This paper shows that the altimeters measure waves over a footprint that has a doughnut shape and discusses what should be the proper definition for the significant wave height definitions altimetry
https://doi.org/10.1029/2023JC020832
Errata:
I just realized (thanks to Daniele Hauser) that there is an error in the caption of fig. 13: “and against Qkk” should be removed. (this is because fig 13 and 14 used to be part of the same figure).