Some 600 kilometres above the Earth, Nasa satellites are
watching you. They're not actually interested inyou or
anything you do, of course -- they're just making measurements of
environmental variables like temperature or cloud cover. While most
people equate space agencies like Nasa or the ESA with the
exploration of extraterrestrial destinations, a critical part of
their mission is to study our own planet from the unique orbital
point of view.
The advantages are obvious: rapid coverage of large areas using
the same instrument and continuous data collection. But there are
challenges, too. It's not exactly cheap to get the instrument up
there, and if a component breaks, it's game over. Then there's the
not insignificant task of finding a way to measure the thing you're
interested in from hundreds of kilometres away.
One of the newest members of the Earth-observing club is Aquarius (along with its
friends aboard the SAC-D satellite). Launched on
20 June, 2011, the satellite is a collaborative effort between the
US and Argentina. Its job? To map surface ocean salinity around the
globe and improve our understanding of ocean circulation and the
The Aquarius instrument consists of two main components. The
actual salinity measurement is made by a microwave radiometer that
surveys the radiation emitted by the ocean surface. Because
salinity affects the electrical conductivity of ocean water, it
changes the microwave emissions perceptibly.
If the sea surface were perfectly smooth and calm, that would be
pretty straightforward. But because waves affect the way that the
radiation is emitted, it's necessary to account for the roughness
of the sea. This is accomplished by using a radar scatterometer
that bounces energy off the surface and measures how much returns
directly to the satellite.
The funny looking visor on the front of the satellite
(interactive schematic here) is a
reflector that focuses the signal, which is funnelled through three
"feed horns." The satellite is on a Sun-synchronous orbit, meaning
it orbits from pole to pole in a plane that rotates westward around
the Earth. Simply put, wherever it passes over the surface, it's
roughly 6pm. The business end of the satellite is pointed toward the dark side of the Earth, away from the Sun. As a
result, the solar panels at the rear of the spacecraft point
backwards, and the dog cone collar on the front is meant to shade
the Aquarius instruments.
The underbelly of the satellite houses other
instruments hitching a ride on SAC-D, which were built by
the French, Italians, Canadians, and Argentinians. This includes
another microwave radiometer tuned to study atmospheric conditions
and sea ice, an infrared sensor to spot wildfires and measure sea
surface temperature, and a camera looking for fires and aurorae.
Two French instruments have very different tasks -- investigating
how cosmic radiation affects electronics and scanning for space
Oceanographers had planned ahead to make the most of this unique
new data source. The Salinity Processes in the
Upper Ocean Regional Study (SPURS) involves
well-orchestrated data gathering cruises. (One cruise was blogged at Nasa Earth Observatory's "Notes from the
Field.") Using the Aquarius data for large-scale context, the
cruises measure ocean water properties at extremely high-resolution
over a small study area.
The research vessels deploy a veritable armada of data
collection tools. Many measurements are made by the ships' crews
themselves, of course, but they cast a much wider net by utilising
a number of automated vehicles, anchored instruments, drifters, and
floats from the Argo
network. By swarming the study area, a coherent picture of
ocean mixing from macro to
micro is captured, which will provide oceanographers a
treasure trove to sift through.
Between the SPURS project and Aquarius' global data set,
oceanographers hope to both answer some nagging questions and
discover the unexpected. Monitoring salinity over time could help
researchers better understand how humans interact with ocean
processes and the water cycle, including the effects of changing
climate. It could also improve forecasts of the El Niño Southern Oscillation, which affects weather patterns
from year to year.
So once a week around 6pm, tip your hat to the salt-seeking eye
in the sky as it streaks overhead. It won't notice, but it's the
thought that counts.
This story originally appeared on ars technica