Passive active L- and S-band (PALS) microwave sensor for ocean salinity and soil moisture measurements

Abstract

A passive/active L/S-band (PALS) microwave aircraft instrument to measure ocean salinity and soil moisture has been built and tested. Because the L-band brightness temperatures associated with salinity changes are expected to be small, it was necessary to build a very sensitive and stable system. This new instrument has dual-frequency, dual polarization radiometer and radar sensors. The antenna is a high beam efficiency conical horn. The PALS instrument was installed on the NCAR C-130 aircraft and soil moisture measurements were made in support of the Southern Great Plains 1999 experiment in Oklahoma from July 8-14, 1999. Data taken before and after a rainstorm showed significant changes in the brightness temperatures, polarization ratios and radar backscatter, as a function of soil moisture. Salinity measurement missions were flown on July 17-19, 1999, southeast of Norfolk, VA, over the Gulf Stream. The measurements indicated a clear and repeatable salinity signal during these three days, which was in good agreement with the Cape Hatteras ship salinity data. Data was also taken in the open ocean and a small decrease of 0.2 K was measured in the brightness temperature, which corresponded to the salinity increase of 0.4 psu measured by the M/V Oleander vessel.

Selecting an optimal configuration for the Soil Moisture and Ocean Salinity mission

“…[10] While the theories of measurement still require further developments, the feasibility of SM and OS retrievals has been proven in many cases using ground and air borne experiments [Chanzy et al, 1997;Jackson et al, 1995;Lagerloef et al, 1995;Wilson et al, 2001]. So as to apply such results to a spaceborne instrument, the issue of spatial resolution, hence of the antenna size, becomes a central one.…”

Comparing soil moisture retrievals from SMOS and ASCAT over France

“…Various airborne and field campaigns were performed, showing the high potential of L-band (∼1.4 GHz) measurements for the estimation of surface parameters (Skou, 1989;Raju et al, 1995;Chanzy et al, 1997;Wigneron et al, 1997;Wilson et al, 2001;Saleh et al, 2004;Calvet et al, 2011;Zribi et al, 2011;Albergel et al, 2011). Moreover, L-band is the optimal wavelength range to observe soil moisture as higher frequencies are more significantly affected by perturbing factors such as atmospheric effects and vegetation cover (Schumugge, 1983;Kerr et al, 2001).…”

Aquarius and Remote Sensing of Sea Surface Salinity from Space

“…However, emission from the atmosphere must be taken into account in order to achieve the accuracy needed for remote sensing of salinity. This includes both upwelling emission (about 1.9 K at nadir; Yueh et al, 2001) and downwelling radiation that is reflected from the surface (dashed lines). The approach adopted for Aquarius is to use conventional radiative transport theory to model attenuation and emission from the atmosphere [e.g.…”

“…At L-band, scattering in the atmosphere and the dependence on clouds and water vapor are small [Yueh et al, 2001]. However, emission from the atmosphere must be taken into account in order to achieve the accuracy needed for remote sensing of salinity.…”