Ocean sensing with decameter and microwave radar: Correspondence and contrast over 40 years

The author first met Werner Alpers in the late 1970s at the time of the first comprehensive-microwave-remote-sensing satellite, SeaSat. We were both interested in ocean remote sensing and have been friends ever since. Over the past 40 years remote sensing using both decameter (HF) and microwave radar has thrived even though the wavelengths are separated by about a factor of a thousand — 10 MHz vs. 10 GHz. This report shows correspondences and contrasts between developments at these two wavelength regimes with an emphasis on the role of Werner Alpers and his colleagues in this research area. Understanding of the dominance of Bragg scatter for ocean radar echoes has led to effective remote sensing of ocean waves, surface currents, slicks and winds by both microwave and HF sensors. We focus on brief examples from both HF and microwave, synthetic aperture radar (SAR), in particular ocean waves, ship tracking and surface films.