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{at}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