Michiko Yamashita

Phase disturbance of 100-kHz groundwaves by a circular island in the open sea

The hyperbolic position fixing of ships by the Decca navigation system utilizes groundwave propagation. But the hyperbolic pattern will be disturbed by an isolated island. The instantaneous phase velocity and the bearing error are calculated from the equiphase lines near the island. It is found numerically and experimentally that the hyperbolic pattern is bent towards the island that is situated on the baseline between the master station and the slave one.

Position determination at sea using low-frequency radio waves--The effect of a nearby mountain

Measurements of direction-finding (DF) error in position determination based on low-frequency (LF) radio waves were made at sea employing the Decca navigation system. The measured DF error is found to be large when a mountain is nearby. Assuming the mountain to be a hemisphere with complex dielectric constant, we evaluate DF error using electrostatic theory. Analytical predictions of the expected position error caused by the mountains presence are qualitatively in fairly good agreement with measured values. In connection with this, fluctuations of the electric field intensities are also described.

Disturbances of 100-kHz band Decca waves caused by a large bridge

Hyperbolic navigation methods in the 100-kHz band are normally used for fixing the position of a ship. Because many large bridges have been built in recent years, an analysis of the positioning error caused by these bridges is required. From experiments and theoretical considerations for Decca waves near the Minami-Bisan Seto-Ohhashi Bridge, it was confirmed that the main sources of the secondary wave are the two main towers of the suspension bridge. It was also shown that the bridge can be assumed macroscopically as a pair of vertical electric dipoles with a static capacitance of approximately 0.2 /spl mu/F.