Sea Level Measurements

Altimetry: Basic Principles

The first step in the QA4EO Five Steps is to define the measurand. Therefore, on this page we consider the basics of what an altimeter over the ocean measures.

The radar instrument transmits a pulsed signal from the satellite to the Earth’s surface with a high repetition frequency and then measures the returned (backscattered) signal from the surface. The basic principle of radar altimetry is that the time taken for the pulse to return gives the necessary information to deduce the range from the satellite to the surface, after corrections are made for how that is slowed down by the atmosphere and how it interacts with a real ocean surface. In order to allow small distance variations to be observed, the pulse needs to be extremely short. A short pulse is achieved through the use of a chirped signal (the radar frequency is varied over the transmission). Practically, the returned signal is convolved with a replica of the transmitted chirp using fast Fourier transforms on board the satellite. 

More information is given in later sections, and the best source of training material to explain these basic principles is on the AVISO website.  More information on altimeter signal processing is given here.

The time it takes for the pulse to reach the surface and return provides the 'range'. In order to obtain a surface elevation, this range must be linked to the satellite orbit height above the Earth's reference ellipsoid. As shown in the slides below, and expanded on individual pages, corrections are also made to account for the slowing down of the radar by the ionosphere and wet and dry troposphere, to account for tides and to account for changes in sea level height due to changes in atmospheric pressure, which affect how much the ocean is 'pushed down' by the weight of atmosphere above it. One of the most important corrections is the sea state bias correction. 

Finally, the sea level anomaly is determined by comparing observations to an average of observations from 1993 - 2021, which provides the 'mean sea surface'. The mean sea surface includes both the geoid and the mean dynamic topography of the oceans.