Satellite with Extraordinary Antenna to Study Soil Moisture
Dec 31, 2014: It's active. It's passive. And it's got a big, spinning
lasso.
Scheduled for launch on Jan. 29, 2015, NASA's Soil Moisture Active Passive
(SMAP) instrument will measure the moisture lodged in Earth's soils with an
unprecedented accuracy and resolution. The instrument's three main parts
are a
radar, a radiometer and the largest rotating mesh antenna ever deployed in
space.
https://www.youtube.com/watch?v=C1FX5AmeD2M#t=45
Launching in January 2015, NASA's Soil Moisture Mapping satellite (SMAP)
will
track water in the soil. Data gathered with help forecast weather, floods,
drought, crop yield and landslides - all from outer space. Music video
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 be stowed into a space of only one foot by four feet (30 by 120
centimeters) for launch. The dish is 19.7 feet (6 meters) in diameter.
"We call it the spinning lasso," said Wendy Edelstein of NASA's Jet
Propulsion
Laboratory, Pasadena, California, the SMAP instrument manager. Like the
cowboy's lariat, 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 dish was provided by
Northrop Grumman Astro Aerospace in Carpinteria, California. The motor that
spins the antenna was provided by the Boeing Company in El Segundo,
California.
"The antenna caused us a lot of angst, no doubt about it," Edelstein noted.
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 (a few
millimeters).
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 test, and we test, and we test some more.
We
have a very stable and robust system now."
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. 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 at NASA's Goddard Space Flight
Center,
Greenbelt, Maryland, 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
Goddard 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.
Credits:
Production editor: Dr. Tony Phillips | Credit: Science@NASA
More information:
For more about the SMAP mission, visit: http://www.nasa.gov/smap/
NASA monitors Earth's vital signs from space, air and land with a fleet of
satellites and ambitious airborne and ground-based observation campaigns.
NASA
develops new ways to observe and study Earth's interconnected natural
systems
with long-term data records and computer analysis tools to better see how
our
planet is changing. The agency shares this unique knowledge with the global
community and works with institutions in the United States and around the
world that contribute to understanding and protecting our home planet.
For more information about NASA's Earth science activities this year, visit:
http://www.nasa.gov/earthrightnow
Regards,
Roger
--- D'Bridge 3.99
* Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)
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