Built with spare parts and without a moment to spare, the International Space Station (ISS)-RapidScat isn't your average NASA Earth science mission.
Short for Rapid Scatterometer, ISS-RapidScat will monitor ocean winds from the vantage point of the space station.
It will join a handful of other satellite scatterometer missions that
make essential measurements used to support weather and marine
forecasting, including the tracking of storms and hurricanes. It will
also help improve our understanding of how interactions between Earth's
ocean and atmosphere influence our climate.
Scientists study ocean winds for a variety of reasons. Winds over
the ocean are an important part of weather systems, and in severe storms
such as hurricanes they can inflict major damage. Ocean storms drive
coastal surges, which are a significant hazard for populations. At the
same time, by driving warm surface ocean water away from the coast,
ocean winds cause nutrient-rich deep water to well up, providing a major
source of food for coastal fisheries. Changes in ocean wind also help
us monitor large-scale changes in Earth's climate, such as El Niño.
Scatterometers work by safely bouncing low-energy microwaves - the
same kind used at high energy to warm up food in your kitchen - off the
surface of Earth. In this case, the surface is not land, but the ocean.
By measuring the strength and direction of the microwave echo,
ISS-RapidScat will be able to determine how fast, and in what direction,
ocean winds are blowing.
"Microwave energy emitted by a radar instrument is reflected back to
the radar more strongly when the surface it illuminates is rougher,"
explains Ernesto Rodríguez, principal investigator for ISS-RapidScat at
NASA's Jet Propulsion Laboratory, Pasadena, Calif. "When wind blows over
water, it causes waves to develop along the direction of wind. The
stronger the wind, the larger the waves."
ISS-RapidScat continues a legacy of measuring ocean winds from space that began in 1978 with the launch of NASA's SeaSat satellite. Most recently, NASA's QuikScat scatterometer, which launched in 1999, gave us a dynamic picture of the world's ocean winds.
But when QuikScat lost its ability to produce ocean wind
measurements in 2009, science suffered from the loss of the data. In the
summer of 2012, an opportunity arose to fly a scatterometer instrument
on the space station. ISS-RapidScat was the result.
Most scatterometer-carrying satellites fly in what's called a
sun-synchronous orbit around Earth. In other words, they cross Earth's
equator at the same local time every orbit. The space station, however,
will carry the ISS-RapidScat in a non-sun-synchronous orbit. This means
the instrument will see different parts of the planet at different times
of day, making measurements in the same spot within less than an hour
before or after another instrument makes its own observations. These
all-hour measurements will allow ISS-RapidScat to pick up the effects of
the sun on ocean winds as the day progresses. In addition, the space
station's coverage over the tropics means that ISS-RapidScat will offer
extra tracking of storms that may develop into hurricanes or other
tropical cyclones.
Anywhere the wind blows
"We'll be able to see how wind speed changes with the time of day,"
said Rodríguez. "ISS-RapidScat will link together all previous and
current scatterometer missions, providing us with a more complete
picture of how ocean winds change. Combined with data from the European
ASCAT scatterometer mission, we'll be able to observe 90 percent of
Earth's surface at least once a day, and in many places, several times a
day."
ISS-RapidScat's near-global coverage of Earth's ocean -- within the
space station's orbit inclination of 51.6 degrees north and south of the
equator -- will make it an important tool for scientists who observe
and predict Earth's weather. "Frequent observations of the winds over
the ocean are used by meteorologists to improve weather and hurricane
forecasts and by the operational weather communities to improve
numerical weather models," said Rodríguez.
Space-based scatterometer instruments have been built before, but
much of what makes ISS-RapidScat unusual is how it came to be. "Space
Station Program Manager Michael Suffredini offered us a mounting
location on the space station and a free ride on a SpaceX Dragon cargo
resupply mission launching in early 2014," explained Howard Eisen, the
ISS-RapidScat project manager at JPL. "So we had about 18 months to put
together an entire mission."
This accelerated timeline is a blink of an eye at NASA, where the typical project is years or decades in the making.
Free ride
Next, Eisen and his team turned to getting creative and crafty with
the mission's hardware. In lieu of using newly-designed instruments,
which would be expensive and take too long to develop, ISS-RapidScat
reuses leftover hardware originally built to test parts of the QuikScat
mission. That process involved dusting off and testing pieces of
equipment that hadn't seen the light of day since the 1990s. Fortunately
the old hardware seems ship-shape and ready to go. "Even though they
were spares, they've done an excellent job so far," said Simon Collins,
ISS-RapidScat's instrument manager at JPL. Despite their age, the old
parts are more than capable of collecting the ocean wind data that
ISS-RapidScat need to be a success.
In addition to old spare parts, some new hardware was needed to
interface this instrument to the space station and the Dragon
spacecraft. ISS-RapidScat will use off-the-shelf, commercially-available
computer hardware instead of the expensive, hardened-against-radiation
computer chips that are typically used in space missions. "If there's an
error or something because of radiation, all we have to do is reset the
computer. It's what we call a managed risk," said Eisen. The radiation
environment on the space station is much less severe than that
experienced en route to Mars, for example, or in more traditional
sun-synchronous orbits.
Science bounty
Cost-saving decisions like this are shaping up to make ISS-RapidScat
an exceptional bargain of a space mission. "We're doing things
differently, and we're trying to do them quickly and cheaply," said
Eisen. Considering that the typical launch alone can cost $200 million,
ISS-RapidScat's estimated $26 million price tag seems like a bargain.
Last year, NASA estimated the cost of a new, free-flying scatterometer
satellite mission at approximately $400 million.
The real challenges of getting ISS-RapidScat into space lie in the
details. One of the major headaches of such a hurried schedule has been
getting the special connectors that will allow ISS-RapidScat to
physically attach to the International Space Station. "They're special
robotically-mated connectors that haven't been made in years," Eisen
said. "We're having to convince the company that produces these
connectors to make us a small run in time for the mission, and it hasn't
been easy."
The logistics of operating an instrument on the space station are
also tricky. "Typically, spacecraft are designed for the instruments
they carry," said Collins. "In this case, it's the other way around."
For example, ISS-RapidScat's docking point on the space station faces
outward toward space - not down toward Earth and the ocean that the
instrument is looking at. The space station's flying angle will also
change as new pieces are added to it, in response to changes in the
station's drag profile. ISS-RapidScat's mount can compensate for both of
these challenges.
Another concern the ISS-RapidScat team confronted early on was that
one of the space station's docking ports lies squarely within the field
of view of the scatterometer. "Bombarding astronauts and visiting supply
vehicles with microwave radiation from the instruments was out of the
question, and turning the instrument off when there were things docked
there would take away too much science," explained Collins. The
project's engineers instead devised a plan where the instrument avoids
irradiating docking vessels, but continues to scan across the vast
majority of its viewing range.
Rodríguez is confident that the reward for overcoming such
difficulties will be a bounty of vital science information. "Because it
uses much of the same hardware QuikScat did, ISS-RapidScat will allow us
to continue the observations of ocean winds already started," said
Rodriguez. "Extending this data record will help us observe and
understand weather patterns and improve our preparedness for tropical
cyclones."
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