In recent years on-air acoustic wave and tsunami gauges are getting popular for the coastal disaster prevention purposes in Japanese ports and harbors. It is because that they are cheaper than the traditional tide gauges installed in the tidal wells. Nevertheless, on-air acoustic type measurement has not been supposed to be applicable to the long-term tide observation, for that calibration method was not established with sufficient precision against the space and time temperature change.
　This note introduces a newly developed on-air acoustic tide gauge that is able to obtain long-term sea level fluctuation with sufficient reliability. Following devices are the points of the invention, and field and indoor experiments showed proper result.
(1) Calibration by the multi reflection boards to revise a measurement error by the change of the temperature and the atmospheric pressure
(2) Application of the newly developed zero-up-time detection method to specify the signal reception time which corresponds to the shortest course
(3) Adoption of the best reflection signal by the multiplicity of the supersonic signal level