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)