Multi-Frequency VLBI (MFV) is one of the most powerful methods for precise positioning of spacecrafts. The system transmits three carrier waves at S-band and one wave at X-band. These frequencies are set to resolve the cycle ambiguity of carrier wave at X-band from two group delays between carrier waves and one phase delay of the carrier wave at S-band. The procedure to resolve the cycle ambiguity is proposed in this article. Some conditions about frequency variation and prediction of position and the ionosphere are also clarified for resolving the cycle ambiguity. The dedicated recording system for MFV is developed. A preliminary observation of MFV is carried out with this system by using Lunar Prospector. As a result of the experiment, residual phases from predicted ones are within ±2π, and the RMS of the residual for the period of several seconds is about 4 degrees. This result supports realization of the MFV.

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Using Australian records, we studied six isolated geomagnetic storms for the regional effects of their associated magnetic fields, emphasizing the linear regression correlation coefficient between station locations and indices. An innovation used here was to consider both the full magnetic disturbance field and a form of this field with the smoothed storm-time Dst values removed to feature the small variations. Magnetic H-component data from paired stations throughout the continent showed correlation values of +0.90 to +0.97. These results indicate the adequacy of the present density of observatories in Australia and their value for use with aeromagnetic surveys. The planetary magnetic ap index was found to have an inconsistent relationship to Australian H-component fields. The planetary magnetic Dst index was successfully represented by data from four Australian observatories. The discovered capability of Australian magnetic observatories to represent the magnetic storm Dst index in real time, combined with the established lognormal-form characteristic of Dst, means that Australian solar-terrestrial disturbance centers should be able to predict the time to recovery of quieter fields once an on-line Austrialian magnetic field Dst mainphase computation has been made.

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One-second resolution geomagnetic data from 5 stations located at low-latitudes (i.e., L = 1.2-1.6) were used to examine the local time dependence of the dominant frequency of Pi2 pulsations. We analyzed 183 Pi2 events simultaneously recorded at the 5 stations and discussed their possible generation mechanisms. The averaged dominant frequency of the H (horizontal) component is higher on the dawn side than that on the dusk side and shows a peak value in the post-midnight at around 03 LT, which confirms the LT dependence previously suggested from single station data (i.e., non-simultaneous observation). However, some of the events have no LT dependence. For the events which show the LT dependence at low-latitudes, we infer that the cavity resonance mode is a plausible generation mechanism, but we do not rule out the possibility of the plasmaspheric surface wave mode which has also been suggested. For the events having a common frequency between the eastern and western stations, we suggest the cavity resonance mode to be the mechanism. The averaged dominant frequency of the D component does not show any clear LT dependence, and only about 20% of Pi2s have identical frequency for both the H and D components, therefore we suggest that the H and D oscillations of Pi2s are generated from different mechanisms. We also found that the frequency of different parts of a Pi2 pulsation, i.e., the dominant frequency for leading part and trailing part of the pulsation, is different, and the frequency of the trailing part is lower than that of the leading part.

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Ionospheric equivalent currents are defined as spherical sheet currents, which reproduce the observed magnetic disturbances below the ionosphere. One way of determining these currents is to place several so called spherical elementary current systems (SECS) in the ionospheric height and to solve an inversion problem for the amplitudes of these systems. In previous studies this method has been applied to two-dimensional data sets, having both latitudinal and longitudinal spatial coverage (2D SECS method). In this paper a one-dimensional variant of this method (1D SECS) is developed. The 1D SECS method can be used even in those situations where the data set is one dimensional, e.g. with one meridionally aligned magnetometer chain. The applicability of the 1D SECS method is tested using both synthetic and real data. It is found that in real situations the errors in the 1D SECS results are 5-10% in current density profiles and -5% in integrated currents, when compared to the results of the more accurate 2D SECS method.

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Tweek atmospherics are ELF/VLF pulse signals with frequency dispersion characteristics that originate from lightning discharges and propagate in the Earth-ionosphere waveguide mode over long distances. In this paper, we estimate equivalent nighttime electron densities at reflection heights in D-region ionosphere at low-middle latitudes by accurately reading the first-order mode cut-off frequency of tweek atmospherics. The estimation method was applied to tweek atmospherics received simultaneously at Moshiri and Kagoshima in Japan. Equivalent electron densities ranged from 20-28 el./cm³ at ionospheric reflection heights of 80-85 km. Comparing our estimates with electron density profiles obtained from the IRI-95 model, MF radar measurements, and rocket experiments revealed almost consistent results for the lower part of the D-region ionosphere. The tweek method has the unique advantage of enabling reflection-height (equivalent electron densities) monitoring over a wide area of several thousand kilometers.

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The nature and location of the present EUR/AFR boundary configuration in the Azores region is controversial and, in particular, the pattern of the present day deformation associated with plate boundary processes, is still unknown. We present here GPS data from 10 stations, in a segment of this boundary (Terceira Island), where data at 2 epochs (1999 and 2001) are available. GPS data were processed using GAMIT and FONDA, with horizontal position repeatability circa 3 mm and 2 mm, respectively, for 1999 and 2001 epochs. Stations horizontal displacements range from 0.96 to 4.60 mm/yr. Strain analysis indicates compression for four sub-networks. The rates of local rotation are consistent for three sub-networks on the western part of Terceira, exhibiting a clockwise rotation of about 8° per million years. Although they are still preliminary, these results are discussed in relation with the expected behaviour from regional geodynamic constraints and past geodetic studies.

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We estimate long-term/permanent crustal deformation in the Nankai forearc, southwest Japan, that accumulated over one earthquake deformation cycle in the Nankai subduction zone. A short-term deformation due to an oblique subduction of the Philippine Sea plate is modeled based on coseismic fault slips associated with interplate thrust earthquakes at the Nankai Trough, and subtracted from the interseismic crustal velocity field observed by GPS. The long-term/permanent deformation left in the data shows arc-parallel movement of the forearc at a rate of 5-10 mm/yr. We interpret that the forearc movement, driven by the oblique plate convergence, is accommodated by steady aseismic slip on the deep portion of the fault plane of the Median Tectonic Line (MTL) on the rear boundary of the Nankai forearc. The rate of the forearc movement is consistent with the geological slip rate of the MTL in the late Quaternary, and also with that expected from deflection of the slip vector of the 1946 Nankai earthquake from the current plate convergence vector.

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A GPS/Acoustic experiment on the southeastern slope of Hawaii Island presented precise seafloor positioning in the condition of large water depth (2.5-4.5 km) and large velocity variations. We estimated sound velocity variations from acoustic ranging, and found that temperature variation can well explain the velocity variation. The effect of daily variation in the sound velocity amounted to +/- 0.7 m on acoustic ranging of 4-7 km with a fixed velocity structure. CTD data observed about every 3 hours could decrease the range residuals to +/- 0.4 m. These large residuals were fairly well canceled in the positioning of the array center of three acoustic transponders. The estimated precision of the array center positioning was about 3 cm in latitude and longitude.

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