4.12　Wave Measurement by Radar

4.12 Wave Measurement by Radar

While ordinary measurements of waves by a wave gauge are used for a time variation of wave height at a point, remote sensing techniques gives information over a broader area.

There are two methods of wave measurement used in remote sensing. a. space borne sensors such as SAR and microwave altimeter b. ground base radar such as X band radar and HF Doppler radar

As space borne sensors have a lower resolution (of the order of a few ten meters to a few kilometers), large size currents, typhoons over wide areas, global wave distribution, etc., will be better monitored by these systems.

On the other hand, ground based radar is suitable for monitoring the waves in near offshore or shallow zones, with the wave field of a few ten centimeters to a few hundred meters.

Figure 4.12.1 shows the distribution of average significant wave height which was measured by GEOSAT-ALT, with an accuracy of +-0.5 m or 10 % of the wave height.

Space borne SAR and ground based X band radar are used for measurement of reflectivity from the sea clutter at similar wavelengths to the sensors based on Bragg scattering (see 4.11).

Figure 4.12.2 explains the effect of capillary waves with respect to slope change or incident angle for Bragg scattering and wave height.

Figure 4.12.3 shows the sea surface conditions measured by SAR and X band radar. The measurement of wave direction and wave length is already operational as a ship borne radar but the measurement of wave height is still being researched. HF Doppler radar using high frequency band (10-100 m) , which is longer than microwave can measure the Bragg scattering from the sea clutter with wave lengths of 5-50 m.

Wave conditions such as wave direction, significant wave height, predominant wave, current etc. of wind waves with longer wave lengths, can be measured by using the Doppler effect with phase velocity of the wave crest. HF Doppler radar is already operational.