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Winds sensor opens door for Earth science from ISS

Posted: October 12, 2014

A $26 million science instrument carried to the International Space Station last month by SpaceX's Dragon cargo capsule has been switched on and is measuring winds over the world's oceans to help forecasters track the intensity of tropical cyclones, NASA officials said.

The International Space Station-Rapid Scatterometer instrument is mounted on the space station's European Space Agency Columbus module. Credit: NASA
Made of leftover parts from a satellite developed in the 1990s, the instrument package was mounted on the outside of the space station to fill a data gap that could degrade the ability of meteorologists to monitor hurricanes.

Without the need for a dedicated launcher or a standalone satellite, NASA saved more than $300 million by recycling spare parts launching the wind monitoring sensor to the space station, according to Howard Eisen, the mission's project manager at the Jet Propulsion Laboratory.

"RapidScat is the ultimate effort in recycling," Eisen said. "We took hardware, some of which was 17 or 18 years old, and we put it to new use."

The International Space Station-Rapid Scatterometer, or ISS-RapidScat, instrument launched from Cape Canaveral on Sept. 21 in the unpressurized trunk section of an unmanned SpaceX Dragon supply ship.

The Dragon spacecraft, carrying more than 2.5 tons of pressurized and unpressurized cargo such as food, experiments and spare parts, arrived at the space station Sept. 23.

Under the control of engineers at NASA's Johnson Space Center in Houston, the station's Canadian-built robot arm and Dextre manipulator -- a two-armed device with mechanical hands -- completed a two-step procedure to pull the RapidScat instrument and its mounting adapter from the Dragon spaceship's trunk.

The first step on Sept. 29 attached an adapter for RapidScat to an external platform on the space station's European Columbus laboratory module. After engineers made sure the adapter had a firm mechanical and electrical attachment to the station, the outpost's robotics system extracted the RapidScat sensor system and mated it to the adapter plate on Columbus.

The instrument was powered up Oct. 1, according to a NASA press release, and it should be supplying weather forecasters with operational data by the end of the month.

Part of the RapidScat instrument assembly is seen attached to the space station's Dextre robot during the transfer from the SpaceX Dragon spacecraft. Credit: NASA
RapidScat's primary sensor is a 100 watt, 2.5-foot-diameter microwave antenna that spins at nearly 20 rpm, emitting and receiving signals bounced off the ocean's surface.

From those signals, scientists can process data on wind speed and direction by analyzing returns reflected off the ocean at different angles, helping hurricane forecasters and climate researchers keep track of short-term and long-term trends.

One of the first weather systems observed by RapidScat was then Tropical Storm Simon off the west coast of Mexico.

"Most satellite missions require weeks or even months to produce data of the quality that we seem to be getting from the first few days of RapidScat," said Ernesto Rodriguez, RapidScat project scientist Ernesto Rodriguez at JPL. "We have been very lucky that within the first days of operations we have already been able to observe a developing tropical cyclone."

Engineers constructed RapidScat out of components originally manufactured for NASA's QuikScat mission, which launched in 1999 but stopped producing wind speed data in 2009 when the satellite's main antenna quit rotating.

The space station does not have the same global coverage as a satellite in a sun-synchronous orbit, which flies over a given location at roughly the same time each day.

But with the end of QuikScat's mission and the loss of a sensor on India's Oceansat 2 satellite in February, scientists faced a hole in ocean wind measurements from space.

"We've lost the capability, due to some losses in the international constellation, to do this global monitoring on a daily basis," Rodriguez said in a press briefing before RapidScat's launch.

RapidScat's microwave radar can see a 500-mile-wide swath of the ocean as the space station flies overhead, but it must be switched off during certain space station operations such as spacewalks where astronauts will pass close to the instrument. The radar's coverage will also be restricted when unmanned supply ships are attached to the station's Harmony module, Eisen said.

Coupled with data from a similar instrument on Europe's polar-orbiting MetOp weather satellites, RapidScat will give scientists a daily view of winds in the same region.

"Right now, I think the biggest impact it will have is the ability to close the gap on seeing things that change quickly -- like hurricanes," Rodriguez said. "Right now, it can happen and it does happen, that the (European) ASCAT scatterometer will completely miss a hurricane that's intensifying. By having an additional platform that helps bridge that gap, we will have at least daily observations of hurricanes.

Tropical Storm Simon as seen by RapidScat as it approached Mexico's Baja California peninsula at 0210 GMT Oct. 4 (7:10 p.m. PDT Oct. 3). The tropical storm is easily identified by its high winds and closed circulation pattern. Wind speeds are shown in color, with reds and yellows being highest, while wind directions are shown as arrows. One meter per second equals approximately 2.24 miles per hour. Credit: NASA/JPL-Caltech
"This is especially important not as it approaches land, where we have airborne facilities, but when it's forming and when it's actually starting to move. Predicting things like where it's going to move from Africa all the way to America is really hard with satellites, and having that daily observation really helps."

RapidScat is scheduled for a two-year mission on the space station. By then, scientists hope India can launch a satellite with another scatterometer for maritime wind measurements.

RapidScat's arrival at the International Space Station marks the first of at least five Earth observing instruments to launch to the complex over the next few years.

The space station's viewpoint "doesn't get to the poles, but it does view the lower latitudes, between plus or minus 50 or so degrees, with much more frequent repeat cycle and also at different times of day" than polar-orbiting free-flying satellites, said Steve Volz, associate director for flight programs in NASA's Earth science division.

Next up is the Cloud Aerosol Transport System, a laser instrument to measure the location and distribution of clouds, pollutant particles, dust and smoke in the atmosphere. The CATS instrument is set to launch on SpaceX's next cargo resupply flight in December for attachment outside the station's Japanese experiment module.

Flying remote sensing instruments on the space station comes with cost savings, but it has forced engineers to rethink the design of Earth-watching payloads after tailoring them to operate on traditional standalone satellites.

"It was a surprise for us to realize that the toughest thermal environment that we'll ever see for the instrument is from that period of time where it goes from the (cargo) spacecraft and is attached to the station," Volz said. "It's an unpowered period of time. We're not used to that. When we fly our satellites on robotic missions, we have power from the start. We don't have to worry about thermal extremes.

"Understanding the steps in an ISS installation, there are a lot of things that are different from how we do it on free-flyers. They're all surmountable, but they need to be understood and carefully looked at one step at a time," Volz said.

In 2016, NASA plans to launch the Stratospheric Aerosol and Gas Experiment 3 (SAGE 3) payload to study the ozone layer and climate change, and the Lightning Imaging Sensor to detect and locate lightning strikes over the tropics and mid-latitude regions.

Two more instruments will launch to the station later this decade: the Global Ecosystem Dynamics Investigation will study forest canopy structure, and the Ecosystem Spaceborne Thermal Radiometer Experiment on Space Station will study water use and water stress in vegetation.

There are also plans to expand commercial Earth observation missions on the space station. The Canadian company UrtheCast recently announced it will develop a high-resolution optical camera and radar imager to be placed outside the station's Tranquility module in 2017.

"Now that we realize the capability of the ISS, we're taking advantage of it," Volz said.

Follow Stephen Clark on Twitter: @StephenClark1.