QuikSCAT's Replacement, the RapidScat Ocean Wind Sensor, Installed on Space Station

In November 2009, one of the greatest success stories in the history of satellite meteorology came to an end when the venerable QuikSCAT satellite failed. Launched in 1999, the QuikSCAT satellite became one of the most useful and controversial meteorological satellites ever to orbit the Earth. It carried a scatterometer--a radar instrument that can measure near-surface wind speed and direction over the ocean. Forecasters world-wide came to rely on QuikSCAT wind data to issue timely warnings and make accurate forecasts of tropical and extratropical storms, wave heights, sea ice, aviation weather, iceberg movement, coral bleaching events, and El Niño. Originally expected to last just 2 - 3 years, QuikSCAT made it past ten, a testament to the skill of the engineers that designed the satellite. A QuikSCAT replacement called ISS-RapidScat was funded in 2011 and built in just 18 months. RapidScat was successfully launched on September 20, 2014 on a SpaceX Dragon cargo spacecraft, which docked last week with the International Space Station (ISS.) This morning, RapidScat was plucked out of the Dragon and install it on the Space Station. The heaters have been turned on, and full activation of RapidScat is expected on Wednesday. In a clever reuse of hardware originally built to test parts of NASA's QuikScat satellite, RapidScat cost NASA just $30 million--80% lower than if the instrument had been built new.



ISS-RapidScat is a radar scatterometer designed to sense near-surface winds over the ocean. The instrument sends a pulse of 13.4 gigahertz microwaves towards the Earth’s surface and measures the intensity of the return pulse that reflects back from the surface. In general, strong radar return signals represent rough surfaces, while weak radar return signals represent smooth surfaces. Stronger winds produce larger waves and therefore stronger radar return signals. The return signal also tells scientists the direction of the wind, since waves line up in the same direction the wind is blowing. The ISS orbit takes the space station between 51.6°N - 51.6°S latitude, and RapidScat will not be able to "see" ocean winds at high latitudes beyond 57°. QuikSCAT measured winds in a swath 1,800 km wide centered on the satellite ground track, but RapidScat's swath will be only 900 km wide, since it is orbiting at a lower altitude (375 - 435 km high versus 800 km for QuikSCAT.) The instrument will be able to "see" with a resolution of up to 12.5 km (7.8 miles.) It completes 15.51 orbits per day, and revisits the same part of the ocean beneath it once every two days. This compares with QuikSCAT, which covered 93% of Earth's surface in 24 hours. The advertised accuracy of RapidScat winds: for wind speeds 7 to 45 miles per hour (3 to 20 meters per second), an accuracy of about 4.5 miles per hour (2 meters per second); for wind speeds of 45 to 70 miles per hour (20 to 30 meters per second), an accuracy within 10 percent; for wind direction, an accuracy of 20 degrees. Precipitation generally degrades the wind measurement accuracy, and accuracy is also reduced at the edge of the swath. Useful data from RapidScat will likely not be available for several months, to allow time for the scientists to validate and calibrate the data being taken. RapidScat's lifetime will be relatively short--just a two-year mission is planned. Scatterometer data is extremely valuable for many aspects of hurricane forecasting, providing early detection of surface circulations in developing tropical depressions, and helping define gale (34 kts) and storm-force (50 kts) wind radii. The information on wind radii from scatterometers is especially important for tropical storms and hurricanes outside the range of aircraft reconnaissance flights conducted in the Atlantic and Eastern Pacific basins, and for the regions where there are no reconnaissance flights (Central Pacific, Western Pacific, and Indian Ocean). Accurate wind radii are critical to the National Hurricane Center (NHC), Central Pacific Hurricane Center (CPHC), and Guam Weather Forecast Office (WFO) watch and warning process, since they affect the size of tropical storm and hurricane watch and warning areas. Between 2003 and 2006, QuikSCAT data were used at NHC 17% of the time to determine the wind radii, 21% of the time for center fixing, and 62% of the time for storm intensity estimates.