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Compact and strong radio sources were repeatedly observed in regular geodetic very long baseline interferometry experiments under the Key Stone Project. The flux densities of these sources in the S and X bands were estimated from a set of correlated amplitudes obtained through the correlation processing of the observed data. From ∼5 years of the observed data, irregular variations in the flux densities were detected for several radio sources using the source 2134+004 as the calibrator. The results showed the monitoring of flux density variations by means of very long baseline interferometry is feasible. Since the correlation processing of the observed data are performed real-time using a high-speed digital communication technique, this method gives a capability to monitor flux density variations of quasars and BL-Lac types of extragalactic radio sources.

Detailed observations of Jovian decimetric radiation (DIM) from relativistic electrons in the inner Jovian radiation belt was carried out to seek the existence of a short-term variation which has information on the electromagnetic disturbances in the inner Jovian magnetosphere. Results demonstrated the DIM flux enhancement continuing for a few days for the case of November 1996. It was shown that the relativistic electrons were highly anisotropic during the DIM flux enhancement. The correlation between the DIM flux enhancement and the solar F10.7 flux enhancement was also detected. A model simulation suggested that the observed DIM flux variation was caused by the enhanced radial diffusion due to the intense solar UV/EUV event which heated the Jupiter's upper atmosphere.

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.

[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.

[1] Currently, there are two types of interferometers in the world: One is the conventional interferometer using the fixed local signal as a reference signal, and the other is the interferometer using the wavefront clock. In this paper, we propose a new method of the wavefront clock system under development. The new wavefront clock system has a great advantage for the Doppler compensation. The Doppler shift, which is caused by the Earth's rotation, is compensated for during the correlation processing or when signals are received in order to detect fringes (interferometer patterns). Also, the proposed system can be effectively applied to high-frequency (millimeter wavelength), wide-bandwidth, and multibaseline interferometry. The main feature of this method is that the reference signals for the front end and back end of the interferometer system are directly controlled from the observing site according to a calculated a priori delay rate. In this method, fringe stopping and delay tracking can be simultaneously performed on all received frequencies as well as on frequency-converted signals in both the upper sideband (USB) and lower sideband (LSB). Furthermore, this method can be introduced with just a slight modification to the current interferometer systems (including very long baseline interferometry) and can also be applied to a pseudofringe (Doppler shift and delay) simulator to check the correlation processor.

[1] In geodetic very long baseline interferometry (VLBI), the observations are performed at two distinct frequencies (2.3 and 8.4 GHz) in order to determine ionospheric delay corrections. This allows information to be obtained from the VLBI observables about the sum of electrons per area unit (total electron content) along the ray path through the ionosphere. Because of the fact that VLBI is a differential technique, the calculated ionospheric corrections depend on the differences of the propagation media over the stations. Additionally, an instrumental delay offset per station causes a bias of the ionospheric measurements. This paper presents a method to estimate ionospheric parameters, that is, values of vertical total electron content from VLBI data, and compares the outcomes to results from other space geodetic techniques. As VLBI observations cover more than two complete solar cycles, the relation to space weather indices on long-term timescales can be shown.