NASA engineers have developed a new instrument for gathering around-the-clock atmospheric carbon dioxide measurements from space. The CO2 Sounder Lidar would beam a laser down to the Earth's surface and measure the wavelengths of light that reflect back to provide a much more accurate measurement of CO2 levels than existing systems, and in a wider range of conditions.
Carbon dioxide absorbs specific wavelengths of infrared light, so space-based carbon-monitoring instruments are able to analyze the light that is reflected from the Earth's surface in order to measure the concentration of the gas in the atmosphere. Essentially, the lower the measurements of a particular wavelength that is reflected back, the more CO2 there is in the atmosphere absorbing it.
Current space-based instruments use reflected sunlight, which limits their effectiveness at night and at certain times of the year. NASA's CO2 Sounder overcomes these issues by providing its own constant light source – a laser.
Developed by NASA's Goddard Space Flight Center, the CO2 Sounder's laser uses a "step-lock" system to quickly switch between 16 specific wavelengths. The light reflected back is picked up by a powerful solid-state detector, made of a mercury-cadmium-telluride alloy which counts each returning infrared photon. The detector is particularly sensitive to near and mid-infrared wavelengths, which is the perfect range for the job of atmospheric greenhouse gas detection.
Running field tests over Nevada and California earlier this year, the CO2 Sounder proved its worth. "During the CO2 Sounder's 2016 (aircraft) campaign, we realized the detector's high sensitivity," says Anand Ramanathan, a member of the team. "It took 10 seconds to accumulate enough light to make a measurement in our 2013 campaign. In 2016, it took just one second. This is like improving the photographic film speed so that camera exposure can get shorter and one can get a higher frames-per-second rate."
The team is currently developing the CO2 Sounder with the goal of it being selected to take part in an upcoming carbon-monitoring mission, the Active Sensing of CO2 Emissions over Nights, Days and Seasons (ASCENDS). Obviously, a requirement of that program is that the system must be able to take measurements of unprecedented accuracy and resolution regardless of the time of day and seasonal weather patterns.
After analyzing the data from the recent field tests, NASA scientists think the technology is ready to meet these criteria. They say the instrument will soon be a flight-ready and ASCENDS compliant. Currently, the laser isn't quite powerful enough to beam the level of light required, but the engineers are developing a new fiber amplifier that should increase the laser's output by 100 times.
Over the next year, the team will continue testing the lab versions, and build towards the CO2 Sounder's final design, which will involve six of those fiber amplifiers working in unison. After that, if it wins the endorsement of the National Research Council, the CO2 Sounder Lidar could be ascending to orbit.
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