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, the US space agency said in a statement.

Remote sensing instruments are called 'active' when they emit their own signals and 'passive' when they record signals that already exist. 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.

SMAP's radar uses the antenna to transmit microwaves toward Earth and receive the signals that bounce back called backscatter. SMAP's radiometer detects differences in Earth's natural emissions of microwaves that are caused by water in soil. The SMAP will be the fifth NASA Earth science mission launched within the last 12 months.


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