3.1.2.7 Footprint size

The radar altimeter receives the reflected wave (or echo), which varies in intensity over time. Where the sea surface is flat (a), the reflected wave's amplitude increases sharply from the moment the leading edge of the radar signal strikes the surface. However, in sea swell or rough seas (b), the wave strikes the crest of one wave and then a series of other crests which cause the reflected wave's amplitude to increase more gradually. We can derive ocean wave height from the information in this reflected wave, since the slope of the curve representing its amplitude over time is proportional to wave height.

The ground footprint size is an important notion to better understand what the altimeter can really observe and measure. The footprint of an antenna is traditionally defined to be the area on the sea surface within the field of view subtended by the beamwidth of the antenna gain pattern. A radar pulse is not narrow such as with a laser, rather it leaves the antenna as a widening beam, getting wider the further it travels.

The oval footprint characteristics for SWH of 1, 5 and 10 m for 1-second averages of altimeter measurements of nadir mean sea level from orbit heights of 1 336 km (solid lines) and 785 km (dashed lines).(Credits CNES)

The beam illuminates a circle of ocean or land surfaces with a 3 to 5 km wide, depending on the sea state, the wave height or the corrugated land. A calm sea or a flat land surface affords a narrower footprint (typically 2 km) than if the sea is very rough (typically 10 km). Significant return signal is available from reflecting surfaces situated up to 18 km off nadir, which makes the exploitation of altimetric data particularly delicate in case of strong variations of the surface reflectivity.

The returning echoes are a blend of thousands of little echoes from within the footprint, some coming from the troughs of waves, some coming up from wave peaks. With waves up to many metres in height, this creates a mish-mash of echoes from varying heights.