3.2.2.1.1. Altimetric range

Range is the distance from the centre of mass of the satellite to the surface of the Earth, as measured by the altimeters. Thus, the altimeter measurements are referred as "range" or "altimeter range", not height. It is the principal measurement of the radar altimeters. Range is estimated from the echo waveforms as part of the processing so-called retracking. This measurement is not the altitude, it is still only a measurement of distance.

Altimetric range in Ku and S band (from Envisat GDR cycle 40) measurements. These maps are drawn using the Basic Radar Altimetry Toolbox.

An altimeter operates by sending out a short pulse of radiation and measuring the time required for the pulse to return from the sea surface (see How altimetry works). This measurement, called the altimeter range, gives the distance between the instrument and the sea surface, provided that the velocity of the propagation of the pulse and the precise arrival time are known. The dual frequency altimeters perform range measurements at the two frequencies, enabling measurements of the range and the total electron content (see ionosphere correction). While both range measurements are usually provided in (I)GDRs, the Ku band range measurement has much higher accuracy than the C or S band measurement. The range reported in (I)GDRs has already been corrected for a variety of calibration and instrument effects, including calibration errors, pointing angle errors, centre of gravity motion, and terms related to the altimeter acceleration such as Doppler shift and oscillator drift. The sum total of these corrections also appears on the (I)GDR for each frequency ranges.