The topography payload is composed of the SAR Radar Altimeter (SRAL), the Microwave Radiometer (MWR) and the Precise Orbit Determination (POD) equipment, namely a GNSS Receiver supplemented by a laser retro-reflector (LRR). Their purpose is to determine very accurately the height of the Earth surface, and in particular the sea surface height relative to a precise Earth reference frame. The radar altimeter determines the range between the satellite and the surface by transmitting microwave pulses, which hit the surface of the Earth and return back after a certain delay. This time delay is derived very precisely after on-ground processing of the altimeter data. Knowing the speed of the propagation, the delay is then converted into range. However, the propagation speed through the atmosphere is variable. The ionosphere and the troposphere introduce additional delays dependent on the density of electrons in the ionosphere, the density of gases (dry troposphere) and the moisture content (wet troposphere) in the troposphere. The wet troposphere delay is removed using the MWR data. The MWR determines the amount of water contained in the propagation path of the radar pulses. The RA transmits pulses alternatively at two different carrier frequencies. Comparing the relative delay of both measurements, the frequency-dependent part introduced by the ionosphere is then derived and compensated for. The influence of the dry troposphere (density
of atmospheric gases) is less variable and can be determined sufficiently accurately using meteorological data and models. In order to achieve the ultimate aim of precision measurement of the surface height relative to the terrestrial reference frame, accurate measurements of the satellite location are needed. To this end, a geodetic-quality GNSS receiver, complemented by the laser retro-reflector, are included and guarantee the overall centimetre accuracy required for the Sentinel-3 topography mission.
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