Prof. Dr. Thomas Hobiger

@article{ASR-Hobiger2012,

title = {A real-time GNSS-R system based on software-defined radio and graphics 

	processing units},

author = {Thomas Hobiger and Jun Amagai and Masanori Aida and Hideki Narita},

doi = {10.1016/j.asr.2012.01.009},

year  = {2012},

date = {2012-01-01},

journal = {Advances in Space Research},

volume = {49},

number = {7},

pages = {1180-1190},

abstract = {Reflected signals of the Global Navigation Satellite System (GNSS) 

	from the sea or land surface can be utilized to deduce and monitor 

	physical and geophysical parameters of the reflecting area. Unlike 

	most other remote sensing techniques, GNSS-Reflectometry (GNSS-R) 

	operates as a passive radar that takes advantage from the increasing 

	number of navigation satellites that broadcast their L-band signals. 

	Thereby, most of the GNSS-R receiver architectures are based on dedicated 

	hardware solutions. Software-defined radio (SDR) technology has advanced 

	in the recent years and enabled signal processing in real-time, which 

	makes it an ideal candidate for the realization of a flexible GNSS-R 

	system. Additionally, modern commodity graphic cards, which offer 

	massive parallel computing performances, allow to handle the whole 

	signal processing chain without interfering with the PC’s CPU. Thus, 

	this paper describes a GNSS-R system which has been developed on 

	the principles of software-defined radio supported by General Purpose 

	Graphics Processing Units (GPGPUs), and presents results from initial 

	field tests which confirm the anticipated capability of the system.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{Tseng2012,

title = {First International TWSTFT Experiment by Employing Dual Pseudo-random 

	Noise Codes},

author = {Wen-Hung Tseng and Yi-Jiun Huang and Tadahiro Gotoh and Jun Amagai and Thomas Hobiger and Miho Fujieda and Shinn-Yan Lin and Huang-Tien Lin and Kai-Ming Feng},

doi = {10.1109/TUFFC.2012.2224},

year  = {2012},

date = {2012-01-01},

journal = {IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control},

volume = {59},

number = {3},

pages = {531 -538},

abstract = {Two-way satellite time and frequency transfer (TWSTFT) is one of the 

	major techniques to compare atomic time scales over long distances. 

	In order to both improve the precision of TWSTFT and decrease the 

	satellite link fee, a new software-defined modem with dual pseudo-random 

	noise (DPN) codes has been developed. In this paper, we demonstrate 

	the first international DPN-based TWSTFT experiment over a period 

	of 6 months. The results of DPN exhibit an excellent performance, 

	which is competitive with the GPS precise point positioning (PPP) 

	technique in the short term and consistent with the conventional 

	TWSTFT in the long term. The time deviations of below 75 ps are achieved 

	for averaging times from 1 s to 1 day. Moreover, the DPN data has 

	less diurnal variations than that of the conventional TWSTFT. Since 

	the DPN-based system has advantages of higher precision and lower 

	bandwidth cost, it is one of the most promising methods to improve 

	the international time transfer links.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{IEEE2012,

title = {Comparison of ray-tracing packages for troposphere delays},

author = {Vahab Nafisi and Landon Urquhart and Marcelo Santos and Felipe Nievinski and Johannes Boehm and Dudy Wijaya and Harald Schuh and Alireza Ardalan and Thomas Hobiger and Ryuichi Ichikawa and Florian Zus and Jens Wickert and Pascal Gegout},

doi = {doi:10.1109/TGRS.2011.2160952},

year  = {2012},

date = {2012-01-01},

journal = {IEEE Transactions on Geoscience and Remote Sensing},

volume = {50},

number = {2},

pages = {469-481},

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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JOG2011,

title = {Multi-technique comparison of troposphere zenith delays and gradients 

	during CONT08},

author = {Kamil Teke and Johannes Boehm and Tobias Nilsson and Harald Schuh and Peter Steigenberger and Rolf Dach and Robert Heinkelmann and Pascal Willis and Rüdiger Haas and Susana Garcia-Espada and Thomas Hobiger and Ryuichi Ichikawa and Shingo Shimizu},

doi = {doi:10.1007/s00190-010-0434-y},

issn = {0949-7714},

year  = {2011},

date = {2011-01-01},

journal = {Journal of Geodesy},

volume = {85},

number = {7},

pages = {395-413},

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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{IEEEIM11,

title = {Development of a GPU-Based Two-Way Time Transfer Modem},

author = {Tadahiro Gotoh and Jun Amagai and Thomas Hobiger and Miho Fujieda and Masanori Aida},

doi = {10.1109/TIM.2010.2091313},

year  = {2011},

date = {2011-01-01},

journal = {IEEE Transactions on Instrumentation and Measurement},

volume = {60},

number = {7},

pages = {2495-2499},

abstract = {We have developed a new two-way time transfer modem to improve the 

	time transfer precision of remote clock comparison. As a timing signal, 

	we apply a binary offset carrier, which is similar to those signals 

	used for the next-generation Global Navigation Satellite Systems. 

	We took advantage of versatile A/D and D/A converters, and most of 

	the digital signal processing stages were realized by software, running 

	on an off-the-shelf PC. This enabled us to realize the complete system 

	with cheaper equipment, leading to an affordable low-cost modem. 

	For the real-time digital signal processing stages implemented in 

	software, we relied on a graphics processing unit (GPU) developed 

	for computer game enthusiast. The developed modem can receive four 

	channels at the same time with a single GPU card. We performed two-way 

	satellite time transfer experiments using these modems between Japan 

	and Taiwan. The obtained results are consistent within 200 ps with 

	respect to the results of GPS carrier phase time transfer. As a consequence, 

	we improved the time transfer precision by nearly one order of magnitude 

	as compared to a conventional two-way modem without increasing the 

	connection fees caused by commercial communication satellites.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{EPS10,

title = {Fully automated VLBI analysis with c5++ for ultra-rapid determination 

	of UT1},

author = {Thomas Hobiger and Toshimichi Otsubo and Mamoru Sekido and Tadahiro Gotoh and Toshihiro Kubooka and Hiroshi Takiguchi},

url = {http://www.terrapub.co.jp/journals/EPS/abstract/6212/62120933.html},

year  = {2010},

date = {2010-01-01},

journal = {Earth Planets Space},

volume = {45},

number = {2},

pages = {75-79},

abstract = {VLBI is the only space-geodetic technique which gives direct access 

	to the Earth's phase of rotation, i.e. universal time UT1. Beside 

	multi-baseline sessions, regular single baseline VLBI experiments 

	are scheduled in order to provide estimates of UT1 for the international 

	space community. Although the turn-around time of such sessions is 

	usually much shorter and results are available within one day after 

	the data were recorded, lower latency of UT1 results is still requested. 

	Based on the experience gained over the last two years, an automated 

	analysis procedure was established. The main goal was to realize 

	fully unattended operation and robust estimation of UT1. Our new 

	analysis software, named c5++, is capable of interfacing directly 

	with the correlator output, carry out all processing stages without 

	human interaction and provide the results for the scientific community 

	or dedicated space applications. Moreover, the concept of ultra-rapid 

	VLBI sessions can be extended to include further well-distributed 

	stations, in order to obtain the polar motion parameters with the 

	same latency and provide an up-to-date complete set of Earth orientation 

	parameters for navigation of space and satellite missions.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{AI10,

title = {Ultra-Rapid dUT1-Observations with e-VLBI},

author = {Rüdiger Haas and Mamoru Sekido and Thomas Hobiger and Tetsuro Kondo and Shinobu Kurihara and D. Tanimoto and Kensuke Kokado and Jan Wagner and J. Ritakari and Mujunen A.},

doi = {10.2478/v10018-010-0007-6},

year  = {2010},

date = {2010-01-01},

journal = {Artificial Satellites},

volume = {45},

number = {2},

pages = {75-79},

abstract = {We give a short overview about the achievements of the Fennoscandian-Japanese 

	ultra-rapid dUT1-project that was initiated in early 2007. The combination 

	of real-time data transfer, near real-time data conversion and correlation, 

	together with near-real time data analysis allows to determine dUT1 

	with a very low latency of less than 5 minutes after the end of a 

	VLBI-session. The accuracy of these ultra-rapid dUT1-results is on 

	the same order than the results of the standard rapid-service of 

	the International Earth Rotation and Reference Frame Service (IERS). 

	The ultra-rapid approach is currently extended to 24 hour sessions 

	and is expected to become an important contribution for the future 

	next generation VLBI system called VLBI2010.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JOG10b,

title = {On the importance of accurately ray-traced troposphere corrections 

	for Interferometric SAR data},

author = {Thomas Hobiger and Youhei Kinoshita and Shingo Shimizu and Ryuichi Ichikawa and Masato Furuya and Tetsuro Kondo and Yasuhiro Koyama},

doi = {doi:10.1007/s00190-010-0393-3},

year  = {2010},

date = {2010-01-01},

journal = {Journal of Geodesy},

volume = {84},

number = {9},

pages = {537-546},

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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JOG10a,

title = {Asymmetric tropospheric delays from numerical weather models for 

	UT1 determination from VLBI Intensive sessions on the baseline 

	Wettzell-Tsukuba},

author = {Johannes Boehm and Thomas Hobiger and Ryuichi Ichikawa and Tetsuro Kondo and Yasuhiro Koyama and Andrea Pany and Harald Schuh and Kamil Teke},

doi = {doi:10.1007/s00190-010-0370-x},

year  = {2010},

date = {2010-01-01},

journal = {Journal of Geodesy},

volume = {84},

number = {5},

pages = {319-325},

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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{GPSSOL10,

title = {A GPU based real-time GPS software receiver},

author = {Thomas Hobiger and Tadahiro Gotoh and Jun Amagai and Yasuhiro Koyama and Tetsuro Kondo},

doi = {10.1007/s10291-009-0135-2},

year  = {2010},

date = {2010-01-01},

journal = {GPS Solutions},

volume = {14},

number = {2},

pages = {207-216},

abstract = {Off-the-shelf graphics processing units provide low-cost massive parallel 

	computing performance, which can be utilized for the implementation 

	of a GPS software receiver. In order to realize a real-time capable 

	system the crucial stages of the receiver should be optimized to 

	suit the requirements of a parallel processor. Moreover, the receiver 

	should be capable to provide wider correlation functions and provide 

	easy access to the spectral domain of the signals. Thus, the most 

	suitable correlation algorithm, which forms the core part of each 

	receivers should be chosen and implemented on the graphics processor. 

	Since the sampling rate of the received signal limits the real-time 

	capabilities of the software radio it is necessary to determine an 

	optimum value, considering that the precision of the observable varies 

	with sampling bandwidth. We are going to discuss details and present 

	our single frequency multi-channel implementation, which is capable 

	of operating in real-time mode. Our implementation differs from other 

	solutions by the wideness of the correlation function and allows 

	simple handling of data in the spectral domain. Comparison with output 

	from a commercial hardware receiver, which shares the antenna with 

	the software radio, confirms the consistency and accuracy of our 

	development.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JASTP10,

title = {Improving GPS positioning estimates during extreme weather situations 

	by the help of fine-mesh numerical weather models},

author = {Thomas Hobiger and Seiji Shimada and Shingo Shimizu and Ryuichi Ichikawa and Yasuhiro Koyama and Tetsuro Kondo},

doi = {doi:10.1016/j.jastp.2009.11.018},

year  = {2010},

date = {2010-01-01},

journal = {Journal of Atmospheric and Solar-Terrestrial Physics},

volume = {72},

number = {2-3},

pages = {262-270},

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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{IEEE2009,

title = {Computation of Troposphere Slant Delays on a GPU},

author = {Thomas Hobiger and Ryuichi Ichikawa and Yasuhiro Koyama and Tetsuro Kondo},

doi = {doi:10.1109/TGRS.2009.2022168},

year  = {2009},

date = {2009-01-01},

journal = {IEEE Transactions on Geoscience and Remote Sensing},

volume = {47},

number = {10},

pages = {3313-3318},

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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JASR2009b,

title = {The effect of neglecting VLBI reference station clock-offsets on 

	UT1 estimates},

author = {Thomas Hobiger and Yasuhiro Koyama and Johannes Boehm and Tetsuro Kondo and Ryuichi Ichikawa},

doi = {doi:10.1016/j.asr.2008.11.005},

year  = {2009},

date = {2009-01-01},

journal = {Advances in Space Research},

volume = {43},

number = {4},

pages = {910-916},

abstract = {Very Long Baseline Interferometry (VLBI) allows to monitor universal 

	time (UT1) by conducting regular international experiments. Such 

	dedicated observation networks are equipped with different hardware 

	components, which require different processing strategies when the 

	data are correlated. As the timing units at each stations are usually 

	offset with respect to universal time (UTC) this effect should be 

	considered during correlation processing. Thus, it is investigated 

	how neglecting of these offsets theoretically impacts the estimation 

	of UT1. Three different strategies for the proper handling of the 

	timing offset will be discussed and their advantages/drawbacks will 

	be pointed out. Moreover, it is studied how neglecting of these timing 

	offsets affects UT1 time-series and how such a missing correction 

	can be applied a posteriori. Although the discussed effect is for 

	most of the UT1 experiments smaller than the formal error of the 

	estimates, it is important to consider station clock offsets properly 

	in next-generation VLBI systems, which are expected to improve accuracy 

	of results by about one order of magnitude.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{EPS2009a,

title = {Estimation of scan-gap limits on phase delay connections in Delta 

	VLBI observations based on the phase structure function at a short 

	time period},

author = {Tetsuro Kondo and Thomas Hobiger and Mamoru Sekido and Ryuichi Ichikawa and Yasuhiro Koyama and Hiroshi Takaba},

url = {http://www.terrapub.co.jp/journals/EPS/abstract/6102/61030357.html},

year  = {2009},

date = {2009-01-01},

journal = {Earth, Planets and Space},

volume = {61},

number = {3},

pages = {357-371},

abstract = {The maximum scan-gap length which connects phase delays from scan 

	to scan over a gap is an important issue in Delta Very Long Baseline 

	Interferometry (D-VLBI), and it is affected by delay fluctuations 

	caused by the wet troposphere. It has recently become possible to 

	obtain near real-time fringe phases by using an e-VLBI technique 

	that realizes real-time VLBI by connecting stations through high-speed 

	Internet. Such real-time VLBI raises the possibility of dynamic D-VLBI 

	scheduling, which changes scan and gap length dynamically according 

	to the weather condition of the date. We have investigated this possibility 

	by using phase structure functions obtained from continuous VLBI 

	observations at S- and X-bands for 1–2 h at the Kashima, Gifu, and 

	Koganei stations (not real-time ones). Five VLBI sessions were conducted 

	during this study between March and July 2006 under different weather 

	conditions. At first a simple method was developed to evaluate phase 

	connectivity from a phase structure function. A model was also proposed 

	to estimate a phase-structure function at longer time periods from 

	a short time period. Finally, an available gap length was estimated 

	using the model. Our results show that it is possible to estimate 

	an available scan gap length by using a structure function at a time 

	period of 10 s. This suggests that it is possible to control scan 

	length and gap length dynamically in order to achieve the best performance 

	of D-VLBI observations.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JASR2009a,

title = {Integer phase ambiguity estimation in next-generation geodetic Very 

	Long Baseline Interferometry},

author = {Thomas Hobiger and Mamoru Sekido and Yasuhiro Koyama and Tetsuro Kondo},

doi = {doi:10.1016/j.asr.2008.06.004},

year  = {2009},

date = {2009-01-01},

journal = {Advances in Space Research},

volume = {43},

number = {1},

pages = {187-192},

abstract = {Next-generation Very Long Baseline Interferometry (VLBI) system designs 

	are aiming at 1 mm global position accuracy. In order to achieve 

	this, it is not only necessary to deploy improved VLBI systems, but 

	also to develop analysis strategies that take full advantage of the 

	observations taken. Since the new systems are expected to incorporate 

	four independent radio frequency bands, it should be feasible to 

	resolve phase ambiguities directly from post-correlation data, providing 

	roughly an order of magnitude improvement in precision of the delay 

	observable. As the unknown ambiguities are of integer nature, it 

	is discussed here how they the can be resolved analytically using 

	algorithms which have been developed for Global Navigation Satellite 

	System (GNSS) applications. Furthermore, it will be shown that ionosphere 

	contribution and source structure effects, so-called core-shifts, 

	can be solved simultaneously with the delay, which is the main geodetic 

	observable for follow-on analysis. In order to verify the proposed 

	algorithm, simulated observations were created using parameters from 

	actual design studies. It is shown that, even in the case of low 

	signal-to-noise ratio observations, reliable phase ambiguity resolution 

	can be achieved and it is discussed how the integer ambiguity recovery 

	depends on the number of observations and signal-to-noise ratio.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JGR2008,

title = {Fast and accurate ray-tracing algorithms for real-time space geodetic 

	applications using numerical weather models},

author = {Thomas Hobiger and Ryuichi Ichikawa and Tetsuro Kondo and Yasuhiro Koyama},

doi = {doi:10.1029/2008JD010503},

year  = {2008},

date = {2008-01-01},

journal = {Journal of Geophysical Research},

volume = {113},

number = {D203027},

pages = {1-14},

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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{EPS2008,

title = {Constrained simultaneous algebraic reconstruction technique (C-SART) 

	- a new and simple algorithm applied to ionospheric tomography},

author = {Thomas Hobiger and Tetsuro Kondo and Yasuhiro Koyama},

url = {http://www.terrapub.co.jp/journals/EPS/abstract/6007/60070727.html},

year  = {2008},

date = {2008-01-01},

journal = {Earth, Planets and Space},

volume = {60},

number = {7},

pages = {727-735},

abstract = {A simple and relatively fast method (C-SART) is presented for tomographic 

	reconstruction of the electron density distribution in the ionosphere 

	using smooth fields. Since it does not use matrix algebra, it can 

	be implemented in a low-level programming language, which speeds 

	up applications significantly. Compared with traditional simultaneous 

	algebraic reconstruction, this method facilitates both estimation 

	of instrumental offsets and consideration of physical principles 

	(expressed in the form of finite differences). Testing using a 2D 

	scenario and an artificial data set showed that C-SART can be used 

	for radio tomographic reconstruction of the electron density distribution 

	in the ionosphere using data collected by global navigation satellite 

	system ground receivers and low Earth orbiting satellites. Its convergence 

	speed is significantly higher than that of classical SART, but it 

	needs to be speeded up by a factor of 100 or more to enable it to 

	be used for (near) real-time 3D tomographic reconstruction of the 

	ionosphere.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{JASR2008,

title = {Using the Global Navigation Satellite System and satellite 

	altimetry for combined Global Ionosphere Maps},

author = {Sonja Todorova and Thomas Hobiger and Harald Schuh},

doi = {doi:10.1016/j.asr.2007.08.024},

year  = {2008},

date = {2008-01-01},

journal = {Advances in Space Research},

volume = {42},

pages = {727-736},

abstract = {For deriving global maps of the Total Electron Content (TEC) from 

	space geodetic techniques usually observations from the Global Navigation 

	Satellite System (GNSS) are taken. However, the GNSS stations are 

	inhomogeneously distributed, with large gaps particularly over the 

	sea surface. Within this study we create Global Ionosphere Maps (GIM) 

	from GNSS data and additionally introduce satellite altimetry observations, 

	which help to compensate the insufficient GNSS coverage of the oceans. 

	The obtained global maps are in 2 h intervals and daily values of 

	Differential Code Biases (DCB) for all the GNSS satellites and receivers 

	are also estimated. The combination of the data from around 160 GNSS 

	stations and two satellite altimetry missions – Jason-1 and TOPEX/Poseidon 

	– is performed on the normal equation level. The comparison between 

	the integrated ionosphere models and the GNSS-only maps shows a higher 

	accuracy of the combined GIM over the seas. The study aims at improved 

	combined global TEC maps, which should make best use of the advantages 

	of each particular type of data and have higher accuracy and reliability 

	than the results derived by the two methods if treated individually.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{EPSe2008,

title = {Ray-traced troposphere slant delays for precise point positioning},

author = {Thomas Hobiger and Ryuichi Ichikawa and Tomoji Takasu and Yasuhiro Koyama and Tetsuro Kondo},

url = {http://www.terrapub.co.jp/journals/EPS/pdf/2008e/6005e001.pdf},

year  = {2008},

date = {2008-01-01},

journal = {Earth, Planets and Space},

volume = {60},

number = {5},

pages = {e1-e4},

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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{GNSS2008a,

title = {Kashima Ray-tracing Service (KARATS) - On-line provision of total 

	troposphere slant delay corrections for East Asian sites},

author = {Thomas Hobiger and Ryuichi Ichikawa and Yasuhiro Koyama and Tetsuro Kondo},

year  = {2008},

date = {2008-01-01},

journal = {Proc. of the International Symposium on GPS/GNSS 2008 (peer-reviewed)},

number = {1},

pages = {40-44},

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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@article{GNSS2008b,

title = {An Evaluation of the Practicability of Current Mapping Functions 

	Using Ray-traced Delays from JMA Mesoscale Numerical Weather Data},

author = {Ryuichi Ichikawa and Thomas Hobiger and Yasuhiro Koyama and Tetsuro Kondo},

year  = {2008},

date = {2008-01-01},

journal = {Proc. of the International Symposium on GPS/GNSS 2008 (peer-reviewed)},

number = {1},

pages = {5-12},

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.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}