SAN FRANCISCO — NASA’s upcoming wind measurement mission is a significant departure from the space agency’s traditional approach to Earth observation. With a price tag of approximately $30 million, the Rapid Scatterometer, or RapidScat, which seeks to gather data using an instrument mounted on the exterior of the international space station, is far less costly than most space-based investigations. It also is likely to remain in orbit for only two years, a fraction of the lifespan of its predecessor, the Quick Scatterometer or QuikScat, which gathered data for more than a decade.
In spite of those constraints, RapidScat will make important scientific contributions by measuring changing wind patterns throughout the day and helping international organizations calibrate existing and planned scatterometers, said Ernesto Rodriguez, RapidScat project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California.
“It was a challenge to take an instrument made of spare parts from a previous mission and tailor the science campaign to the space station platform,” Rodriguez said. “But we found two key things we could do that were not being done by any of the other wind measurement mission in space.”
RapidScat was built primarily with engineering models and flight spares designed for the SeaWinds instrument, which launched onboard QuikScat in 1999. Although engineers could have improved the instrument with updated technology, its similarity to the version flying on QuikScat will make it easier to cross-calibrate the two instruments, said Howard Eisen, RapidScat project manager at JPL.
“QuikScat is the gold standard for scatterometers,” Rodriguez said. “It lasted 10 years and is extremely stable.”
Weather forecasters and climate researchers relied heavily on data provided by QuikScat’s SeaWinds instrument to measure wind speed and direction from July 1999 until November 2009, when its antenna bearings wore out. At that point, instead of gathering data over 90 percent of the world’s oceans as it had done previously, SeaWinds began to observe a single narrow swath.
When the space station carries RapidScat over the same narrow swath observed by QuikScat, researchers will use data acquired by the two instruments to calibrate RapidScat. “Since RapidScat is a clone of QuikScat, it will inherit that calibration and use it to cross-calibrate other instruments in the constellation,” Rodriguez said.
That constellation includes the Advanced Scatterometer on the European Meteorological Satellite Organization’s polar-orbiting Metop A and B satellites. The Indian Space Research Organisation also flew a scatterometer on its OceanSat-2 spacecraft until February when the instrument’s onboard traveling wave tube failed.
ISRO plans to launch OceanSat-3 with a new scatterometer approximately one year after RapidScat begins its planned two-year mission, NASA officials said. An ISRO spokesman did not respond to requests for comment.
If India’s satellite launches as planned, QuikScat’s calibration will have been passed on to three satellites capable of sharing it with future spacecraft. That cross-calibration is important because many of the climate variables researchers study are changing over decades, Rodriguez said.
“Earth’s climate is changing slowly,” Rodriguez said. “To have a coherent vision of how it is changing, all these space-based systems have to work together.”
In addition to RapidScat’s serving as a calibration tool, the space station’s unique orbit will help researchers use RapidScat to learn how wind patterns change in response to solar heat. “Winds on Earth, which are the main source of heat transfer, change significantly during the day,” Rodriguez said. “We need to understand how this daily motion of wind triggers clouds, precipitation and the motion of water in the atmosphere.”
Like other wind-measurement instruments flying in sun-synchronous orbits, QuikScat provided two daily snapshots of wind speed and direction for various points on Earth. “We got snapshots at the beginning of the day and the end of the day, but what happened in between was not well known,” Rodriguez said.
RapidScat’s perch on the space station will enable it to gather wind measurements at various times of the day for the area the space station passes over, from 51.6 degrees north latitude to 51.6 degrees south latitude. Over the course of approximately two months, RapidScat will gather data on every point in that region at every time of day, Rodriguez said.
Those observations are expected to come at a price far lower than most NASA missions because RapidScat was built using leftover SeaWinds hardware with the exception of its antenna subsystem, electronic interface with the space station and trusses designed to mount the instrument on the European Space Agency’s Columbus module. For the additional hardware, NASA purchased commercially available industrial versions of items whenever possible and eliminated much of the redundancy common to space missions, Eisen said.
“There is one amplifier tube and one processing card,” Eisen said. “We are accepting more risk.”
RapidScat also is getting a free ride into orbit. NASA space station managers offered to carry the instrument in the Dragon capsule of Space Exploration Technologies’ fourth cargo resupply mission, which is likely to occur in September. Once it arrives at the space station, RapidScat will be removed by a robotic arm and positioned on the Columbus module, a process that is expected to take three to five hours.