The GPM satellite is an Earth science mission that will study global precipitation (rain, snow, ice) with one Core spacecraft and a host of eight other vehicles in Earth orbit.

The instrument, known as GPM Microwave Imager (GMI), will measure the Earth's atmospheric moisture.

GMI will have a scanning 1.2-meter (3.9-foot) dish antenna that rotates with the instrument at a rate of approximately 30 revolutions per minute. GMI will run on 162 watts of power and weighs 160.5 kilograms (353.8 pounds).

The instrument will have a life expectancy of three years after launch.

Goddard intends to purchase the second instrument from Ball Aerospace and Technology Corp. (BATC) as a sole source procurement. The original GMI contract was awarded to BATC on March 22, 2005 to develop one flight unit for what is known as the Core spacecraft with an option to buy a second flight unit one year later.

In addition to the Core spacecraft, a constellation of up to eight satellites will comprise the GPM mission. Other vehicles in the constellation are called satellites of opportunity, contributed by domestic agency partners such as the National Oceanic and Atmospheric Administration, the Department of Defense, and GPM international partners.

One specific example of a potential satellite of opportunity is the proposed French/Indian mission known as Megha-Tropiques.

Each satellite of opportunity has its own unique scientific mission but will also contribute precipitation measurements to GPM. Each satellite in the constellation will carry one or more precipitation sensing instruments.

At a minimum, to be a support satellite for the GPM constellation, a mission has to carry some type of passive microwave radiometer measuring several precipitation frequencies.

"To achieve full potential of the GPM mission, it is paramount that the two instruments on the Core and Low Inclination Spacecrafts must be identical in performance," NASA said in contract documents released Thursday.

"Since BATC is currently under contract to develop the first GMI flight unit for the Core Spacecraft, they are uniquely suited to produce the second instrument for Low Inclination Spacecraft."

The second flight instrument is needed in late 2012 to support a low inclination spacecraft launch, NASA said.