Video At the end of the month, NASA will launch one of its most unusual satellites yet: a radar hurling a 19-foot-long (6-metre) mesh antenna around its head every four seconds.
"We call it the spinning lasso," said Wendy Edelstein, instrument manager for the Soil Moisture Active Passive (SMAP) sensor built by NASA's Jet Propulsion Laboratory in Pasadena, California.
The antenna will be folded up into a 1ft x 4ft (30cm x 120cm) sheet and launched into orbit on January 29. When in position, it must be unfolded as flat and smooth as possible, with no wrinkles or kinks deforming it more than a few millimetres from its design.
NASA will use graphite struts and a novel mechanism to unfold the lasso so that it can capture all the data needed.
"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 test, and we test, and we test some more. We have a very stable and robust system now."
SMAP will use an active microwave radar and a passive radiometer to map out how moist the planet is. The antenna's active radar beams down microwaves that penetrate a few inches of soil in swaths a mile wide, and measures the backscatter [PDF] to determine how much water is interfering with the signal.
The radiometer will be doing its best to pick up microwaves emitted naturally from the planet caused by wet soil, which will be compared to the active radar's results.
NASA has to be very careful that these passive signals aren't being ruined by inteference. The problem is that humankind is producing a lot of microwaves ourselves these days. SMAP uses microwave lengths that are reserved for scientists, meaning no other equipment should be using this spectrum, ideally.
Measuring the infinitesimal changes in the Earth's natural microwave emissions is a delicate business, and one blast from a human-made microwave emitter can wreck the measurements for an entire scanning run – so NASA's worked out some signal-processing algorithms to filter out the noise as much as possible.
"The radiometer provides more accurate soil moisture but a coarse resolution of about 40 kilometres [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 orbit the planet every three days for as long as the instruments and funding permits, mapping out the moist nooks and crannies of the planet to help weather and crop prediction, as well as gathering data on the carbon cycles of Earth's vegetation. ®