The complex dielectric constant of sea water is a function of salinity at 21-cm wavelength, and sea-water salinity can be determined by a measurement of radiometric temperature at 21 cm along with a measurement of thermodynamic temperature. Three aircraft and two helicopter experiments using two different 21-cm radiometers were conducted under different salinity and temperature conditions. Ground-truth measurements were used to calibrate the data in each experiment. RMS deviations of between 2 and 3%0 were found between remote and ground-truth boat measurements. Part of this deviation is attributed to position mislocation between the aircraft and boats. It is inferred from these experiments that accuracies of 1 to 2%o are possible with a single surface calibration point necessary only every two hours if the following conditions are met-water temperatures about 20°C, salinities above 10%0, level aircraft flight, and extreme care near land masses.