In Orbit
COMMENTARYBy Frank Morring, Jr.Senior Editor Frank
Morring, Jr., blogs at:
AviationWeek.com/onspace
[email protected]Airborne sensors flying over the
Sierra Nevada suggest the situation is
unlikely to improve quickly, because
the snowpack last year was only half
what it had been estimated to be. The
overhead data give a much better
picture of what farmers in the state’s
Central Valley breadbasket already
know. The crops are dying.
“Spaceborne and airborne mea-
surements of Earth’s changing shape,
surface height and gravity field now
allow us to measure and analyze key
features of droughts better than ever
before, including determining precise-
ly when they begin and end and what
their magnitude is at any moment
in time,” says JPL’s Jay Famiglietti.
“That’s an incredible advance and
something that would be impossible
using only ground-based sensors.”
The view is likely to improve later
this year, if NASA’s new Soil Mois-
ture Active Passive (SMAP) satel-
lite reaches orbit and performs as
planned. Designed and built at JPL,
it is intended to give detailed maps of
the water content of topsoil worldwide,
updated every three days. That would
be a dramatic improvement over what
is available today.
“There are significant activities
going on right now combining surface
soil moisture measurements, which
respond on... very small spatial scales,
and Grace measurements, which are
columnar and with much coarserR
esearchers at the Jet Propulsion Laboratory calculate the
State of California is short about 11 trillion gallons of water,
more than the state’s 38 million residents have used over the
past three years of severe drought for drinking, washing and
handling their sewage. Based on data from NASA’s twin Gravity
Recovery and Climate Experiment (Grace) satellites, which de-
tect subtle shifts in gravity below their tandem orbits by precise-
ly measuring the distance between them with laser rangefinders,
JPL scientists say it would take 42 cu. km. of water to replace
the supply lost during the drought.Water World
JPL spacecraft will add climate detail
resolution,” says Dara Entekhabi of the
Massachusetts Institute of Technology,
the SMAP science team lead. “Together
with models, these two very diferent
resolution and very diferent sensing
depth measurements can get integrated
in order to produce a much better pic-
ture of the terrestrial water cycle.”
The relatively small amount of water
in topsoil, compared to the oceans, riv-
ers, lakes, underground reservoirs and
ice, is important because three major
climate factors—water, the energy
it takes to vaporize it, and the plants
that need it to grow, “are intimatelylinked through the water variable,” says
Entekhabi.
The three-year, $916 million SMAP
mission also will give weather fore-
casters in agricultural regions an
early warning signal for drought. And
because soil that is too wet or too dry
doesn’t absorb rainfall very quickly, the
SMAP maps will allow better near-
term flood warnings.
Following liftof Jan. 29 from Van-
denberg AFB, California, on a United
Launch Alliance Delta II vehicle, the
SMAP spacecraft will deploy a 20-ft.-
dia. mesh antenna and spin it like a
cowboy’s lasso (see illustration). Based
on larger antennas developed by
Northrop Grumman for commercial
communications satellites, the SMAP
antenna will spin at 14.5 rpm to serve as
a shared aperture for SMAP’s L-band
radar and passive radiometer. Com-
bined, the two instruments will map soil
moisture across a 1,000-km-wide swath
from its 680-km polar orbit.
Depending on latitude, the space-
craft will overfly each point on the sur-
face every 2-3 days, producing a 40-km
resolution with the passive instrument
and higher resolution—as fine as 1
km—with the radar. When combined
electronically, the moisture data from
the top 3-5 in. of soil will have a resolu-
tion of about 10 km.
“You have this very noisy active
radar sharing the same aperture with
a very sensitive scientific receiver that
wants to live in a very quiet neighbor-
hood,” says Kent Kellogg, SMAP proj-
ect manager at JPL. “The way we deal
with that is the two measurements are
at diferent frequencies, so that helps.
The radiometer actually listens in
between the radar pulses.”
While it cannot measure the water
content of frozen soil, SMAP will be
able to determine if the soil is frozen.
That will be invaluable for tracking the
role of the vast boreal forests in the
Northern Hemisphere in the worldwide
carbon cycle, an important factor in
gauging the greenhouse-gas role of
human-generated carbon dioxide in
global warming.
“That is a very important compo-
nent of the global carbon budget, and
in fact is referred to as the ‘missing
carbon problem’ because we can’t ac-
count for it,” says Entekhabi. cNASAAviationWeek.com/awst AVIATION WEEK & SPACE TECHNOLOGY/JANUARY 15-FEBRUARY 1, 2015 21