SAN FRANCISCO — If the Global Precipitation Measurement (GPM) Core Observatory were designed to operate alone, it would provide researchers with the most accurate data ever obtained of rain and snow falling over much of the world. By operating as part of an international constellation of nine spacecraft, however, the U.S.-Japanese satellite scheduled for launch Feb. 27 on a Japanese H-2A rocket from Tanegashima Space Center will provide even more, including detailed observations of global precipitation every three hours with a resolution of 5 kilometers, said Gail Skofronick-Jackson, GPM deputy project scientist at the Goddard Space Flight Center in Greenbelt, Md.
That unprecedented measurement of global rain and snowfall will dramatically improve the ability of forecasters to pinpoint hurricanes, blizzards, floods, droughts and landslides, said Steve Neeck, deputy associate director of flight programs in NASA’s Earth Science Division, in a Jan. 27 NASA briefing.
The GPM mission also is expected to help researchers improve their understanding of Earth’s water cycle. “In a changing climate, precipitation patterns are likely to change,” Skofronick-Jackson said in a Feb. 6 interview. “We need to know if the rain is falling over the ocean instead of over the land and if the rain is moving from one location to another. We need to be able to follow the way the water moves all over the Earth.”
The GPM Core Observatory includes two instruments, the GPM Microwave Imager built by Ball Aerospace & Technologies Corp. of Boulder, Colo., and the Dual-frequency Precipitation Radar built by the Japan Aerospace Exploration Agency (JAXA) and Japan’s National Institute of Information and Communications Technology, to detect precipitation ranging from rain as light as 0.2 millimeters per hour to 110 millimeters per hour, a rate likely to be associated with hurricanes or monsoons, Skofronick-Jackson said.
Both instruments are designed for extreme accuracy due to their role in unifying and calibrating data drawn from other satellites in the GPM constellation, which includes research and operational spacecraft operated by U.S., French, Indian and Japanese government agencies as well as the Europe’s meteorological satellite organization, Eumetsat.
GPM’s Dual-frequency Precipitation Radar employs a Ka-band frequency for measuring heavy precipitation and Ku-band frequency to observe light rain and falling snow. “The power of the radar will allow us to see three dimensions within clouds,” Skofronick-Jackson said. “For every 500 meters [of altitude], we will know characteristics of the particles.”
The GPM Microwave Imager is more sophisticated than the radiometer flown on its predecessor, the NASA-JAXA Tropical Rainfall Measuring Mission (TRMM) launched in 1997. In contrast to the TRMM mission, which gathered data on areas within 35 degrees of the equator, the GPM Core Observatory is destined for a 407-kilometer circular orbit with an inclination of 65 degrees, enabling it to observe higher latitudes.
Precipitation particles at higher latitudes typically are much larger than those near the equator. As a result, the GPM Microwave Imager is equipped with two high-frequency channels optimized to detect light rain and ice crystals. “This will be the first time in history that we will be able to get data on how much it’s snowing and where it’s snowing in remote locations,” Don Figgins, Ball Aerospace GPM Microwave Imager program manager, said in a Feb. 7 interview.
Ball Aerospace also equipped the GPM Microwave Imager with a unique set of self-calibration features. When the instrument is not focused on Earth it uses traditional hot and cold calibration targets as well as noise diodes emitting signals at selected channels to verify its accuracy.
“We believe the GPM Microwave Imager will be the best calibrated and most accurate radiometer out there,” Skofronick-Jackson said.
The GPM Core Observatory’s orbit is designed to cross the orbits of other space-based radiometers in the GPM constellation to sample the same data and confirm the accuracy of measurements taken by those instruments. Data from all radiometers in the constellation will be sent to the Precipitation Processing Center at NASA Goddard where they will be merged to produce global information on rain and snow falling worldwide every three hours.
The U.S. National Oceanic and Atmospheric Administration also intends to use data from the GPM mission to enhance weather forecasting, including efforts to track the movement of hurricanes. “We take whatever data we can get and use it to obtain accurate information on precipitation,” Ralph Ferraro, chief of NOAA’s Satellite Climate Studies Branch, said in a Feb. 11 interview. “The more observations we have from satellites the better products we can offer.”
Because the GPM Core Observatory covers higher latitudes than TRMM, its data are likely to help forecasters improve weather prediction models. “Quite a few active weather systems come in from high latitudes,” Ferraro said. “So having more information at higher latitudes, especially measurements that are sensitive to cold-season precipitation, will be useful.”
In addition to the GPM Core Observatory, the GPM constellation will include: Eumetsat’s MetOp-B and planned MetOp-C; the NASA-NOAA Suomi National Polar-orbiting Partnership; France and India’s Megha-Tropiques; NOAA’s Polar-orbiting Operational Environmental Satellites; Japan’s first Global Change Observation Mission-Water; U.S. Defense Department meteorological satellites; NOAA’s Joint Polar Satellite System; and TRMM.
Although TRMM is running low on fuel, its instruments continue to work well, Skofronick-Jackson said. When the fuel supply is exhausted, which is expected to occur within the next one to two years, the NASA-JAXA team will continue to gather data with the spacecraft’s instruments for approximately 18 months as it descends, she said.
