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Active Microwaves
Published in Iain H. Woodhouse, Introduction to Microwave Remote Sensing, 2006
On Earth there is one context where spaceborne altimeters have been used to map solid Earth topography and that is the topography of the ocean floor. This is achieved by analysis of long-term (yearly) averages of ocean surface topography — this should corresponds to the geoid, a virtual surface of equal potential energy. (Long averages are required to even out the effects of atmospheric forcing (wind and pressure), currents and tidal effects.) Assuming that the Earth’s surface water rotates at the same angular speed as the solid Earth the ocean surface would match the equipotential surface that equates to mean sea level and we define this to be the geoid. Practically, the geoid is a very effective reference surface for defining vertical location because it relates to a measurable physical property, namely the mean level of the sea. The geoid
Trends of sea level in the Bay of Bengal using altimetry and other complementary techniques
Published in Journal of Spatial Science, 2018
S. Ghosh, S. Hazra, S. Nandy, P. P. Mondal, T. Watham, S. P. S. Kushwaha
The altimetry data sets from various missions (e.g. Topex/Poseidon, ERS-1/2, Jason- 1⁄2, Envisat, SARAL/AliKa etc.) have been extensively used for studying ocean surface topography (Sharma and Ali 1993, Gopalan et al. 2000, Srinivasan and Leben 2006, Willis et al. 2008, Kusche et al. 2016, Parekh et al. 2017, Taqi et al. 2017) and continental hydrology (Birkett 1998, Calmant and Seyler 2006, Crétaux et al. 2011, Abarca-Del-Rio et al. 2012, Ghosh et al. 2015, Singh et al. 2015, Sulistioadi et al. 2015, Sai krishnaveni et al.2016, Ghosh et al. 2016). A satellite altimeter is a nadir-viewing non-imaging radar sensor which emits high-frequency (2–40 GHz) electromagnetic pulses and records the travel time, the magnitude and the shape of each return signal after reflection from the sea surface. The height of the satellite is calculated relative to a reference level, then the height of the water surface above the reference level is determined. The reference level is a regular ellipsoid-shaped surface defined within a frame of reference fixed in the rotating Earth. It is chosen to match approximately the shape of the earth at sea level and provides a convenient datum from which to measure the sea/water level (Cheney 1995, Chelton et al. 2001). Satellite radar altimetry measures significant wave height (via return pulse shape characteristics), height above the ellipsoid and surface backscatter co-efficient (with received level energy) (Brooks et al. 1998, Fu and Cazenave 2001).