| Abstract: |
A transmission type cavity is employed to determine accurately the seawater dielectric constant at L-band [Lang et al. 2016]. Based on the dielectric constant data, a dielectric model function has been generated and applied to retrieve the seawater surface salinity from the Aquarius satellite data [Zhou et al. 2017]. The comparison between the retrieved salinity and the in-situ data, however, indicates that more accuracy is needed for the dielectric measurements of seawater at low temperatures.
To check the accuracy of the measurements at low temperatures, an electromagnetic cavity model has been developed. The model and perturbation results are then compared to check the accuracy of the perturbation method. The model takes into account the changes in the cavity dimensions and the conductivity of the cavity walls as the temperature changes. This model provides the exact solution of the electromagnetic fields inside the cavity; these fields can be used to compute the systematic errors in the perturbation results. Another factor that can affect the measurement accuracy occurs when the tube passes through the center exit-hole on the cavity endplates. This 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 by using a multimode waveguide analysis. Both the seawater and the methanol measurements have this frequency shift, and the shifts partially cancel one another out.
The experimental system has been upgraded recently in preparation for the low temperature measurements. The coaxial RF probes have been replaced with new ones that are tin coated to prevent corrosion in the coolant. New thermistors with 0.01° C accuracy have been purchased and installed. The accuracy is valid throughout an extensive temperature range from -3° C to 35° C; this will improve the seawater dielectric measurements at low temperature. A new Matlab script has been developed that uses a least-square fit to determine the resonant frequency and cavity Q from the raw network analyzer data. These results are more accurate than those obtained by the network analyzer. With all these upgrades and theoretical corrections, the low temperature measurements are currently being made from -1.5° C to 5° C. The new data will be used to adjust the model function to improve its accuracy at low temperatures. We will compare the new more accurate data with past measurements.
References
Zhou, Y., R. Lang, E. Dinnat and D. Le Vine (2017), “L-band model function of the dielectric constant of seawater”, IEEE Transactions on Geoscience and Remote Sensing, vol: 55, issue: 12, doi: 10.1109/TGRS.2017.2737419
Lang, R., Y. Zhou, C. Utku, and D. Le Vine (2016), “Accurate measurements of the dielectric constant of seawater at L-band”, Radio Science, vol: 51, pp. 2-24, doi:10.1002/ 2015RS005776
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