NASA Tests Global Warming Sensor for Use on Future Satellite

The makers of a prototype satellite sensor designed to test the accuracy of certain calculations underlying global warming predictions said they will start looking for opportunities to fly the instrument on a future NASA satellite.

The Far-Infrared Spectroscopy of the Troposphere (FIRST) instrument measures the cooling effect of water vapor in the lower atmosphere, a phenomenon climate scientists currently calculate indirectly from other readings, said Marty Mlynczak, an atmospheric scientist and the FIRST principal investigator at NASA’s Langley Research Center. Langley, located in Hampton, Va., managed development of the prototype instrument.

The FIRST team mimicked a spaceflight June 7 by flying the $5 million instrument to an altitude of 33,000 meters over New Mexico aboard a NASA research balloon.

“From a quick look at the data, it looks like the machine was operating just as we expected,” said Stan Wellard, the FIRST program manager at Utah State University’s Space Dynamics Laboratory in Logan, which designed the instrument for NASA.

The team will analyze the data in more detail and compare the results to infrared readings gathered simultaneously by the Atmospheric Infrared Sounder on NASA’s Aqua climate satellite. The FIRST instrument covers some of the same wavelengths as the sensor on Aqua, which was passing over New Mexico at the time of the balloon experiment , Mlynczak said.

Mlynczak said he is encouraged enough by the early results to begin searching for opportunities to build and fly a space version of FIRST, perhaps through NASA’s Earth System Science Pathfinder Program, which launches new kinds of environmental satellites and instruments.

“The climate is not going away as an issue. We will have an opportunity to compete to fly this in space some day,” Mlynczak said.

The Space Dynamics Laboratory developed FIRST after winning a contract in 2001 under NASA’s Instrument Incubator Program. Harvard University’s Smithsonian Astrophysics Laboratory in Cambridge, Mass., is a partner together with engineers at Langley. The incubator program funds development of prototype space sensors and tests them on aircraft and balloons.

The FIRST instrument measures the intensities of infrared emissions from water vapor molecules in the upper level of Earth’s troposphere, the layer of the atmosphere closest to the ground. These emissions cool the atmosphere by radiating heat to space, Mlynczak said. Most of those emissions are in the far-infrared portion of the spectrum at wavelengths from 10 to 100 microns, which FIRST would measure. The Atmospheric Infrared Sounder and Moderate Resolution Imaging Spectroradiometer on Aqua study wavelengths in the 4 to 15 micron range, Mlyczak said.

There is a “big chunk of energy that we’re not looking at routinely from space,” he said. “If we have a temperature and moisture profile, we can go ahead and calculate the far infrared and that’s fine. But given the central role [water vapor] plays in controlling the planet’s heat balance, we want to make sure we’ve got it right by measuring it,” he said.

The challenge facing the FIRST engineering team was to design a single sensor that would take infrared measurements over a wide range of wavelengths and in large enough snapshots to provide global coverage.

The FIRST design calls for an array of 100 infrared-sensitive detectors made of mercury cadmium telluride semiconductor material. Two optical mirrors direct the infrared energy to the far corners of the array, also known as a focal plane.

“Instead of having one detector that would essentially trace a pencil around the Earth everyday, now we have an array of detectors that scan side to side,” Mlynczak said.

To save money on the balloon version of FIRST, the engineering team ordered the installation of only 10 detectors, two at each corner and two in the center of the plane to serve as a control. The team members were confident they could illuminate most of the array with infrared energy and so they focused the test on the corners, Wellard said.

To measure the intensity of the emissions in each wavelength, the instrument divides the incoming infrared signals and directs them back together with the two mirrors. When the signals rejoin they interfere with each other, which is exactly what the engineers want. Sometimes the infrared signals cancel each other, and the signal strength drops to zero at the detectors. Sometimes they interfere constructively, and the signal is enhanced. The result is a pattern of interference for each wavelength called an interferogram. By running the interferograms through a computer back in a laboratory, scientists can figure out the intensity of each signal, Wellard explained.

Engineers at The Smithsonian Astrophysics Laboratory built FIRST’s beam splitter. It consists of a film of polypropylene covered with the dark gray element germanium, a substance used in semiconductors.

“Germanium is opaque in the visible [wavelengths] but in the infrared it works like a champ. It splits the beam without altering it,” Wellard said.

With the intensity data in hand, climatologists will be able to test the accuracy of their global warming predictions. Mlynczak cautioned that the new measurements would not remove all uncertainties or close the disparities among global warming predictions. “We’re going to have a better handle on a portion of a very complicated equation,” he said.