Description: In a recent study we developed a fast and accurate algorithm to compute Global Positioning System (GPS) Slant Total Delay (STDs) utilizing numerical weather model data. Having developed a forward operator we construct in this study the tangent linear (adjoint) operator by application of the chain rule of differential calculus in forward (reverse) mode. Armed with these operators we show in a simulation study the potential benefit of GPS STDs in inverse modeling. We conclude that the developed operators are tailored for three (four)‐dimensional variational data assimilation and/or travel time tomography. A fast and accurate STD forward operator is developed The tangent linear and adjoint code is constructed The operators are tailored for data assimilation and travel time tomography
    Keywords: Gps Slant Total Delay
    ISSN: 0048-6604
    E-ISSN: 1944-799X
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  • 8
    Language: English
    In: Advances in Space Research, 2011, Vol.47(5), pp.886-897
    Description: The German Research Centre for Geosciences (GFZ) operates a GNSS water vapour tomography system using about 350 German GNSS stations. The GNSS data processing at the GFZ works in near real-time and provides zenith total delays, integrated water vapour and slant delay data operationally. This large data set of more than 50,000 slant delays per hour is used to reconstruct spatially resolved humidity fields by means of tomographic techniques. It can be expected that additional observations from the future Galileo system provide more information with improved quality. A simulation study covering 12 h at 14 July 2009 was therefore started to estimate the impact of GPS, Galileo and GLONASS data on the GNSS tomography. It is shown that the spatial coverage of the atmosphere with slant paths is highly improved by combining observations from two or three satellite systems. Equally important for a reliable tomographic reconstruction is the distribution of slant path intersections as they are required to locate the integrated delay information. The number of intersection points can be increased by a factor of 4 or 8 if two or three systems are combined and their distribution will cover larger regions of the atmosphere. The combined data sets can be used to increase the spatiotemporal resolution of the reconstructed humidity fields up to 30 km horizontally, 300 m vertically and 15 min. The reconstruction quality could not be improved considerably using the currently available techniques.
    Keywords: Gps/Gnss ; Tomography ; Water Vapour ; Galileo ; Art ; Engineering ; Astronomy & Astrophysics ; Physics
    ISSN: 0273-1177
    E-ISSN: 1879-1948
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  • 9
    In: Tellus A, 03/2011
    ISSN: 0280-6495
    E-ISSN: 1600-0870
    Source: CrossRef
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  • 10
    Language: English
    In: Advances in Space Research, 15 January 2014, Vol.53(2), pp.272-279
    Description: On 21 June 2010 the TerraSAR-X satellite was joined by the TanDEM-X satellite. A Global Positioning System (GPS) radio occultation (RO) experiment using the twin satellites has been carried out to estimate the precision of GPS atmospheric soundings. For the Day Of Year (DOY) 330–336, 2011, we analyze phase and amplitude data recorded by GPS receivers separated by a few hundred meters in a low earth orbit and derive collocated atmospheric refractivity profiles. In the altitude range 10–20 km the standard deviation between TerraSAR-X and TanDEM-X refractivity does not exceed 0.15%. The standard deviation is rapidly increasing for lower and higher altitudes; close to the surface and at an altitude of 30 km the standard deviation reaches 0.8% and 0.5%, respectively. Systematic deviations between TerraSAR-X and TanDEM-X refractivity in the considered altitude range (0–30 km) are negligible. The results confirm the anticipated high precision of the GPS RO technique. However, the difference in the retrieved refractivity in the lower troposphere for different Open Loop (OL) signal tracking parameters, altered onboard TanDEM-X for DOY 49–55, 2012, calls for an in depth analysis. At the moment we can not exclude that a potential bias in the OL Doppler model introduces a bias in our retrieved refractivity at altitudes km.
    Keywords: Tandem-X ; Gps Radio Occultation ; Ol Signal Tracking ; Engineering ; Astronomy & Astrophysics ; Physics
    ISSN: 0273-1177
    E-ISSN: 1879-1948
    Source: ScienceDirect Journals (Elsevier)
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