Journal of the Geodetic Society of Japan

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Laser interferometers are widely used for accurate measurements in fundamental physics, metrology, telecommunication, and so on. The most outstanding feature of laser light is its coherent optical waves, and once the laser wavelength is stabilized it provides a significantly stable and accurate standard of length for measurements. The excellent wavelength stability in various time scales is useful, especially for geodetic observations that often measure small crustal deformations for a long period of time. In this article, after describing the principles of a laser interferometer and its possible errors, three geodetic applications are shown: a laser strainmeter, an absolute gravimeter, and a satellite gravity mission. A laser strainmeter installed 1000 m underground in Kamioka Mine showed low background noise and clearly detected earth tides, whose waveforms agreed with theory within 5%. With the instrument, far-field crustal deformations associated with earthquakes were successfully observed with sufficient precision to provide geodetic constraints on fault parameters and dislocation theory. Absolute gravimeters, meanwhile, are commercially available at present. Their dimensions, weight, and price, however, are not suitable for field observation, such as dense network observation surrounding a volcano and installation in boreholes and the ocean bottom. A compact absolute gravimeter has been developed using newly devised methods for signal processing, seismic-noise correction, and a compact free-fall mechanism. A prototype worked almost as designed and showed 0.8 µgal precision during a 2-day observation. These achievements will be utilized to develop a more compact, practical absolute gravimeter in the near future. Finally, laser interferometers are useful for a satellite gravity mission that needs to measure the precise displacement of a reference mass due to gravity, in which compact instruments are necessary. The development of an accelerometer to be used in the DECIGO Pathfinder project is also presented.

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Phased Array type L-band Synthetic Aperture Radar (PALSAR) onboard “Daichi” is the world unique L-band imaging radar observing the earth surface operationally. While four years have passed since its launch, no degradation at the sensor nor the satellite allows its daily PALSAR data collection of more than 1,000 scenes per day. Higher coherence shown for the vegetation covered land can be maintained by the frequent orbital maintenance and the L-band signal nature that penetrates the vegetation over the land surface penetration. Thus the PALSAR is not only a research tool but also the operational instrument for monitoring the crustal deformation. Recently the ionospheric investigation initiated by the PALSAR increases the spread of the researches based on the L-band frequency. In this paper, we will introduce the current status of the Advanced Land Observing Satellite (ALOS) and the interferometric researches progressed by the PALSAR.

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