The Multifrequency Future for Remote Sensing of Sea Surface Salinity from Space

David M Le Vine,Emmanuel P Dinnat

doi:10.3390/rs12091381

Abstract

Passive microwave remote sensing of sea surface salinity from space is done with measurements in the 27 MHz wide spectral window at 1.413 GHz (L-band) which is protected for passive use only. The frequency, 1.413 GHz, is near the peak in sensitivity to changes in salinity and modern L-band instruments, such as the radiometers on SMOS and Aquarius, have demonstrated the feasibility of monitoring surface salinity from space. They have also demonstrated the need for better accuracy, especially in cold water. Proposals to improve accuracy have largely involved adding more frequencies. For example, adding higher frequencies to improve the correction for sea surface temperature and lower frequencies to improve the sensitivity to salinity in cold water. These strategies involve trade-offs, some obvious such as the effects of interference outside the protected band and loss of spatial resolution at lower frequencies, but some are more subtle because of the interdependence of the measurement on other parameters of the ocean surface, in particular, the interdependence of salinity, water temperature and roughness (wind speed). The objective of this manuscript is to describe these interdependencies in a quantitative way with documented assumptions to support the design of future instruments for remote sensing of salinity.

Highlights

The spectral window at 1.413 GHz is near the peak in sensitivity to sea surface salinity (SSS) and protected for passive use only, and a frequency near 6.9 GHz is close to the peak in response to sea surface temperature (SST)
Experiments have been made to measure the radiometric response of rough water surfaces (e.g., [22,26,27,28]) but the experiments are limited to selected frequencies and/or incidence angles and tend to have large error bars, they do reveal the general behavior with polarization, incidence angle and frequency
For purposes of this study, the empirical model developed by Meissner and Wentz [22] is used to model the emissivity as a function of wind speed for frequencies above 6 GHz, and a two-scale scattering model developed for SMOS by Yin et al [29] is used for frequencies below 6 GHz

Summary

Introduction

Passive microwave remote sensing of the ocean surface from space is done at a small set of frequencies selected for a combination of protection from radio frequency interference (RFI) and sensitivity to parameters of importance. For purposes of this study, the empirical model developed by Meissner and Wentz [22] is used to model the emissivity as a function of wind speed for frequencies above 6 GHz, and a two-scale scattering model developed for SMOS by Yin et al [29] is used for frequencies below 6 GHz. The empirical model [22] was chosen because it is based on radiometric measurements, covers a large range of frequencies (6–90 GHz), and because it appears to agree reasonably well with available observations ([22] Section 7). The two-scale model [29] is a physical optics model assuming tilted planes (tilted in two dimensions) with small scale roughness (Bragg scatter) superimposed on each planar facet and modified to include foam

Results

Discussion

Conclusion