Introduction to the Special Issue on “Satellite Altimetry: New Sensors and New Applications”

Spaceborne radar altimeters provide information on the Earth's surface by transmitting a series of radio-frequency pulses and recording their echoes. Although the beamwidth of the instrument is typically more than a degree, a finer ground footprint is achieved by only recording the return within a narrow window. Geophysical information on the sub-surface pixel is then derived from analysing the characteristics of the return signal, known as a 'waveform'. The delay in return of the signal indicates the mean height of the feature below the instrument (once a large number of corrections have been applied); the amplitude of the signal records the backscatter strength of the target, which is dependent upon the surface type and its roughness. Finally the shape of the waveform contains information on the variations in topography within the footprint. Further details about the principles of altimetry are given by Chelton et al. [1].