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Nasa is set to launch a new satellite that will act like a 'lasso' to measure the moisture in Earth's soils with unprecedented accuracy later this month.

The Soil Moisture Active Passive (SMAP) instrument on board the Soil Moisture Mapping satellite will spin around 14 times per minute as it orbits Earth.

The goal is to come up with a global map of Earth's soil moisture to help cope with drought and flood monitoring.

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A new Nasa Earth satellite will launch from California on 29 January. Called the Soil Moisture Mapping satellite (illustrated) it will map global soil moisture. The satellite largest rotating mesh antenna ever used in space. It will be used to map wide areas and cover entire Earth every three days

The SMAP satellite is set to launch at 9.20am EST (2.20pm GMT) on 29 January from Vandenberg Air Force Base in California.

The instrument's three main parts are 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 will use a sensor of each type to create the highest-resolution, most accurate measurements ever made of soil moisture.

Soil moisture is a tiny fraction of Earth's water that has a disproportionately large effect on weather and agriculture.

The ambitious mission is designed not only to be accurate, though, but also to cover the entire globe every three days or less.

Engineers at Nasa's Jet Propulsion Laboratory in Pasadena, California, designed and built the largest rotating antenna that could be stowed into a space of only one by four feet (30 by 120 cm) for launch. 

When unfolded the dish measures a huge 19.7 feet (six meters) in diameter.

'We call it the spinning lasso,' said Nasa's Dr Wendy Edelstein, the SMAP instrument manager.

Like the cowboy's rope, 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). 

'The antenna caused us a lot of angst, no doubt about it,' Dr Edelstein noted.

'We test, and we test, and we test some more. We have a very stable and robust system now.'

Pictured is the Soil Moisture Active Passive (SMAP) instrument as the protective covering is removed in the Astrotech payload processing facility on Vandenberg Air Force Base in California during a post-shipment inspection. The launch of SMAP is targeted for 29 January 2015

SMAP's radar, developed and built at JPL, uses the antenna to transmit microwaves toward 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.

Pictured is the first stage of the Delta II rocket that will take SMAP into orbit at  Vandenberg Air Force Base in California

Using a complex technique called synthetic aperture radar processing, the radar can produce ultra-sharp images with a resolution of about half a mile to a mile and a half (one to three kilometers).

SMAP's radiometer detects differences in Earth's natural emissions of microwaves that are caused by water in soil.

To address a problem that has seriously hampered earlier missions using this kind of instrument to study soil moisture, the radiometer designers developed and built one of the most sophisticated signal-processing systems ever created for such a scientific instrument.

The problem is radio frequency interference. The microwave wavelengths that SMAP uses are officially reserved for scientific use, but signals at nearby wavelengths that are used for air traffic control, cell phones and other purposes spill over into SMAP's wavelengths unpredictably.

Conventional signal processing averages data over a long time period, which means that even a short burst of interference skews the record for that whole period.

The engineers devised a new way to delete only the small segments of actual interference, leaving much more of the observations untouched.

Combining the radar and radiometer signals allows scientists to take advantage of the strengths of both technologies while working around their weaknesses.

'The radiometer provides more accurate soil moisture but a coarse resolution of about 40 kilometers [25 miles] across,' said JPL's Eni Njoku, a research scientist with SMAP.

'With the radar, you can create very high resolution, but it's less accurate. To get both an accurate and a high-resolution measurement, we process the two signals together.'

SMAP will be the fifth Nasa Earth science mission launched within the last 12 months.