Principal Investigator: Eni Njoku, Jet Propulsion Laboratory

Thomas Campbell, NASA Langley Research Center
Robert Freeland, Jet Propulsion Laboratory
Wes Lawrence, NASA Langley Research Center
Yahya Rahmat-Samii, University of California, Los Angeles
William Wilson, Jet Propulsion Laboratory
Simon Yueh, Jet Propulsion Laboratory

Proposal Title: Study of a Large-Aperture, Deployable, Mesh Antenna System for Spaceborne Microwave Sensing of the Earth Surface

A concept has been developed for a space mission that will provide accurate, global measurements of ocean salinity and soil moisture. The work proposed here will develop specific technology elements of this concept in readiness for a space mission with a new start in the 2001 time frame. The concept will address key objectives of the Instrument Incubator Program, namely: (a) new earth observation measurements will be enabled–soil moisture and ocean salinity; (b) other existing measurements (such as, potentially, all-weather ocean winds and surface temperatures) will be enabled at higher spatial resolution and lower cost.

The concept features a rotating, offset-fed 6- to 10-m-diameter deployable mesh reflector antenna to measure accurately the microwave emission and backscatter from the Earth’s surface. The system operates nominally at three frequencies and two polarizations, with a conical scan, providing a constant incidence angle and footprint size over a wide swath. These capabilities enable highly accurate geophysical retrievals. The radiometric measurements have low RMS noise per pixel and well calibrated antenna temperatures, providing the primary data for the ocean salinity and soil moisture retrievals. The scatterometer measurements provide direct information on surface roughness, augmenting the radiometric measurements. This system will provide ocean salinity to an accuracy of 0.2 psu at 100-km spatial resolution, and soil moisture to an accuracy of 0.04 g cm-3 at 25-km spatial resolution. This will be the first instrument to make such measurements from space. An important feature of the design is the lightweight and low-cost antenna system which can be scaled straightforwardly to larger diameters and multiple, higher frequencies. This broadens the future applications of this technology to include global measurements of ocean winds, surface temperatures, sea-ice, snow, and other environmental parameters at much higher spatial resolutions than currently feasible. Such measurements are critical components of NASA’s Earth Science Enterprise scientific objectives.

Four areas have been identified which require development prior to a new-start space mission: (1) Optimization of trade-offs between specifications of channels, sensitivities, beam-pointing, sampling, and other characteristics that drive the overall design. (2) Radiometric analysis of the mesh antenna reflector material and design, as these relate to the thermal emissivity and reflectivity of the antenna surface and the required performance. (3) Design and development of lightweight, multichannel feedhorns and electronics, accommodating passive and active channels in the range of 1 to 6 GHz. (4) An antenna/spacecraft configuration, integration, and optimization study, including deployment mechanisms, mechanical and thermal modeling, and attitude control analysis.

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