Aquarius/SAC-D is a cooperative international mission developed between the National Aeronautics and Space Administration (NASA) of United States of America (USA) and the Comisión Nacional de Actividades Espaciales (CONAE) of Argentina.
The overall mission objective is to contribute to the understanding of the total Earth system and the consequences of the natural and man-made changes in the environment of the planet. Major themes are: ocean surface salinity, carbon, water cycle, geo-hazards, and cryosphere.
Salinity is important for understanding ocean dynamics, energy exchange with the atmosphere and the global water cycle. Existing data is limited and much of the ocean has never even been sampled. Sea surface salinity can be measured remotely by satellite and a three year mission for this purpose called Aquarius/SAC-D has recently been selected by NASA's Earth System Science Pathfinder (ESSP) program. The objective is to map the salinity field globally with a spatial resolution of 100 km and a monthly average accuracy of 0.2 psu. The mission, scheduled for launch in 2008, is a partnership of the United States National Aeronauatics and Space Agency (NASA) and the Argentine Comision Nacional de Actividades Epaciales (CONAE).
A major application for a 21 cm radiometer is the remote sensing of soil moisture which is possible because of the large contrast between the dielectric constant of dry soil (approximately equals 3.5) and that of liquid water ( approximately equals 80). One of the major problems with the utilization of long wavelength radiometers from satellite platforms has been the large antenna size required with its substantial mass. For example, at satellite altitudes an antenna size of at least 10 m is required to obtain resolutions in the 10-20 km range. The size requirement is fundamental but the mass can be reduced by using unfilled arrays or as will be described here a thinned array antenna. Such a system operating at L-Band ((lambda) equals 21 cm or 1.42 GHz) has been developed and tested from an aircraft platform. It is called ESTAR (Electronically Scanned Thinned Array Radiometer) and it uses linear (stick) antennas in the along-track direction and aperture synthesis between pairs of sticks separated by odd multiples on half wavelengths in the cross track direction. The approximate dimensions of the antenna are 1 meter by 1 meter. Results from an evaluation series of flights over a study watershed in Oklahoma indicate that such a system can provide useful soil moisture information.
A model is developed to relate the attenuation through a vegetation canopy, to the geometric and dielectric parameters of the canopy constituents. The model is designed to operate over a wide frequency band in the microwave region and include both deciduous and coniferous trees. The vegetation canopy is represented by a discrete random layer of cylinders and disks having the same geometric and dielectric properties as the vegetation canopy constituents. The Foldy-Twersky Integral Equation is used to relate the attenuation to the scattering amplitude of the vegetation constituent evaluated in the forward direction. Numerical results are presented to show how the level and frequency dependance of the attenuation depend on type of vegetation constituent and its orientation.
A mathematical evaluation of the potential of active microwave sensors for monitoring underground permafrost is carried out. For this purpose, the Helmholtz Integral is used to relate the scattered field to the field inside the permafrost. A mathematical formation is derived for the scattered field by estimating the inner field through a generalized Rayleigh- Gans approximation. Numerical calculation show that the freezing thawing process is the dominant factor determining the relative level of the backscattered signal.