Search

The frequency accuracy of optical atomic clocks has dramatically increased over the past 15 years, improving by more than two orders of magnitude from 16 digits of precision to 18 or even 19 digits of precision. Since around 2015 researchers from around the world began to consider a redefinition of the second that uses optical atomic clocks. Since then, the development of optical atomic clocks has progressed and the recent results demonstrated to detect the frequency change with 18 digits of precision.The National Institute of Information and Communications Technology (NICT) has developed the Sr optical lattice clock and optical ion clocks employing In+ and Ca+, as well as a Sr optical lattice clock that provides calibration data to BIPM. On the other hand, the centimeter-level uncertainty of site elevation has caused 10-18-level frequency uncertainties of optical frequency standards. Therefore, it is significantly important to understand frequency changes caused by solid-earth tides that often range from 10 to 20 cm in amplitude, by oceanic tidal loading, crustal deformations due to earthquakes, and ground movements with groundwater changes for the stable operation of optical atomic clocks.NICT, in collaboration with partners including the Geospatial Information Authority of Japan (GSI), has begun observations and data analysis to evaluate how these effects interact with optical atomic clocks. Since early 2021, NICT and GSI have been jointly conducting leveling surveys and relative gravimeter observations at NICT’s headquarters in Koganei. These observations reduce the contribution of gravitational redshift to the total uncertainty of the NICT-Sr1 optical lattice clock has been reduced to the 10-19 level.With the support of National Institute of Polar Research (NIPR), absolute gravity measurements were performed in the August 2019 and May 2022 to evaluate the effects of the 2011 March 11 Tohoku megaquake on coseismic vertical crustal movement. The obtained absolute gravity change between the two periods was -43.8 μGal. This matches the trend of GNSS result obtained by GSI, which show a vertical movement of up to 31.5 mm from August 2019 to May 2022, equivalent to about -10 μGal gravity change, even though the values do not agree precisely.We have introduced a Micro-g LaCoste's gPhoneX gravimeter for continuous gravity measurements near by the optical clocks in the end of 2021. The preliminary results over seven months detects stable gravity change due to solid-earth tide with about 22 μGal precision. In addition, we have started to investigate the temporal variation of the ground water level at Koganei. We are also monitoring vertical crustal movements by geodetic GNSS measurements. We will investigate uncertainties of optical clocks due to vertical movements caused by geodetic phenomena using continuous gravity and GNSS measurements.

The comparison of distant atomic clocks is foundational to international timekeeping, global positioning and tests of fundamental physics. Optical-fibre links allow the most precise optical clocks to be compared, without degradation, over intracontinental distances up to thousands of kilometres, but intercontinental comparisons remain limited by the performance of satellite transfer techniques. Here we show that very long baseline interferometry (VLBI), although originally developed for radio astronomy and geodesy, can overcome this limit and compare remote clocks through the observation of extragalactic radio sources. We developed dedicated transportable VLBI stations that use broadband detection and demonstrate the comparison of two optical clocks located in Italy and Japan separated by 9,000 km. This system demonstrates performance beyond satellite techniques and can pave the way for future long-term stable international clock comparisons. Very long baseline interferometry is used to compare two optical clocks located in Japan and Italy through the observation of extragalactic radio sources. This approach overcomes limitations of the performance of satellite transfer techniques.

We have developed a broadband VLBI (very long baseline interferometry) system inspired by the concept of the VLBI Global Observing System (VGOS). The new broadband VLBI system was implemented in the Kashima 34 m antenna and in two transportable stations utilizing 2.4 m diameter antennas. The transportable stations have been developed as a tool for intercontinental frequency comparison but are equally useful for geodesy. To enable practical use of such small VLBI stations in intercontinental VLBI, we have developed the procedure of node-hub style VLBI: In joint observation with a large, high sensitivity ‘hub’ antenna, the closure delay relation provides a virtual delay observable between ‘node’ stations. This overcomes the limited sensitivity of the small diameter node antennas, while error sources associated with large diameter antennas, such as gravitational deformation and delay changes in necessarily long signal cables, are eliminated. We show that this scheme does not result in an increased sensitivity to radio source structure if one side of the baseline triangle is kept short. We have performed VLBI experiments utilizing this approach over both short range and intercontinental distance. This article describes the system components, signal processing procedure, experiment, and results in terms of baseline repeatability. Our measurements reveal signatures of structure effects in the correlation amplitude of several of the observed radio sources. We present a model of the frequency-dependent source size for 1928+738 derived from correlation amplitude data observed in four frequency bands.

Space-Time Standards laboratory in National institute of Information and Communications Technology (NICT), Japan, has various activities of time and frequency metrology. NICT generates and disseminates Japan Standard Time (JST. In addition to the headquarter in Koganei, Tokyo, we built a substation of JST in Kobe, which is in western part of Japan. Every place in Japan has a potential to be attacked by a large earthquake. The combination of two stations provides a resilience. Two optical frequency standards are being developed. One is a strontium lattice clock and the other is a single In+ trap. The lattice clock is recognized as a secondary represenataion of the Second. Various technique of the time and frequency transfer is under developing, including GNSS, TWSTFT, VLBI and fiber link. We developed the two-way carrier phase technique and developed a modem for it, which is now commercially available in Japan. VLBI does not need satellites. We can compare the clocks without depending on GNSS providers or communication satellites. These various activity will be briefly introduced.

For the aim to enable precise frequency transfer over earth radius scale distance, NICT developed a broadband VLBI system observing 3-14 GHz radio frequency range, and that was implemented in a pair of transportable 2.4m diameter VLBI stations and Kashima 34m diameter VLBI station. The small diameter VLBI stations installed in Medicina observatory of INAF (Italy) and NICT headquarters in Koganei (Japan) were used compare the atomic frequency standards. The poor sensitivity of VLBI observation with the small antenna pair was compensated by participation of the large diameter antenna for boosting the signal to noise ratio (SNR) of the VLBI observation. Owing to the broad observing frequency range, a few pico seconds precision of group delay is achieved on intercontinental baseline even with small diameter antenna. Advanced digital data acquisition with highspeed 16 GHz sampling enabled stable group delay measurement. A series of frequency transfer experiments was conducted during Oct. 2018 – Feb. 2019 by using the broadband VLBI system between ytterbium (Yb) optical lattice clock of INRiM (Italy) and strontium (Sr) lattice clock of NICT(Japan) separated by 9000km. The deviation of fractional frequency ratio between Yb and Sr was obtained by VLBI observation as y(Yb/Sr)=2.5(2.8)x10-16. The uncertainty in this experiment is evaluated not to be limited by the VLBI link itself, and further improvement is expected. Prospect of VLBI frequency link and uncertainty budget in VLBI observation is discussed in this report.

Since the establishment in 2013, GGOS Japan, formerly known as GGOS Working Group of Japan, has actively contributed to domestic and international space-geodetic activities. Until it was established, six Japanese agencies with their own backgrounds and missions had individually conducted geodetic observations of GNSS, VLBI, SLR, DORIS and gravimetry in Japan and Antarctica. GGOS Japan was established to strengthen the collaboration between the agencies and to get connected to international organizations such as IAG and GGOS.Its core members consist of a chair, a secretary, a lead of the outreach working group, a lead of the DOI working group and representatives of five core techniques. It is supported by tens of people in Japan and also by its parent entity, IAG subcommittee in Japan.This presentation will cover our achievement such as assembling our site list, hosting various domestic/international meetings, planning a special issue in a domestic journal, producing its leaflet and website and so on. Since 2017 it has been approved as an GGOS Affiliate and it is remarkable that Basara Miyahara was elected as GGOS President in 2019.

We have started to develop a next-generation microwave radiometer to be used in millimeter-wave spectroscopy for the high-resolution and high-precision monitoring of water vapor behavior. The new radiometer will be suitable for not only space geodetic techniques such as VLBI and GNSS, but also field measurements to monitor, for example, volcanic activities and cumulonimbus cloud generation. The planned front-end system for our new microwave radiometer has a wide bandwidth feed of 20–60 GHz. A signal from the feed is separated into two linear orthogonal polarized signals using an orthomode transducer (OMT); one is in the 20–30 GHz feed and the other is in the 50–60 GHz feed. We are now planning to cool the wideband feed, OMT, and LNA for each signal at 77 K using a Stirling cryocooler to improve the signal-to-noise ratio. We assembled a room-temperature 20–30 GHz receiver without the cooling system until the middle of 2019 as a first step of our development. We implemented the new receiver into the 3.7 m dish at Okinawa Electromagnetic Technology Center, National Institute of Information and Communications Technology (NICT), and carried out the first measurements using this receiver for validation tests in October 2019. Quick-look data obtained by the new receiver shows good power signals for the expected receiving band of 18–28 GHz. We are now developing another receiver for a higher band of 50–60 GHz, and we are going to implement the second one into the new prototype radiometer by the end of this fiscal year.

We have developed a state-of-the-art microwave radiometer named KUMODeS (KEK Universal Moisture and Oxygen Detection System) using millimeter-wave spectroscopy to monitor water vapor behavior. We have carried out comparative measurements of precipitable water vapor (PWV) to investigate the potential of KUMODeS/PWV measurements. Although further investigation is required to evaluate the performance of KUMODeS quantitatively, the preliminary results of PWV comparisons imply that the KUMODeS technology will be useful for retrieving the accurate behavior of water vapor with high temporal resolution.

We performed a frequency comparison of lattice clocks in Tokyo and Torino for the first time using broadband VLBI technique. Two portable antennas with 2.4 m diameters were installed at NICT Koganei headquarters and INAF Medicina Radio Observatory, close to Bologna, Italy, realizing a VLBI network together with Kashima large-aperture antenna of 34 m diameter. The clock signal at INAF was evaluated by INRIM using an optical fiber link between Medicina and Torino. The fractional frequency difference that VLBI evaluated was consistent with the difference between two HM frequencies which were separately calibrated by a Sr lattice clock at NICT and a Yb lattice clock at INRIM.

Wideband antennas for VLBI have been under development in NICT Kashima and the status of the project was reported in the poster session. An aim of this wideband observing system is Time and Frequency Transfer, the comparison of atomic clocks separated by long distances of several hundred or thousand kilometers with VLBI, as reported in “Broadband VLBI System GALA-V” by M. Sekido et al. This report focuses on our wideband antenna systems.

Interferometric Synthetic Aperture Radar (InSAR) phase data include not only signals due to crustal movements but also those associated with microwave propagation delay through the atmosphere. In particular, the effect of water vapor can generate apparent signals on the order of a few centimeters or more, and prevent us from detecting such geophysical signals as those due to secular crustal deformation. In order to examine if and to what extent numerical weather model (NWM) outputs are helpful to reduce the tropospheric delay signals at spatial scales of 5～50 km wavelengths, we compared three approaches of tropospheric signal reduction, using 54 interferograms in central Hokkaido, Japan. The first approach is the conventional topography-correlated delay correction that is based on the regional digital elevation model (DEM). The second approach is based on the Japan Meteorological Agency's operational meso-scale analysis model (MSM) data, where we compute tropospheric delays and subtract them from the interferogram. However, the MSM data are available at predefined epochs, and their spatial resolution is about 10 km, and therefore we need to interpolate both temporally and spatially to match with interferograms. Expecting to obtain a more physically plausible reduction of the tropospheric effects, we ran a 1-km mesh high-resolution numerical weather model WRF (Weather Research and Forecasting model) by ourselves, using the MSM data as the initial and boundary conditions. The third approach is similar to the second approach except that we make use of the WRF-based tropospheric data. Results show that if the topography-correlated phases are significant, both the conventional DEM-based approach and the MSM-based approach reveal comparable performances. However, when the topography-correlated phases are insignificant, none of the approaches could efficiently reduce the tropospheric phases. Although it could reduce the tropospheric signals in a local area, in none of the case studies did the WRF model produce the "best" performance. Whereas the global atmospheric model outputs are shown to be effective in reducing long-wavelength tropospheric signals, we consider that further improvements are needed for the initial and boundary condition data for high-resolution NWM, so that the NWM-based approach will become more reliable even in the case of a non-stratified troposphere.

We carry out an error analysis of 24-hour global positioning system (GPS)-very long baseline interferometry (VLBI) (GV) hybrid observation data. In this paper, we focus on the impacts of broadcast and final orbits on the GPS delays of the GV hybrid observation by analyzing the residuals, observed - calculated (O-C) values. The residuals show apparent and consistent biases for L1 and L2 signals, respectively. The scatters of the residuals are around a few nanoseconds. The main cause of those observation errors is the absence of the GPS phase and delay calibration system. Most of the satellites show that the differences between the delays, to which broadcast and final orbits are applied, are about 100 times smaller than the current GV hybrid observation errors. We conclude that GPS delays are not greatly affected by orbit accuracies.

Radio frequency (RF) direct sampling is a technique used to sample RF signals that are higher than the sampling rate, without the use of a frequency converter and an anti-aliasing filter. In the case of geodetic VLBI, the RF frequency is at most 9 GHz. Recently, a digital sampler with high sensitivity at RF frequencies greater than 10 GHz was developed. The sampler enables us to evaluate the use of the RF direct sampling technique in geodetic VLBI. RF direct sampling has the potential to make the system simple and stable because, unlike a conventional system, analog frequency converters are not used. We have developed two sets of RF direct sampling systems and operated them on Kashima and Tsukuba baseline (about 50 km length) in Japan. At first, we carried out the VLBI experiment only for X band (8 GHz) signals and successfully got the first fringes. Aliased signals could be discriminated through correlation processing. Then, we adopted RF direct sampling for mixed signals, i.e., S band (2 GHz) and X band signals are combined with each other to make a geodetic VLBI observation. We carried out a 24 hr geodetic VLBI session on 2011 October 19 and succeeded in fringe detection for both S and X bands. After correlation processing, baseline analysis was carried out and we got results consistent with those obtained by conventional VLBI.

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.

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.

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.

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.

We obtain a set of pointed wet delay observations at a total of 81 stations at 9 separate time epochs by ray tracing through the output fields of a Japanese numerical weather model with a grid spacing of 10 km. For each station-epoch, we invert the simulated data set, consisting of 4000 pointed delays, using an isotropic and an anisotropic delay model. The isotropic model has only one parameter-the zenith wet delay (ZWD). The anisotropic delay model of Chen and Herring has two additional lateral gradient parameters. We find that the anisotropic model leads to a much better fit between the delay model and the delay observations. On the other hand, the ZWD parameters estimated using the differing delay models are almost identical. This implies the ZWD estimates are not greatly perturbed by lateral heterogeneity of the atmosphere.

Terahertz (THz) dual comb spectroscopy (DCS) is a promising method for high-accuracy, high-resolution, broadband THz spectroscopy. In this paper, we have demonstrated adaptive-sampling THz-DCS, allowing the use of unstabilized dual femtosecond lasers.

Accepted: 2006-10-13, 資料番号: SA1000577000

GPS analysis requires statistical information on temporal variations of precipitable water vapor content (PWC), since temporal variability of PWC is assumed in the analyses. Spatial scales of PWC variations also are necessary for incorporation of the analyzed GPS data into numerical weather prediction systems. The objective of the present study is to investigate temporal and spatial characteristics of the PWC variations in Japan. For this purpose, slant-path PWC was observed with ground-based microwave radiometers (MWRs) for the directions of GPS satellites in the eastern part of the Kanto Plain during several observation periods in 2000 and 2001. Three components, vertical PWC, horizontal gradient and higher-order inhomogeneity, were retrieved from the slant-path observation data. Deviations of vertical PWC from ten-day averages (hereafter referred to as vertical deviations) were then calculated in order to remove seasonal changes of this component. Simultaneous observation data at three MWR sites also were used for the rough estimation of horizontal scales of the three PWC components. The results show: 1) The vertical deviations marked were about 20 times as large amplitudes as the other two components, while the variations due to the gradient had even smaller amplitudes than the inhomogeneity; 2) The vertical deviations had large spectral power against periods around 5-6 days and 8-9 days, while the gradient was dominated by diurnal variations; and, 3) It was roughly estimated that the vertical deviations (gradient) had the horizontal scales of several hundred (several ten) kilometers. The horizontal scale of the PWC inhomogeneity was considered to be less than 10 km. The vertical deviations (the gradient) were considered to be closely related with large-scale meteorological disturbances (local-scale circulations) on the basis of the above temporal and spatial characteristics.

In order to study small-scale water vapor variations over distances from a few km to 20 km, two campaign observations with a dense GPS network were carried out for 2.5 months in total at Tsukuba, Japan. For the observations 79 GPS antennas were installed at 75 sites within a 20 km by 20 km square area, at 1 to 3 km intervals. The PCV models provided by the US National Oceanic and Atmospheric Administration (NOAA) were applied to remove unmodeled phase center variation (PCV) specific to GPS antenna type. In addition, new PCV maps (MPS map) were constructed for all the antennas by stacking one-way postfit residuals over both campaign periods, to remove not only azimuth dependent PCV, but also the errors due to multipath effects. After MPS maps were introduced into the analysis, strong elevation dependence as well as azimuth dependence of postfit phase residuals, almost disappeared for all the antennas. In addition, the time variations in postfit residuals which were common to all the GPS sites, were subtracted to remove satellite orbits and/or clock errors. This led to the accurate estimate of slant path delay (SPD), which enabled the SPD to be applied to tomography analyses of water vapor (Seko et al. 2003). The horizontal scale of SPD was estimated using correlation distributions. As a result, the horizontal scale of the zenith total delay, the gradient component, and the postfit residual may be roughly considered as 644±120 km, 62±23 km, and 2-3 km, respectively. Improvement of the postfit residuals following the application of MPS maps also showed a positive impact on PWV estimation. Systematic biases of GPS derived PWV between different antenna types (Trimble and Ashtech) were reduced, resulting in a better agreement of GPS PWV, with RMS errors of 2.0 mm or less relative to PWV by rawinsonde or water vapor radiometer observations. The distribution of time-averaged PWV estimated at the 75 GPS sites showed a systematic pattern which has a negative correlation with the antenna height of each site.

Simultaneous GPS and water vapor radiometer (WVR) observations were carried out in Tsukuba during May–June 1998, for the validation of precipitable water content (PWC) gradients estimated from single-site GPS data. Slant path PWC observed by WVR were fitted into hourly PWC gradients (WVR gradients) using the least-square method. GPS PWC gradients were retrieved from tropospheric delay gradients that were estimated with GIPSY OASYS 2 package (GIPSY gradients). The results indicate that GIPSY gradients had good, linear correlation with WVR gradients, especially for a large gradient range. Both gradients had spike-shaped, short time-scale (∼ hours) peaks which were mostly associated with synoptic fronts. The GIPSY gradients were also compared with meso-scale PWC gradients calculated from zenith wet delay data of GPS network (NET gradients). The results show that GIPSY gradients did not have very good correlation with NET gradients, and that significant meso-scale discrepancy existed between the two gradients for a cold frontal case on 19 June 1998. One possible reason for this discrepancy is vertical differences in RH gradients, because GIPSY gradients are sensitive to RH gradients around the scale height of humidity (∼2500 m) while RH gradients in lowermost level have largest weights for NET gradients. To study PWC gradients associated with the fronts, GPS gradients were compared with other meteorological data over the Kanto Plain for two frontal cases. The results indicate that large PWC gradient zones with horizontal scale of about several tens kilometers in cross-frontal directions were collocated with the surface wind shear zones of the fronts. This suggests that the large PWC gradients were due to humidity discontinuity around the fronts.

Since 1995, VLBI measurements using fixed VLBI stations around the Tokyo metropolitan area have been continually producing data of station positions and baseline lengths. The accuracy of baseline length measurements is evaluated in terms of repeatability, conventionally defined as a standard deviation of those obtained by five continuous sessions. Continuous improvement both in system hardware and in the observation method have resulted in a remarkable improvement in measurement accuracy. Repeatability reaches about a 2-mm level in baseline length in our VLBI network.

Post-seismic deformations in the focal region of the 1995 Hyogo-ken Nanbu earthquake (M=7.2), in southwest Japan, which occurred on January 17 (JST), were observed with GPS observations. We deployed more than 30 GPS stations in and around the area of aftershock activity within several days after the occurrence of the earthquake. First the data were stored and manually collected at the observation sites, but by the middle of February the data at most of sites were started to be telemetered through public phone lines to data centers. GPS observations continued until the end of March, except for 4 sites left active for monitoring long-term deformations. We used two types of GPS observations, usual static and real-time kinematic GPS observations. Static day-by-day analyses, which are based on 24-h data sampled every 30 s, show the deformation associated with post-seismic readjustment with a relaxation time of about 70 days and with an amplitude of 1-2 cm in the area close to the epicenter. Also, rapid movements were observed just after the occurrence of the earthquake. However, since our analyses are only preliminary, further analyses are needed to clarify these observations and to investigate models of post-seismic deformations. Real-time kinematic GPS observations, which is a new type of GPS observations, were made every 1 s from February 9 to March 22 for detecting possible rapid relative movements between two sites across the surface fault trace on Awaji Island and across the other fault, the Arima-Takatsuki Tectonic Fault on the Kobe side. Present real-time kinematic GPS observations show scatters with the amplitude of 0.5-3.0 cm depending on components and baseline lengths. Any motions with amplitudes larger than these noise levels could not be detected during the observation period.

We have been developing a state-of-art tool to estimate the atmospheric path delays by ray-tracing through meso-scale analysis (MANAL data) data, which is operationally used for numerical weather prediction by Japan Meteorological Agency (JMA). The tools, which we have named “KAshima RAytracing Tools (KARAT)”, are capable of calculating total slant delays and ray-bending angles considering real atmospheric phenomena. The KARAT can estimate atmospheric slant delays by an analytical 2-D ray-propagation model by Thayer and a 3-D Eikonal solver. The biases of slant delay estimates between the Thayer model and the modern mapping functions, which are ranging from 18 to 90 mm, are considered to be a deficiency of the mapping functions. We compared PPP solutions using KARAT with that using the Global Mapping Function (GMF) and Vienna Mapping Function 1 (VMF1) for GPS sites of the GEONET (GPS Earth Observation Network System) operated by Geographical Survey Institute (GSI). In our comparison 57 stations of GEONET during the year of 2008 were processed. The KARAT solutions are slightly better than the solutions using VMF1 and GMF with linear gradient model for horizontal and height positions. Our results imply that KARAT is a useful tool for an efficient reduction of atmospheric path delays in radio based space geodetic techniques such as GNSS and VLBI.

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.

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 μs level with maximum differences of up to 50 μs, which is clearly above the formal uncertainties of the UT1 estimates (between 5 and 20 μs). 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.

[1] 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.

[1] Global Navigation Satellite Systems like the Global Positioning System and the Russian counterpart GLONASS permit the estimation of ionosphere parameters in the form of vertical total electron content (vTEC) values. Although the GNSS reference systems are rotation-symmetric ellipsoids, simple approximations of gravitational equipotential surfaces, the modeling of global vTEC maps is carried out in spherical systems, since this simplifies mathematical expressions. Due to the Earth's oblateness this approximation is in slight disagreement to the geometric situation of the observations. Thus it is expected that estimated vTEC values will change when the WGS84 ellipsoid is taken as a reference for all computations. We will discuss both, the direct effect (i.e due to a shift of the ionospheric pierce point) and the indirect effect, caused by a change of the mapping function, on global vTEC values using a single layer model of the ionosphere. We show that the estimated global TEC maps change slightly by up to ±0.4 TECU when refined mathematical models are considered and we discuss how differential code biases are affected by this change.

We simultaneously calculate atmospheric parameters (zenith wet delay and a gradient vector) and positioning errors estimated from tropospheric slant delays using ray tracing technique through a one epoch data set of the mesoscale non-hydrostatic numerical model. In this numerical calculation both isotropic and anistropic mapping functions are evaluated. We find that the positioning errors are not reduced by an anisotropic mapping function, with the exception of the north-south component. Elevation weighting is more important than mapping function type for reducing horizontal errors with the present specific data set. Large horizontal and vertical positioning errors associated with topography and mountain lee wave effect are indicated.

In order to search for new VLBI sources in the Galactic plane that can be used as phase reference sources in differential VLBI, we have conducted 22 GHz observations of radio sources in the Galactic plane using the Japanese VLBI Network (J-Net). We have observed 267 VLBI source candidates selected from existing radio surveys and have detected 93 sources at the signal-to-noise ratio larger than 5. While 42 of the 93 detected sources had already been detected with VLBI at relatively lower frequency (typically 2 to 8 GHz), the remaining 51 are found to be new VLBI sources detected for the first time. These VLBI sources are located within $|b|\le 5^\circ$, and have a large number of Galactic maser sources around them. Thus, they are potential candidates for phase reference sources for VLBI Exploration of Radio Astrometry (VERA), which is the first VLBI array dedicated to the phase referencing VLBI astrometry aiming at measuring the parallax and proper motion of maser sources in the whole Galaxy., Comment: to be published in PASJ (2000, Vol.52, No.5), 13 pages and 2 figures

Communications Research Laboratory has established four space geodetic observation sites in and around Tokyo, Japan under the Key Stone Project (KSP) [KOYAMA et al, 1997]. At each of the four sites (i.e. Koganei, Kashima, Miura, and Tateyama), an 11-m antenna VLBI system, a 75-cm telescope SLR system, and a dual-frequency geodetic GPS receiver system are collocated closely with each other. The relative positions of the three reference points are repeatedly measured by ground survey measurements. The comparisons of the site coordinates and velocities estimated from three different space geodetic techniques are considered to be quite important to understand error sources of each measurement technique and to evaluate consistency of the coordinates and velocities defined in a conventional terrestrial reference frame. In this paper, site velocities estimated from VLBI and GPS observations are compared.

A space geodetic observation network has been established around Tokyo, Japan under a project name of Key Stone Project by Communications Research Laboratory. Three space geodetic methods, i.e. Very Long Baseline Interferometry, Satellite Laser Ranging, and Global Positioning System, are involved in the project. As of September, 1997, VLBI and GPS observation facilities at all four stations are operational, whereas developments of SLR observation facilities are in course of final alignment procedures. Daily VLBI observations began in January 1995 with a single baseline between Koganei and Kashima, and the full network observations with four stations began in September 1996. Observations and data analysis of VLBI measurements are fully automated and the analysis results are produced shortly after all observations of an experiment session finished. GPS observations at four sites began in July 1997 and the automatic data collection and analysis system are under developments.

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.