Water, the stuff of life, can also be deadly if there is too much of it, or not enough, or if it is too cold or hot. A new $1.2 billion international spacecraft mission will give scientists, forecasters and first-responders a map of where the water is on Earth, with unprecedented detail, every 3 hr.

Set for launch on an H-IIA rocket Feb. 27 from the Yoshinobu Launch Complex at Japan's Tanegashima Space Center, the Global Precipitation Mission (GPM) will become the “core” spacecraft in a constellation of 11 other orbiting atmospheric-water monitors that will greatly enhance current capability to know when and where water is falling.

NASA and the Japan Aerospace Exploration Agency (JAXA) have developed the mission over the past decade as a follow-on to the Tropical Rainfall Measurement Mission (TRMM), launched in 1997, which is still operating.

“The water cycle is central to creating an understanding of weather, climate and water resource management,” says Ramesh Kakar, GPM program scientist at NASA headquarters. “You've got to measure precipitation if you want to study the water cycle.”

GPM is designed to do just that, expanding the TRMM dataset from the tropical regions between 35 deg. N. and S. Lat. to the Arctic and Antarctic Circles with its 65-deg.-inclination orbit. Its two instruments—one developed by NASA and one by JAXA—are more advanced than their TRMM counterparts, and NASA's Goddard Space Flight Center is equipped to meld it with data from similar instruments on other satellites for “near-real-time observations of rain and snow every three hours anywhere on the globe.”

Experts at the Greenbelt, Md., NASA field center's Precipitation Processing Center (PPC) also plan to use the expanded database to refine models used in forecasting and climate research and to project rain and snow levels back and forward in time to improve predictions of weather and climate change.

The spacecraft is already at Tanegashima, after a trip from Goddard via C-5A Galaxy and by ship (the runway at the island launch site isn't long enough for the big U.S. Air Force cargo carrier). The 3,855-kg (8,500-lb.) spacecraft, measuring 6.5 meters (21 ft.) tall and 13 meters wide with its two solar arrays deployed, is the largest ever designed, built and tested at Goddard.

If all goes as planned, it will lift off from Tanegashima's Pad 1 at 1:07 p.m. EST Feb. 27 (3:07 a.m. Feb. 28 Japan Standard Time) on an H-IIA operated for JAXA by Mitsubishi Heavy Industries. Engineers estimate the mission can last 12-13 years with the 550 kg (1,212 lb.) of fuel on board, according to Art Azarbarzin, the GPM project manager at Goddard. Arrays and the high-gain antenna should deploy within 1.5 hr. after launch, he says, and within eight or nine days the two instruments will be deployed and active, setting up a 60-day checkout period.

The instrument suite consists of the GPM Microwave Imager (GMI), a scanning microwave radiometer built by Ball Aerospace for NASA that serves the same function as the one on TRMM—but with 13 channels covering frequencies from 10-183 GHz to generate data that can be correlated with the data generated by all of the other radiometers in the GPM constellation—and a two-band precipitation radar built by JAXA and Japan's National Institute of Information and Communications Technology.

That data will give a reading on the total precipitation within all layers of clouds beneath the spacecraft, down to and including light rain and snowfall “which accounts for a significant fraction of precipitation occurrence in the middle and high latitudes,” Azarbarzin says. That will allow the new spacecraft to capture much more of Earth's precipitation than TRMM, which cannot detect snow or light rain.

The Japanese instrument, known as the Dual-frequency Precipitation Radar (DPR), contains Ku-band and Ka-band precipitation radars, aligned so they both cover the same 5-km (3-mi.) swath straight down to provide the highest resolution. Also an update on a TRMM instrument, the DPR will generate 3-D precipitation profiles that scientists and forecasters can use to determine the internal structure of storms. With the high-latitude coverage offered by GPM, they will also be able to monitor changes in those storm structures as they move out of the tropics into the mid-latitudes, a topic of particular interest in Japan.

“GPM will provide critical new information for a number of weather and natural hazards that are unique to Japan's part of the world,” says Riko Oki, JAXA's GPM project scientist. “Japan is located in east-north Asia, and is affected strongly by the Asian monsoon climate. . . . Every year in autumn, our life is affected by several typhoons approaching Japan.”

The GPM will work with satellites operated by French space agency CNES, the Indian Space Research Organization, Eumetsat, U.S. National Oceanic and Atmospheric Administration and U.S. Defense Department to generate the near-real-time precipitation maps. Data from the new core satellite will be routed to Goddard's PPC through the Tracking and Data Relay Satellite System ground station in White Sands, N.M. There it will be fused with temperature-brightness data from the other spacecraft in the constellation, processed into maps and sent back out to the public via the Internet.

“We use the GPM core spacecraft as a transfer standard so we get these next-generation uniform precipitation products everywhere in the world, every three hours,” says NASA's Gail Skofronick-Jackson, the GPM project scientist at Goddard. “Not only do we get the products every three hours, but we have near-real-time release of the data. . . . Can you imagine if flood forecasting and landslide forecasting had estimates within three hours of data collection, every three hours, all day long? That means that the emergency planners can evacuate, if necessary.”

The data also will be useful for climate-change studies, giving researchers another way to test the theory that extreme weather events will become more extreme as the climate changes. “Areas where they're already having heavy rain will have more rain; the areas where there's drought, they'll have more drought, or extended droughts,” says Skofronick-Jackson in outlining the theory.

Analysts at the PPC have algorithms that will allow them to correlate the data they receive through the GPM constellation with data going back to the first radiometers flown in space and forward into the future. The results also will allow climate researchers to better understand the changes over time, as will processing geostationary data of cloud-top movement down to 30-min. 3-D increments.

“They have several different levels of releasing the data,” says Skofronick-Jackson. “The data is downlinked every 90 minutes, and then from one to three hours after that you start getting all these estimates. We'll get another mid-estimate a day later, and then the climate-quality products a week to a couple of months later. They won't be any good for flood forecasting at that point, but the flood forecasters can use the near-real-time data for analysis.”

Project Manager Azarbarzin says NASA will spend $933 million on the GPM spacecraft and its share of the baseline three-year mission, including reserves. Japan's contribution totals $226 million, according to Kinji Furukawa, deputy project manager on JAXA's GPM radar instrument. The collaboration, a continuation of the teamwork that produced TRMM, has been in the works since the late 1990s, giving an idea of how difficult it can be to maintain data continuity over the long haul in government-backed space-science projects.

“It was a very long formulation period, which was a combination of factors involving the Congress and the administrations,” says Steven Neeck, deputy associate director of the Earth science flight program at NASA and a veteran of the GPM effort from the beginning. “It's not uncommon to take some time for science missions to come together and jell. There are also some complexities related to the launch service and the availability of it due to some of the delays, and that took some time to work out, too. . . . Due to good fortune, TRMM continues to operate, so we anticipate some degree of overlap between the two missions.”