The Soil Moisture Active Passive (SMAP) instrument scheduled for launch on January 29, 2015, has three main parts - a radar, a radiometer and the largest rotating mesh antenna ever deployed in space.

Remote sensing instruments are called "active" when they emit their own signals and "passive" when they record signals that already exist.
The mission's science instrument ropes together a sensor of each type to corral the highest-resolution, most accurate measurements ever made of soil moisture - a tiny fraction of
Earth's water that has a disproportionately large effect on  weather and agriculture.
To enable the mission to meet its accuracy needs while covering the globe every three days or less, SMAP engineers at NASA's Jet Propulsion Laboratory in Pasadena, California, designed and built the largest rotating antenna that could bestowed into a space of only one foot by four feet for launch. The dish is 19.7 feet in diameter.

"We call it the spinning lasso," said Wendy Edelstein of JPL, the SMAP instrument manager. The antenna is attached on one side to an arm with a crook in its elbow. It spins around the arm at about 14 revolutions per minute (one complete rotation every four seconds).

Although the antenna must fit during launch into a space not much bigger than a tall kitchen trash can, it must unfold so precisely that the surface shape of the mesh is accurate within about an eighth of an inch.
The mesh dish is edged with a ring of lightweight graphite supports that stretch apart like a baby gate when a single cable is pulled, drawing the mesh outward.
"Making sure we don't have snags, that the mesh doesn't hang up on the supports and tear when it's deploying - all of that requires very careful engineering," Edelstein said.
"We have a very stable and robust system now," Edelstein added. SMAP's radar uses the antenna to transmit microwaves towards Earth and receive the signals that bounce back, called backscatter.
The microwaves penetrate a few inches or more into the soil before they rebound. Changes in the electrical properties of the returning microwaves indicate changes in soil moisture, and also tell whether or not the soil is frozen.

Using a complex technique called synthetic aperture radar processing, the radar can produce ultra-sharp images with a resolution of about one to three kilometres. The SMAP will be the fifth NASA Earth science mission launched within the last 12 months.

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