Significant wave height is computed from the slope of the return radar pulse (the gradient of the leading edge of the radar echo, so called the leading-edge slope), after reflection on the surface.
If the altimeter is a two-frequency instrument, there will be a Significant Wave Height computed for both frequencies.

ku_sig_wv_ht_sm s_sig_wv_ht_sm
Significant wave heights in Ku and S band (from Envisat GDR cycle 40) measurements. The algorithm is computing on the whole Earth, even if the output is not meaningful outside ocean. These maps are drawn using the BRAT.

 

Significant wave height is a variable used in marine meteorology, that matches the mean height of the third of the highest waves. However, a wave in a hundred reaches one and a half this value, and some can even reach twice this. Heights once at the coast are difficult to predict, since they depend on submarine reliefs. Significant wave heights measured during storms or hurricanes can be over 18 m: 18.3 m for a winter storm in the North Atlantic in December 2007, 16.9 m for Katrina in August 2005, 17.1 m for Hurricane Luis in September 1995, or 17.9 m for Ivan in September 2004.