WASHINGTON — After years of foot-dragging and budget-driven delays, NASA has finally given the green light to the $978 million Global Precipitation Measurement (GPM) mission, an international climate-monitoring effort aimed at improving scientific understanding of Earth’s water cycle and the accuracy of climate and weather predictions.

NASA Associate Administrator Christopher Scolese approved the joint NASA-Japan Aerospace Exploration Agency (JAXA) project during a Dec. 2 mission confirmation review here. Composed of a core spacecraft carrying two main instruments — a Dual-frequency Precipitation Radar and a GPM Microwave Imager — and a second, low-inclination spacecraft with just a microwave imager, GPM builds on the civil space agencies’ scientific and technical collaboration on the Tropical Rainfall Measuring Mission, or TRMM, a program that currently monitors tropical rainfall.

“This joint NASA/JAXA mission is scientifically important and stands as a prime example of the power of international cooperation in Earth observations,” said Michael Freilich, NASA’s Earth Science Division director, in a Dec. 8 statement on the GPM mission review. “GPM’s global precipitation measurements will advance our abilities to monitor and accurately predict precipitation on a global basis.”

GPM’s core observatory, which will be built in house by NASA’s Goddard Space Flight Center in Greenbelt, Md., is designed to set new standards that will allow the mission to unify and improve precipitation measurements from a variety of Earth-monitoring satellites carrying microwave sensors, including those from other U.S. and international organizations.

GPM Mission Scientist Arthur Hou said the mission will combine data from participating satellites to provide next-generation global precipitation products for scientific research and societal applications.

“A lot of sensors have the ability to provide information about precipitation, but they’re not specifically designed to do it,” Hou said Dec. 8. “The goal is to take a step beyond TRMM and provide an advanced radar radiometer system in space as a global transfer standard to inter-calibrate all the radiometers.”

The GPM constellation will provide uniformly calibrated precipitation measurements over 90 percent of the globe every two to four hours. Hou said the JAXA-developed Dual-frequency Precipitation Radar will be more sensitive to light rain and snowfall than TRMM. The two sensors include the Ku-band Precipitation Radar, which can detect heavy to moderate rainfall, and the Ka-band  Precipitation Radar, which is sensitive enough to detect light rain or snow. Together they can accurately observe rainfall rates from drizzle to snow in the high-latitude regions to intense rainfall in the tropics.

GPM’s microwave imager is currently in development at Boulder, Colo.-based Ball Aerospace & Technologies Corp.

Hou said the GPM core spacecraft is slated to launch in July 2013 at Tanegashima Island, Japan, atop a Japanese H-2A booster.

A second, low-inclination satellite, slated for launch in November 2014, will carry a second GPM microwave imager to be built under a contract Goddard awarded Ball in October using $10 million of NASA’s $1 billion in 2009 American Recovery and Reinvention Act funds.

The contract followed $18.2 million in economic stimulus money Goddard gave Ball in August to accelerate construction of the first microwave imager.

Although the original instrument contract awarded to Ball in March 2005 was valued at up to $100 million over a period of eight years, Hou said a series of contract modifications, including the addition of the second flight instrument, increased the value to $195 million. Ball has received $138 million of that amount to date.

Although NASA and JAXA are leading the GPM mission, both agencies are considering additional partners.

“European partnerships are under development,” Hou said, citing a draft memorandum of understanding in the works with France’s and India’s space agencies to utilize contributions from their joint Megha-Tropiques satellite as part of the GPM mission.

Hou said other potential European partners, including Eumetsat, have expressed an interest in joining GPM, as has the Brazilian Space Agency, whose current 10-year plan includes contributing a satellite to the GPM constellation. Meanwhile, Hou said NASA is still looking for a partner to contribute the spacecraft bus and launch for the GPM low-inclination observatory.


Rain Delay

Hou said the concept for the GPM constellation dates back to 1998, though project formulation did not officially begin until late 2001.

“The initial proposal was a real constellation mission, a core satellite with eight really light-weight radiometers carrying just a few frequencies,” Hou said. “That didn’t go very far, because of cost.”

GPM was one of several Earth science projects that lost funding to NASA’s manned exploration program and other competing priorities during the administration of former U.S. President George W. Bush, who in 2004 announced a plan to develop new rockets and spacecraft capable of returning humans to the Moon.

NASA continued to shortchange GMP until the National Academy of Science published its 2007 Earth science decadal survey identifying the mission as a top priority.

“Our budget was effectively zeroed out until [NASA] heard from the decadal survey,” he said. The lack of funding caused development of the GPM microwave imager to be delayed and the planned launch of the core GPM spacecraft to slip to 2010. Cost growth ensued, and additional funding reductions further delayed the launch schedule to 2013.

But Hou sees NASA’s formal commitment to the GPM mission as a positive sign, and sees an upside to the mission’s delayed implementation.

“I think we have learned a lot as the mission has gone through this protracted formulation phase,” Hou said. “A lot of things have become more mature, both in terms of the design and the science.”