SPACEBORNE MICROWAVE INSTRUMENT FOR HIGH RESOLUTION REMOTE SENSING OF THE EARTHâS SURFACE USING A LARGE-APERTURE MESH ANTENNA (OSIRIS)

The goal of this project is to develop the Ocean-salinity Soil-moisture Integrated Radiometer-radar Imaging System (OSIRIS) instrument concept for combined passive and active sensing in the 1-3GHz range, using a 6m diameter, lightweight, deployable mesh antenna.

The OSIRIS concept was used as the starting point to determine specifications and error budgets for the instrument subsystems and to evaluate the science performance. Key technology issues were then identified to scope the study. The ability of a tensioned wire mesh to serve as a high- precision, high reflectivity and low emissivity antenna reflector surface at low frequencies is a key requirement. The emissivity must be low enough that uncertainties in the emissivity and physical temperature of the reflecting surface do not give rise to excessive thermal noise. The design of the reflector and feedhorns, and the surface shape accuracy of the reflector, must provide antenna patterns that meet the required performance for beam efficiency, gain, sidelobe levels, and cross-polarization. Mass and power requirements must be kept to a minimum, and the rotational dynamics of the antenna and the attitude control requirements and capabilities of the spacecraft must be well understood. The volume of the combined antenna and spacecraft in the stowed configuration and the total mass must be within the capabilities of a low-cost launch vehicle. To address these issues, the specific objectives of the study were outlined as follows:

(1) Perform a requirements analysis to validate the baseline instrument design (measurement channels, sensitivities, beam-pointing, sampling, and other system and orbital characteristics) and to assess the geophysical retrieval accuracies and allowable error budgets for the instrument subsystems 

(2)  Perform laboratory measurements of wire mesh samples to determine their microwave emissivity, and evaluate the ability of mesh reflectors to meet the required brightness temperature precision and calibration stability

(3)  Design lightweight, multifrequency, dual-polarized feedhorns and electronics subsystems, including passive and active channels at L- and S-band frequencies, for a rotating parabolic mesh reflector system, and analyze the antenna pattern characteristics and performance

(4)  Perform an antenna and spacecraft configuration, integration, and optimization study, including deployment mechanisms, mechanical and thermal modeling, and attitude control analysis of the antenna and spacecraft system.