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. 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.

fig 1. Altimetric range in Ku and S band (from Envisat GDR cycle 40) measurements. These maps are drawn using the BRAT.


The dual frequency altimeters perform range measurements at the two frequencies, enabling measurements of the range and the total electron content. 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, the 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.