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Marine radar
Published in Alexander Arnfinn Olsen, Core Principles of Maritime Navigation, 2023
The term “radar” is an acronym for Radio Detection and Ranging. The marine radar works on the basic principle of electromagnetic waves. The radar antenna sends the high-speed electromagnetic waves to establish the location of a target, which is the distance, the velocity, and the direction the wave travelled together with the altitude of the object, whether moving or stationary. Electromagnetic energy travels through the air at a constant high speed, equivalent to the speed of light, or 186,411 miles (300,000 km) per second. The object may vary from ships, boats, terrain, weather formations, coastal formations, buildings and so forth. The radar system sends out electromagnetic waves as a high-speed signal which travels several miles in the same direction the radar is facing. If there are no objects in the direction of the wave, the radar screen will be blank. If there is an object, this will reflect the wave and bounce it back to the radar receiver. Once the ship has received the returned signal, the onboard radar computer calculates the distance between the ship and the object along with its location. Subsequently, radar provides three critical pieces of information: the location of an object, the range of the object, and the direction the object is travelling.
Ocean wave height inversion under low sea state from horizontal polarized X-band nautical radar images
Published in Journal of Spatial Science, 2021
Usually, X-band marine radar is used to measure the current as well. The current velocity u is firstly determined by an iterative least squares method from the dispersion relation. And then the dispersion relation of linear surface-gravity waves is obtained (Seemann et al. 1999, Senet et al. 2001, Gangeskar 2002). The dispersion relation, including the current contribution to the Doppler shift in frequency, is used as a band-pass filter to separate the energy of ocean waves from the background noise and to remove those components of the image spectrum which do not belong to the wave field (Vicen-Bueno et al. 2012, Wang et al. 2017a). The filtered 3-D image spectrum is denoted as. Then a 3-D wave spectrum is obtained from by using a transform function defined by