Bibliography related to IAG IC-SSG 12

  • L. Urquhart, F. Nievinski, and M. Santos, "Ray-traced slant factors for mitigating the tropospheric delay at the observation level," Journal of Geodesy, vol. NN, iss. NN, 2012.
    @ARTICLE{URQ12,
      author = {Urquhart, Landon and Nievinski, Felipe and Santos, Marcelo},
      title = {Ray-traced slant factors for mitigating the tropospheric delay at the observation level},
      journal = {Journal of Geodesy},
      issn = {0949-7714},
      pages = {1-12},
      url = {http://dx.doi.org/10.1007/s00190-011-0503-x},
      doi = {doi:10.1007/s00190-011-0503-x},
      abstract= {Three-dimensional ray tracing through a numerical weather model has been applied to a global precise point positioning (PPP) campaign for modeling both the elevation angle- and azimuth-dependence of the tropospheric delay. Rather than applying the ray-traced slant delays directly, the delay has been parameterized in terms of slant factors, which are applied in a similar manner to traditional mapping functions, but which can account for the azimuthal asymmetry of the delay. Five strategies are considered: (1) Vienna Mapping Functions 1 (VMF1) and estimation of a residual zenith delay parameter; (2) VMF1, estimation of a residual zenith delay and estimation of two tropospheric gradient parameters; (3) three-dimensional ray-traced slant factors and estimation of a residual zenith delay; (4) using only ray-traced slant factors and no estimation of any tropospheric parameters and; (5) using both ray-traced slant factors and estimating a residual zenith delay and two tropospheric gradient parameters. The use of the ray-traced slant factors (solution 3) showed a 3.8% improvement in the repeatability of the up component when compared to the assumption of a symmetric atmosphere (solution 1), while the estimation of two tropospheric gradient parameters gave the best results showing an 7.6% improvement over solution 1 in the up component. Solution 4 performed well in the horizontal domain, allowing for sub-centimeter repeatability but the up component was degraded due to deficiencies in the modeling of the zenith delay, particularly for stations located at equatorial latitudes. The magnitude of the differences in the mean coordinates between solution 2 and solution 3, and the strong correlation with the differences between the north component and the ray-traced gradients (coefficient of correlation of 0.83), as well as the impact of observation geometry on the gradient solution indicate that the use of the ray-traced slant factors could have an implication on the realization of reference frames. The estimated tropospheric products from the PPP solutions were compared to those derived from ray tracing. For the zenith delay, a root mean square (RMS) of 5.4 mm was found, while for the gradient terms, a correlation coefficient of 0.46 for the N–S and 0.42 for the E–W was found for the north–south and east–west components, suggesting that there are still important differences in the gradient parameters which could be due to either errors in the NWM or to non-tropospheric error sources leaking into the PPP-estimated gradients.},
      year = {2012},
      volume = {NN},
      pages = {NN},
      number = {NN}
    }
  • V. Nafisi, L. Urquhart, M. Santos, F. Nievinski, J. Boehm, D. Wijaya, H. Schuh, A. Ardalan, T. Hobiger, R. Ichikawa, F. Zus, J. Wickert, and P. Gegout, "Comparison of ray-tracing packages for troposphere delays," IEEE Transactions on Geoscience and Remote Sensing, vol. 50, iss. 2, pp. 469-481, 2012.
    @ARTICLE{IEEE2012,
      author = {Nafisi, Vahab and Urquhart, Landon and Santos, Marcelo and Nievinski, Felipe and Boehm, Johannes and Wijaya, Dudy and Schuh, Harald and Ardalan, Alireza and Hobiger, Thomas and Ichikawa, Ryuichi and Zus, Florian and Wickert, Jens and Gegout, Pascal},
      title = {Comparison of ray-tracing packages for troposphere delays},
      journal = {IEEE Transactions on Geoscience and Remote Sensing},
      year = {2012},
      volume = {50},
      pages = {469-481},
      number = {2},
      abstract = {A comparison campaign to evaluate and compare troposphere delays from different ray-tracing software was carried out under the umbrella of the International Association of Geodesy (IAG) Working Group 4.3.3 in the first half of 2010 with five institutions participating: the German Research Centre for Geosciences (GFZ), the Groupe de Recherche de Geodesie Spatiale (GRGS), the National Institute of Information and Communication Technology (NICT), the University of New Brunswick (UNB), and the Institute of Geodesy and Geophysics (IGG) of the Vienna University of Technology. High-resolution data from the operational analysis of the European Centre for Medium-Range Weather Forecasts (ECMWF) was provided to the participants of the comparison campaign for the stations Tsukuba (Japan) and Wettzell (Germany). The data consisted of geopotential differences with respect to mean sea level, temperature, and specific humidity, all at isobaric levels. Additionally, information about the geoid undulations was provided and the participants computed the ray-traced total delays for 5o elevation angle and every degree in azimuth. In general, we find good agreement, with standard deviations and biases at the 1 cm level (or significantly better for some combinations) between the ray-traced slant factors from the different solutions at 5 degrees elevation if determined from the same pressure level data of the ECMWF. Some of these discrepancies are due to differences in the algorithms and the interpolation approaches. If compared to slant factors determined from ECMWF native model level data, the biases can be significantly larger, and when employing different atmospheric models provided by different weather agencies, discrepancies as large as 20 cm show up, indicating the accuracy that could be expected for ray-traced delays.},
      doi = {doi:10.1109/TGRS.2011.2160952},
      domains = {troposphere}
    }
  • K. Teke, J. Boehm, T. Nilsson, H. Schuh, P. Steigenberger, R. Dach, R. Heinkelmann, P. Willis, R. Haas, S. Garcia-Espada, T. Hobiger, R. Ichikawa, and S. Shimizu, "Multi-technique comparison of troposphere zenith delays and gradients during CONT08," Journal of Geodesy, vol. 85, iss. 7, pp. 395-413, 2011.
    @ARTICLE{JOG2011,
      author = {Teke, Kamil and Boehm, Johannes and Nilsson, Tobias and Schuh, Harald and Steigenberger, Peter and Dach, Rolf and Heinkelmann, Robert and Willis, Pascal and Haas, Ruediger and Garcia-Espada, Susana and Hobiger, Thomas and Ichikawa, Ryuichi and Shimizu, Shingo},
      title = {Multi-technique comparison of troposphere zenith delays and gradients during CONT08},
      journal = {Journal of Geodesy},
      issn = {0949-7714},
      domains = {VLBI:troposphere},
      doi = {doi:10.1007/s00190-010-0434-y},
      abstract = {CONT08 was a 15 days campaign of continuous Very Long Baseline Interferometry (VLBI) sessions during the second half of August 2008 carried out by the International VLBI Service for Geodesy and Astrometry (IVS). In this study, VLBI estimates of troposphere zenith total delays (ZTD) and gradients during CONT08 were compared with those derived from observations with the Global Positioning System (GPS), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), and water vapor radiometers (WVR) co-located with the VLBI radio telescopes. Similar geophysical models were used for the analysis of the space geodetic data, whereas the parameterization for the least-squares adjustment of the space geodetic techniques was optimized for each technique. In addition to space geodetic techniques and WVR, ZTD and gradients from numerical weather models (NWM) were used from the European Centre for Medium-Range Weather Forecasts (ECMWF) (all sites), the Japan Meteorological Agency (JMA) and Cloud Resolving Storm Simulator (CReSS) (Tsukuba), and the High Resolution Limited Area Model (HIRLAM) (European sites). Biases, standard deviations, and correlation coefficients were computed between the troposphere estimates of the various techniques for all eleven CONT08 co-located sites. ZTD from space geodetic techniques generally agree at the sub-centimetre level during CONT08, and—as expected—the best agreement is found for intra-technique comparisons: between the Vienna VLBI Software and the combined IVS solutions as well as between the Center for Orbit Determination (CODE) solution and an IGS PPP time series; both intra-technique comparisons are with standard deviations of about 3–6 mm. The best inter space geodetic technique agreement of ZTD during CONT08 is found between the combined IVS and the IGS solutions with a mean standard deviation of about 6 mm over all sites, whereas the agreement with numerical weather models is between 6 and 20 mm. The standard deviations are generally larger at low latitude sites because of higher humidity, and the latter is also the reason why the standard deviations are larger at northern hemisphere stations during CONT08 in comparison to CONT02 which was observed in October 2002. The assessment of the troposphere gradients from the different techniques is not as clear because of different time intervals, different estimation properties, or different observables. However, the best inter-technique agreement is found between the IVS combined gradients and the GPS solutions with standard deviations between 0.2 and 0.7 mm.},
      year = {2011},
      volume = {85},
      pages = {395-413},
      number = {7}
    }
  • P. Gegout, R. Biancale, and L. Soudarin, "Adaptive mapping functions to the azimuthal anisotropy of the neutral atmosphere," Journal of Geodesy, vol. 85, pp. 661-677, 2011.
    @ARTICLE{AMF11,
      author = {Gegout, P. and Biancale, R. and Soudarin, L.},
      title = {Adaptive mapping functions to the azimuthal anisotropy of the neutral atmosphere},
      journal = {Journal of Geodesy},
      issn = {0949-7714},
      pages = {661-677},
      volume = {85},
      issue = {10},
      url = {http://dx.doi.org/10.1007/s00190-011-0474-y},
      doi= {doi:10.1007/s00190-011-0474-y},
      year = {2011},
      domain = {GPS:troposphere},
      abstract = {The anisotropy of propagation of radio waves used by global navigation satellite systems is investigated using high-resolution observational data assimilations produced by the European Centre for Medium-range Weather Forecast. The geometry and the refractivity of the neutral atmosphere are built introducing accurate geodetic heights and continuous formulations of the refractivity and its gradient. Hence the realistic ellipsoidal shape of the refractivity field above the topography is properly represented. Atmospheric delays are obtained by ray-tracing through the refractivity field, integrating the eikonal differential system. Ray-traced delays reveal the anisotropy of the atmosphere. With the aim to preserve the classical mapping function strategy, mapping functions can evolve to adapt to high-frequency atmospheric fluctuations and to account for the anisotropy of propagation by fitting at each site and time the zenith delays and the mapping functions coefficients. Adaptive mapping functions (AMF) are designed with coefficients of the continued fraction form which depend on azimuth. The basic idea is to expand the azimuthal dependency of the coefficients in Fourier series introducing a multi-scale azimuthal decomposition which slightly changes the elevation functions with the azimuth. AMF are used to approximate thousands of atmospheric ray-traced delays using a few tens of coefficients. Generic recursive definitions of the AMF and their partial derivatives lead to observe that the truncation of the continued fraction form at the third term and the truncation of the azimuthal Fourier series at the fourth term are sufficient in usual meteorological conditions. Delays’ and elevations’ mapping functions allow to store and to retrieve the ray-tracing results to solve the parallax problem at the observation level. AMF are suitable to fit the time-variable isotropic and anisotropic parts of the ray-traced delays at each site at each time step and to provide GPS range corrections at the measurement level with millimeter accuracy at low elevation. AMF to the azimuthal anisotropy of the neutral atmosphere are designed to adapt to complex weather conditions by adaptively changing their truncations.}
    }
  • J. Boehm, T. Hobiger, R. Ichikawa, T. Kondo, Y. Koyama, A. Pany, H. Schuh, and K. Teke, "Asymmetric tropospheric delays from numerical weather models for UT1 determination from VLBI Intensive sessions on the baseline Wettzell-Tsukuba," Journal of Geodesy, vol. 84, iss. 5, pp. 319-325, 2010.
    @ARTICLE{JOG10a,
      author = {Boehm, Johannes and Hobiger, Thomas and Ichikawa, Ryuichi and Kondo, Tetsuro and Koyama, Yasuhiro and Pany, Andrea and Schuh, Harald and Teke, Kamil},
      title = {Asymmetric tropospheric delays from numerical weather models for {UT1} determination from {VLBI} Intensive sessions on the baseline {W}ettzell-{T}sukuba },
      journal = {Journal of Geodesy},
      year = {2010},
      volume = {84},
      pages = {319-325},
      number = {5},
      abstract = {One-baseline 1-h Very Long Baseline Interferometry (VLBI) Intensive sessions are carried out every day to determine Universal Time (UT1). Azimuthal asymmetry of tropospheric delays around the stations is usually ignored and not estimated because of the small number of observations. In this study we use external information about the asymmetry for the Intensive sessions between Tsukuba (Japan) and Wettzell (Germany), which are carried out on Saturdays and Sundays (1) from direct ray-tracing for each observation at Tsukuba and (2) in the form of linear horizontal north and east gradients every 6 h at both stations. The change of the UT1 estimates is at the 10 mu s level with maximum differences of up to 50 mu s, which is clearly above the formal uncertainties of the UT1 estimates (between 5 and 20 mu s). Spectral analysis reveals that delays from direct ray-tracing for the station Tsukuba add significant power at short periods (1-2 weeks) w.r.t. the state-of-the-art approach, and comparisons with length-of-day (LOD) estimates from Global Positioning System (GPS) indicate that these ray-traced delays slightly improve the UT1 estimates from Intensive sessions.},
      doi = {doi:10.1007/s00190-010-0370-x},
      domains = {VLBI, troposphere}
    }
  • T. Hobiger, Y. Kinoshita, S. Shimizu, R. Ichikawa, M. Furuya, T. Kondo, and Y. Koyama, "On the importance of accurately ray-traced troposphere corrections for Interferometric SAR data," Journal of Geodesy, vol. 84, iss. 9, pp. 537-546, 2010.
    @ARTICLE{JOG10b,
      author = {Hobiger, Thomas and Kinoshita, Youhei and Shimizu, Shingo and Ichikawa, Ryuichi and Furuya, Masato and Kondo, Tetsuro and Koyama, Yasuhiro},
      title = {On the importance of accurately ray-traced troposphere corrections for Interferometric {SAR} data },
      journal = {Journal of Geodesy},
      year = {2010},
      volume = {84},
      pages = {537-546},
      number = {9},
      abstract = {Numerical weather models offer the possibility to compute corrections for a variety of space geodetic applications, including remote sensing techniques like interferometric SAR. Due to the computational complexity, exact ray-tracing is avoided in many cases and mapping approaches are applied to transform vertically integrated delay corrections into slant direction. Such an approach works well as long as lateral atmospheric gradients are small enough to be neglected. But since such an approximation holds only for very rare cases it is investigated how horizontal gradients of different atmospheric constituents can evoke errors caused by the mapping strategy. Moreover, it is discussed how sudden changes of wet refractivity can easily lead to millimeter order biases when simplified methods are applied instead of ray-tracing. By an example, based on real InSAR data, the differences of the various troposphere correction schemes are evaluated and it is shown how the interpretation of the geophysical signals can be affected. In addition, it is studied to which extend troposphere noise can be reduced by applying the exact ray-tracing solution.},
      doi = {doi:10.1007/s00190-010-0393-3},
      domains = {SAR, troposphere}
    }
  • F. G. Nievinski and M. C. Santos, "Ray-tracing options to mitigate the neutral atmosphere delay in GPS," Geomatica, vol. 64, pp. 191-207, 2010.
    @ARTICLE{FN2010,
      author = {Nievinski, F. G. and M. C. Santos},
      title = {Ray-tracing options to mitigate the neutral atmosphere delay in GPS},
      journal = {Geomatica},
      abstract = {One of the most rigorous way of quantifying the neutral atmosphere radio propagation delay is with ray-tracing, i.e., supposing the signal to be a ray and tracing its path, from satellite to receiver. We demonstrate how one can find significant discrepancies in ray-traced neutral atmosphere delays, due to reasonable variations in the underlying models. We offer a three part contribution. The first part is the separation of the ray-tracing options into three orthogonal groups: atmospheric source, atmospheric structure, and ray-path model. The second contribution is the systematization of model alternatives within each group, namely, atmospheric sources made of climate models, radiosondes, and numerical weather models; the atmospheric structures called spherical concentric, spherical osculating, ellipsoidal, gradient, and 3d; and the ray-path models bent-3d, bent-2d, straight-line, and zenithal. The third part of this contribution is the experimental comparison of these different models, in which we quantified the resulting discrepancy in terms of delay. Our findings are as follows. (i) Regarding ray-path models, the bent-2d model, albeit not strictly valid in a 3d atmosphere, introduces only negligible errors, compared to the more rigorous bent-3d model (in a 15-km horizontal resolution atmospheric model). Regarding atmospheric structures, we found that (ii) the oblateness of the Earth cannot be neglected when it comes to predicting the neutral atmosphere delay, as demonstrated by the poor results of a spherical concentric atmosphere; (iii) the spherical osculating model is the only one exhibiting azimuthal symmetry; (iv) the oblateness of the Earth is adequately accounted for by a spherical osculating model, as demonstrated by the small discrepancy between a spherical osculating and a more rigorous ellipsoidal model; and (v) a gradient atmosphere helps in accounting for the main trend in azimuthal asymmetry exhibited by a 3d atmosphere, but there remains secondary directions of azimuthal asymmetry that only a full 3d atmosphere is able to capture.},
      year = {2010},
      pages = {191-207},
      volume = {64},
      issue = {2}
    }
  • T. Hobiger, S. Shimada, S. Shimizu, R. Ichikawa, Y. Koyama, and T. Kondo, "Improving GPS positioning estimates during extreme weather situations by the help of fine-mesh numerical weather models," Journal of Atmospheric and Solar-Terrestrial Physics, vol. 72, iss. 2-3, pp. 262-270, 2010.
    @ARTICLE{JASTP10,
      author = {Hobiger, Thomas and Shimada, Seiji and Shimizu, Shingo and Ichikawa, Ryuichi and Koyama, Yasuhiro and Kondo, Tetsuro},
      title = {Improving {GPS} positioning estimates during extreme weather situations by the help of fine-mesh numerical weather models },
      journal = {Journal of Atmospheric and Solar-Terrestrial Physics},
      year = {2010},
      volume = {72},
      pages = {262-270},
      number = {2-3},
      abstract = {Space geodetic applications require to model troposphere delays as good as possible in order to achieve highly accurate positioning estimates. However, these models are not capable to consider complex refractivity fields which are likely to occur during extreme weather situations like typhoons, storms, heavy rain-fall, etc. Thus it has been investigated how positioning results can be improved if information from numerical weather models is taken into account. It will be demonstrated that positioning errors can be significantly reduced by the usage of ray-traced slant delays. Therefore, meso-scale and fine-mesh numerical weather models are utilized and their impact on the positioning results will be measured. The approach has been evaluated during a typhoon passage using global positioning service (GPS) observations of 72 receivers located around Tokyo, proving the usefulness of ray-traced slant delays for positioning applications. Thereby, it is possible reduce virtual station movements as well as improve station height repeatabilities by up to 30% w.r.t. standard processing techniques. Additionally the advantages and caveats of numerical weather models will be discussed and it will be shown how fine-mesh numerical weather models, which are restricted in their spatial extent, have to be handled in order to provide useful corrections.},
      doi = {doi:10.1016/j.jastp.2009.11.018},
      domains = {GPS, troposphere}
    }
  • T. Hobiger, R. Ichikawa, Y. Koyama, and T. Kondo, "Computation of Troposphere Slant Delays on a GPU," IEEE Transactions on Geoscience and Remote Sensing, vol. 47, iss. 10, pp. 3313-3318, 2009.
    @ARTICLE{IEEE2009,
      author = {Hobiger, Thomas and Ichikawa, Ryuichi and Koyama, Yasuhiro and Kondo, Tetsuro},
      title = {Computation of Troposphere Slant Delays on a {GPU}},
      journal = {IEEE Transactions on Geoscience and Remote Sensing},
      year = {2009},
      volume = {47},
      pages = {3313-3318 },
      number = {10},
      abstract = {The computation of ray-traced troposphere delays which can be utilized for space geodetic applications is a time-consuming effort when a large number of rays has to be calculated. On the other hand, computation time can be tremendously reduced when algorithms are capable of supporting parallel processing architectures. Thus, by the use of an off-the-shelf graphics processing unit (GPU), it is demonstrated that troposphere slant delays can be computed very efficiently, without loss of accuracy. An adopted ray-tracing algorithm is presented, and results from GPU computations are compared with those obtained from calculations on a standard personal computer's CPU.},
      doi = {doi:10.1109/TGRS.2009.2022168},
      domains = {GPU, GPS, troposphere}
    }
  • A. Pany, J. Boehm, H. Schuh, T. Hobiger, and R. Ichikawa, "Modeling azimuthal asymmetries of the troposphere delay during a 14-days typhoon period in Tsukuba," in Proceedings of the 19th European VLBI for Geodesy and Astrometry Working Meeting, 24-25 March 2009, 2009, pp. 44-48.
    @INPROCEEDINGS{EVGA09b,
      author = {Pany, Andrea and Boehm, Johannes and Schuh, Harald and Hobiger, Thomas and Ichikawa, Ryuichi},
      title = {Modeling azimuthal asymmetries of the troposphere delay during a 14-days typhoon period in Tsukuba},
      booktitle = {Proceedings of the 19th European VLBI for Geodesy and Astrometry Working Meeting, 24-25 March 2009},
      year = {2009},
      number = {-},
      pages = {44-48},
      abstract = {},
      domains = {VLBI, Troposphere},
      url = {}
    }
  • T. Gotoh, T. Hobiger, R. Ichikawa, T. Feldmann, and D. Piester, "Application of ray-traced troposphere delays to GPS time transfer," in Proc. 52nd Meeting of the Japan Society for Aeronautical and Space Sciences, 5-7 Nov 2008, Awaji Yumebutai International Conference Center, Hyogo, Japan, 2008, pp. 1322-1326.
    @INPROCEEDINGS{JSASS08,
      author = {Gotoh, Tadahiro and Hobiger,Thomas and Ichikawa,Ryuichi and Feldmann,Thorsten and Piester,Dirk},
      title = {Application of ray-traced troposphere delays to {GPS} time transfer},
      booktitle = {Proc. 52nd Meeting of the Japan Society for Aeronautical and Space Sciences, 5-7 Nov 2008, Awaji Yumebutai International Conference Center, Hyogo, Japan},
      year = {2008},
      editor = {D. Behrend and K. Baver},
      pages = {1322-1326},
      domains = {GPS:troposphere}
    }
  • R. Ichikawa, T. Hobiger, Y. Koyama, and T. Kondo, "A Comparison between Current Mapping Functions and Ray-traced Slant Delays from JMA Mesoscale Numerical Weather Data," in NICT IVS Technical Development Center News, 2008, pp. 3-7.
    @INPROCEEDINGS{TDC08a,
      author = {Ichikawa, Ryuichi and Hobiger, Thomas and Koyama, Yasuhiro and Kondo,Tetsuro},
      title = {A Comparison between Current Mapping Functions and Ray-traced Slant Delays from JMA Mesoscale Numerical Weather Data},
      booktitle = {NICT IVS Technical Development Center News},
      year = {2008},
      number = {29},
      pages = {3-7},
      abstract = {We have estimated atmospheric slant delays using the KAshima RAytracing Tools (KARAT) through the JMA 10 km MANAL data.The comparisons between KARAT-based slant delays and empirical mapping functions indicate large biases ranging between 18 and 90 mm for summer season, which are considered to be caused by a significant variability of water vapor. We also compared PPP processed position solution using KARAT with that using the latest mapping function for the two week GEONET data sets. The KARAT solution were almost identical to the solution using GMF with linear gradient model, but some cases were slightly worse under the extreme atmospheric condition. Though we need further investigations to evaluate the capability of KARAT to reduce atmospheric path delays under the various topographic and meteorological regimes, the KARAT will be the powerful tool to reduce atmospheric path delay with the numerical weather model improvement.},
      domains = {VLBI:GPS:troposphere},
      url = {http://www2.nict.go.jp/w/w114/stsi/ivstdc/news_29/pdf/tdcnews_29.pdf}
    }
  • R. Ichikawa, T. Hobiger, Y. Koyama, and T. Kondo, "An Evaluation of the Practicability of Current Mapping Functions Using Ray-traced Delays from JMA Mesoscale Numerical Weather Data," Proc. of the International Symposium on GPS/GNSS 2008 (peer-reviewed), iss. 1, pp. 5-12, 2008.
    @ARTICLE{GNSS2008b,
      author = {Ichikawa, Ryuichi and Hobiger,Thomas and Koyama,Yasuhiro and Kondo, Tetsuro},
      title = {An Evaluation of the Practicability of Current Mapping Functions Using Ray-traced Delays from {JMA} Mesoscale Numerical Weather Data},
      journal = {Proc. of the International Symposium on GPS/GNSS 2008 (peer-reviewed)},
      year = {2008},
      pages = {5-12},
      number = {1},
      abstract = {We have developed a new tool to obtain atmospheric slant path delays by ray-tracing through the meso-scale analysis data for numerical weather prediction developed by Japan Meteorological Agency (JMA) with 10 km horizontal resolution (hereafter, we call this "JMA 10km MANAL data"). These data is operationally used for the purpose of weather forecast and considered for our study. We have created ray-tracing routines and named the tools "KAshima RAytracing Tools (KARAT)". We evaluated atmospheric parameters (equivalent zenith wet delay and linear horizontal delay gradients) derived from slant path delays using KARAT. We also estimate position changes caused by the horizontal variability of the atmosphere by running simulations using the ray-traced slant delays in order to examine the position error magnitude and its behavior under meso-scale atmospheric disturbances. Finally, we assessed empirical mapping functions, developed for use in space geodesy, by comparison with KARAT slant delays.},
      domains = {GPS:troposphere}
    }
  • T. Hobiger, R. Ichikawa, Y. Koyama, and T. Kondo, "Kashima Ray-tracing Service (KARATS) – On-line provision of total troposphere slant delay corrections for East Asian sites," Proc. of the International Symposium on GPS/GNSS 2008 (peer-reviewed), iss. 1, pp. 40-44, 2008.
    @ARTICLE{GNSS2008a,
      author = {Hobiger, Thomas and Ichikawa, Ryuichi and Koyama,Yasuhiro and Kondo, Tetsuro},
      title = {Kashima Ray-tracing Service ({KARATS}) - On-line provision of total troposphere slant delay corrections for East Asian sites},
      journal = {Proc. of the International Symposium on GPS/GNSS 2008 (peer-reviewed)},
      year = {2008},
      pages = {40-44},
      number = {1},
      abstract = {Numerical weather models have undergone a significant improvement of accuracy and spatial resolution, which makes it feasible to utilize such models for the correction of troposphere excess path delays. In our presentation we will discuss results from our recent studies which confirm the benefit from the appliance of ray-traced data within geodetic analysis. Moreover, we present the Kashima Ray-Tracing Service (KARATS) which will allow the user to reduce atmospheric delays from the observations taken at stations across East Asia. We will discuss all aspects of this service which is expected to be operational at the beginning of November 2008.},
      domains = {GPS:troposphere}
    }
  • T. Hobiger, R. Ichikawa, T. Kondo, and Y. Koyama, "Fast and accurate ray-tracing algorithms for real-time space geodetic applications using numerical weather models," Journal of Geophysical Research, vol. 113, iss. D203027, pp. 1-14, 2008.
    @ARTICLE{JGR2008,
      author = {Hobiger, Thomas and Ichikawa, Ryuichi and Kondo, Tetsuro and Koyama, Yasuhiro},
      title = {Fast and accurate ray-tracing algorithms for real-time space geodetic applications using numerical weather models},
      journal = {Journal of Geophysical Research},
      year = {2008},
      volume = {113},
      pages = {1-14},
      number = {D203027},
      abstract = {The atmospheric excess path delay is a major contributor to the error budget of space geodetic positioning applications and should therefore be reduced to the maximum possible extent. Numerical weather models are undergoing improvements with regard to their spatial resolution, which enables the compensation of troposphere propagation errors by applying corrections obtained from ray-tracing through three-dimensional meteorologic fields. Since in the selection of the locations of the grid points priority is given to the requirements of meteorologists rather than the facilitation of efficient ray-tracing algorithms, we propose a method that can resample and refine the large data cubes onto regular grids using a sophisticated and fast method developed at the National Institute of Information and Communications Technology (NICT). Once these data sets are generated, ray-tracing algorithms can be applied in order to compute atmospheric excess path delays in real time for several users using off-the-shelf PCs. We present three different ray-tracing strategies and discuss their advantages and bottlenecks with regard to accuracy and data throughput.},
      doi = {doi:10.1029/2008JD010503},
      domains = {troposphere:GPS}
    }
  • T. Hobiger, R. Ichikawa, T. Takasu, Y. Koyama, and T. Kondo, "Ray-traced troposphere slant delays for precise point positioning," Earth, Planets and Space, vol. 60, iss. 5, p. e1-e4, 2008.
    @ARTICLE{EPSe2008,
      author = {Hobiger, Thomas and Ichikawa, Ryuichi and Takasu, Tomoji and Koyama, Yasuhiro and Kondo, Tetsuro},
      title = {Ray-traced troposphere slant delays for precise point positioning},
      journal = {Earth, Planets and Space},
      year = {2008},
      volume = {60},
      pages = {e1-e4},
      number = {5},
      abstract = {Precise satellite orbits and clock information for global navigation satellite systems (GNSS) allow zero-difference position solutions, also known as precise point positioning (PPP) to be calculated. In recent years numerical weather models (NWM) have undergone an improvement of spatial and temporal resolution. This makes them not only useful for the computation of mapping functions but also allows slant troposphere delays from ray-tracing to be obtained. For this study, such ray-traced troposphere corrections have been applied to code and phase observations of 13 sites from the International GNSS Service (IGS) receiver network, which are located inside the boundaries of the Japanese Meteorological Agency (JMA) meso-scale weather model, covering a period of 4 months. The results from this approach are presented together with a comparison to standard PPP processing results. Moreover the advantages and caveats of the introduction of ray-traced slant delays for precise point positioning are discussed.},
      domains = {GPS:troposphere},
      url = {http://www.terrapub.co.jp/journals/EPS/pdf/2008e/6005e001.pdf}
    }