Absolute salinity is defined as the concentration of dissolved salts in seawater, historically expressed in grams of salt per kilogram of seawater (g salt per 1000 g of water), also known as parts per thousand (ppt, or notated as 0/00). Determining salinity in traditional ways (e.g., titrating seawater) is very time consuming. Thus, salinity measurements were very sparse prior to the development of electronic sensors within oceanographic instruments. These instruments can be attached to moorings, floats, docks, and ship hulls, as well as deployed with cables overboard ships to measure salinity over great ocean depths.

Modern oceanography uses the Practical Salinity Scale (PSS) to derive salinity from measurement of the electrical conductivity, temperature, and pressure (related to depth) of a sample of seawater (usually measured in situ or in the water). Also used is the Practical Salinity Unit (PSU). Both PSS and PSU are roughly equal to parts per thousand (0/00).

With the advent of satellite-based salinity measurements, collected by the pioneering Aquarius satellite starting in 2011, and currently collected by the SMAP satellite since 2015, global ocean sea surface salinity (SSS) mapping is now possible (view the global data maps on our home page). In order to match up these different salinity data sets, many scientists and sensors were needed to intercalibrate satellite SSS measurements (a measurement of seawater brightness temperature) with ocean (in situ) SSS measurements. Now satellite-made SSS data can be reported reliably in units of PSU and 0/00 as well!