Taizoh Yoshino

Displacement measuring technique for satellite-to-satellite laser interferometer to determine Earth's gravity field

We present a new displacement measuring technique with simplicity, robustness, high sensitivity and wide measurement range. A set of a frequency shifter and a voltage–frequency converter is used to lock a homodyne interferometer on the half-bright fringe by eliminating the Doppler fringe resulting from mirror motion. The mirror displacement is directly retrieved from the feedback signal of a fringe control loop. By developing a table-top interferometer, we successfully demonstrated signal recovery without significant degradation. The achieved displacement sensitivity and measurement range of the interferometer were 24 nm Hz−1/2 and 1.3 mm at a Fourier frequency of 0.1 Hz, respectively. This technique was found to have a potential for application to precise displacement measurements. It is, in particular, suitable for a satellite-to-satellite laser interferometer to determine Earths gravity field.

Evaluation of repeatability of baseline lengths in the VLBI network around the Tokyo metropolitan area

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.

Japan-US time comparison experiment for realizing better than 1-ns accuracy by using a radio interferometric technique

A zero baseline interferometry (ZBI) experiment was conducted to determine the differential instrumental delay between two stations (Kashima, Japan, and Richmond, USA). It enabled the authors to find an absolute time comparison with an accuracy of better than 1 ns. ZBI experiments in Japan were conducted in winter and summer to find the delay dependency on temperature. >

Lunar laser ranging by optical transponder collocated with VLBI radio sources on the Moon

The lunar laser ranging (LLR) has been dependent on the retro-reflectors placed on the Moon since the past three decades before. In spite of the technical improvement, regular observations are performed by a limited number of stations due to the weak echo. To allow more opportunities for observations, it is most effective to place modern device on the Moon. Since a Japanese second lunar probe is planned to be launched in 2006, the ideas are collected for scientific purposes. It is described here that an optical transponder on the Moon would enhance the possibility of LLR observations to a large degree. It is also proposed to have three radio sources on the Moon to determine the angular component perpendicular to the Earth-Moon vector and librations. One of the most important scientific targets for the proposed mission is a relativistic experiment which was not attained by a conventional LLR.

Far-Field Deformation due to Volcanic Activity and Earthquake Swarm

The KSP network of the Communications Research Laboratory, a regional VLBI network around Tokyo, detected significant displacements from volcanic and seismic activities in the Izu islands, located 100-300 km from the network. A 13 0km baseline revealed a gradual shortening of 4.5 cm over two months. Of particular importance is that the real-time VLBI monitoring has been operational on a daily basis, which enables us to investigate the development of source activity. The finding also raises interesting questions. Can we model the far-field deformation in terms of dislocations buried in a homogeneous half-space ? May we safely neglect the earths sphericity, stratification and self-gravitation? The Izu islands event provides us with a rare opportunity to answer these questions because it includes both tensile dislocation and shear dislocations. We compute horizontal displacements for several cases: (1) an elastic homogenous half-space, and (2) a radially stratified perfectly elastic earth. The two models give remarkably different results for tensile dislocation. The result clearly indicates that far-field displacement should be analyzed in the framework of spherical earth theory.

Development of VLBI time transfer system toward 0.1 nsec accuracy using transportable ground terminal for calibration

To improve the accuracy of time transfer in very long baseline interferometry (VLBI) technique, it is planned to perform a short baseline interferometry (SBI) experiment instead of zero baseline interferometry (ZBI) which was demonstrated in Japan-US baseline in 1986. A transportable VLBI receiving system which consists of 1.2 m antenna and delay calibration system is under development. The source flux available in SBI is weaker than ZBI because extended radio sources are resolved with distance. However, an improved VLBI data recorder with recording speeds up to 256 Mbps provides us higher sensitivity to observe weak sources in spite of small aperture antenna. Using the above system, it is easier to apply any VLBI stations for time transfer. It is expected that the new system will have 0.1 nsec level accuracy in time transfer. To confirm time transfer accuracy, a closure test will be made using more than three stations.<<ETX>>

The first comparative experiment of VLBI and two-way time transfer with better than 1 nsec precision

Time transfer technique comparison between very long baseline interferometry (VLBI) and two way satellite method was firstly performed between Koganei (Tokyo) and Kashima (Ibaragi) over 110 km baseline in November, 1992. Clock variation in two stations are compared by both techniques with a precision of better than 1 nsec. Obtained clock variation by both techniques behaves in the same manner. However, significant scale difference was found in the results. The most likely explanation is that the VLBI clock parameters are coupled with the other adjusted parameters.<<ETX>>

Development of a simulator of a satellite-to-satellite interferometer for determination of the Earth’s gravity field

A satellite-to-satellite laser interferometer is essential for a future gravity-field mission to improve the accuracy and spatial resolution of the Earth’s gravity model. We have designed a ground-based simulator of a satellite-to-satellite interferometer, which aims to evaluate instrument error reflecting on the accuracy of the gravity-field retrieval and achieve the essential technologies required to measure the intersatellite range rate. The developed simulator was successfully operated with a range-rate sensitivity of 40nm∕s∕Hz in the measurement band of 10−2–1Hz. From a primary demonstration, it was confirmed to be available for experimental simulation of the satellite-to-satellite interferometer in the laboratory. The simulator will be a powerful tool for establishing guidelines for the development of future gravity missions.

Very long baseline connected interferometry via the ATM network

The Communications Research Laboratory (CRL), the National Astronomical Observatory (NAO), the Institute of Space and Astronautical Science (ISAS), and the Telecommunication Network Laboratory Group of Nippon Telegraph and Telephone Corporation have developed a real-time VLBI array, maximum baseline-length was 208 km. The very long baseline interferometry (VLBI) observed data is transmitted through a high-speed asynchronous transfer mode (ATM) network (2,488- Gbps [STM-16/OC-48] ATM network) instead of being recorded onto magnetic tapes. The system was composed of two real-time VLSI networks: the Keystone Project network of CRL (which is used for measuring crustal deformation in the Tokyo metropolitan area), and the OLIVE (optically linked VLBI experiment) network of NAO and ISAS which is used for astronomy (space-VLBI). The acquired VLBI data were corrected via the ATM network and the cross-correlation processing were done simultaneously. A radio flares on the weak radio source (HR1099) and weak radio sources were detected.

Timing precision of active Q-switched mode-locked laser and fire control system for the synchronous satellite laser ranging

We have developed a laser fire synchronization system which controls the firing time of the laser so that each laser pulse reaches a target satellite exactly within 100 ns at a specified epoch. A conventional Nd:YAG active-passive laser has been modified to an active Q-switched mode-locked laser to reduce the laser delay jitters with respect to commanded time. An epoch-triggered fire control system has also been introduced to ensure commanded time to take account of the target range prediction, station clock error, and other fixed delays. Timing jitters of 70 ns RMS have been demonstrated on a geodetic satellite after evaluating laser delay offset, predicted range error, and station clock drift. Improvements to the 10 ns level by further laser control upgrades are possible. >