Accurate Measurements and Model Function for the Dielectric Constant of Seawater at L-band
Open AccessThis dissertation describes the accurate measurements of the dielectric constant of seawater at L-band (1.413 GHz). This is the center frequency of the protected band (i.e. passive use only) used in the radiometric measurement of sea surface salinity from space. The purpose of this study is to provide an accurate model of the seawater dielectric constant as a function of salinity and temperature. This model function can be used by Earth observing satellites to retrieve seawater salinities from remotely sensed data. To measure the seawater dielectric constant, a cylindrical resonant cavity in transmission mode has been employed in the measurements. A capillary tube with 0.1 mm inner diameter is inserted into the cavity for the purpose of introducing seawater into the cavity. After the seawater enters the tube, the real and imaginary parts of the seawater dielectric constant are determined by the changes in the cavity resonant frequency and Q-factor, respectively. The measurements are made using accurately measured seawater samples at salinities of 30, 33, 35 and 38 psu over a range of temperatures from 0° to 35° C in 5° C intervals. At each temperature and salinity, at least three measurements are made. A careful analysis of the measurement uncertainties has also been performed. The algorithm used for determining the seawater dielectric constant has an unknown coefficient that is measured through a calibration procedure using methanol as the sample solution. Based on a perturbation theory, this coefficient is assumed to be a constant due to the fact that the volume of sample solution is very small. The accuracy of the algorithm can be affected by this assumption. In this dissertation, the accuracy of the algorithm applied in the measurement is examined by a wave analysis. Calculations show that the accuracy is better than 0.016% when the tube inner diameter is 0.1 mm or less. Another factor that might affect the measurement accuracy occurs when the tube passes through the center exit-hole on the cavity endplates. This might causes a shift in the resonant frequency which results in an error in the real part of the measured dielectric constant. The effect has been examined using a multimode waveguide analysis. Both the seawater and the methanol measurements suffer this frequency shift and they partially cancel one another out. The effect of the exit-hole on the seawater measurement results is less than 0.086%. Based on the measurement data, an accurate model function for the seawater dielectric constant as a function of salinity and temperature has been developed for the open ocean. The development of the model function is presented along with an analysis of the goodness of fit to the measurement data. The new model function is applied in the algorithm for retrieving sea surface salinity from NASA Aquarius satellite data. The retrieved salinities are compared with in-situ data measured by Argo floats. Compared with other existing model functions, the model function presented in this dissertation has improved performance at low and high temperatures.
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