NASA’s Lidar Has Potential for Carbon-Monitoring Mission

Scientists and engineers at NASA are set to accomplish a lifetime ambition after many years of research. The team aimed to develop an accurate and powerful instrument that can continuously collect measurements of worldwide atmospheric carbon-dioxide (CO₂) from space.

Dubbed the CO₂ Sounder Lidar, the instrument is a powerful competitor for a promising state-of-the-art carbon-monitoring mission, Days and Seasons, the Active Sensing of CO₂ Emissions over Nights, or ASCENDS.

While such a mission is being backed by the scientific community, researchers claim that the development of this mission relies on the support of the National Research Council in its subsequent Earth Science Decadal Survey. This survey marks an agreement of key research fields and the order in which they must be constructed.

According to scientists, the technology should be strong enough to determine greenhouse gases with the required unparalleled resolution and precision, irrespective of the time or season or time of day.

At NASA’s Goddard Space Flight Center in Greenbelt, Maryland, scientists and engineers believe that this new technology could be used to achieve the above outcomes, as the data collected during the instrument’s latest aircraft operation across Nevada and California has been already assessed earlier this year.

Headed by Goddard scientist Jim Abshire, the researchers outlined these results at the International Workshop on Greenhouse Gas Measurements from Space held in Kyoto, Japan, in early June this year.

Proven Aircraft Demonstrations

Goddard’s Internal Research and Development program, and later NASA’s Earth Science Technology Office supported the study which was started around 10 years ago. In the Lidar instrument, an infrared laser light is bounced from the surface of the Earth.

Just like atmospheric gases, the CO₂ gas takes up light in narrow wavelength bands which are infrared in this case. By adjusting the laser to the infrared light, scientists will be able to sense and examine the amount of CO₂ in that vertical path. The absorption lines will be deeper if more gas is present along the light’s path.

According to new data analysis, the CO₂ Sounder Lidar system will perform its task faster and with a greater level of accuracy, compared to the previous versions of the instrument. This phenomenal success was attributed to several key technologies.

For example, the Lidar instrument is fitted with the first solid-state detector in the world. This advanced detector is made of a special alloy known as mercury-cadmium-telluride and is extremely sensitive to the mid-and near-infrared wavelengths - a spectral sweet spot to sense greenhouse gases in the atmosphere of Earth. The detector can efficiently count all of the returning infrared photons, and thus has unmatched sensitivity.

Faster, More Efficient Retrieval

During the CO₂ Sounder’s 2016 (aircraft) campaign, we realized the detector’s high sensitivity. 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. Besides the high sensitivity, the detector also demonstrated a highly linear dynamic range, which was of great help flying over snow, which is surprisingly dark in the infrared.

Anand Ramanathan, Instrument-Development Team Member

Abshire added that the laser system of the instrument is equally important, which is a unique capability that is not experienced by present space-borne carbon-monitoring missions.

The laser system in the CO₂ Sounder Lidar offers light source, making it possible to determine the CO₂ gas day and night, irrespective of the season or time of day - an essential ability for ASCENDS. This capability is not found in current-generation of instruments, which depend on reflected or passive sunlight to collect measurements.

Laser Demonstrates Pluck

“The laser team deserves many kudos,” Abshire said, referring to Goddard laser engineers Jeffrey Chen, Mark Stephen, and Tony Yu, who are jointly developing the tunable laser system of the Lidar instrument. “The laser keeps getting better all the time.”

In order to determine trace gases such as methane and CO₂, the laser must be able to create different types of light wavelengths, especially those surrounding the wavelength believed to absorb the target gas.

During the study, the Goddard group used a laser that quickly and accurately adjusts to up to 30 varying wavelengths, while the other Lidar groups employed a single laser per wavelength. The laser, known as a “step-lock” system, can quickly change between the 16 wavelengths employed by the CO₂ Sounder Lidar.

It’s so fast you can precisely step through the wavelengths a hundred times, all within the blink of an eye.

Anand Ramanathan, Instrument-Development Team Member

For researchers who wish to know the chemical makeup of the atmosphere, another important benefit provided by the laser is the ability to determine water vapor whose absorption lines lie adjacent to the absorption lines of CO₂. “The step-locked system enabled us to make simultaneous water-vapor measurements along with the CO₂ measurement,” he added.

With the highly sensitive detector together with the speedily improving laser system, the researchers believe that they are at the forefront of advancing a flight-ready ASCENDS-type instrument. According to Abshire, one remaining obstacle is to make sure that the laser is strong enough to deliver a light beam and that it can accurately determine the returning signals for further spectroscopic examination, but despite this fact, Abshire is quite confident.

“We need to increase the power so we can take our instrument into space. These guys have figured a way to that,” Abshire said, attributing to the unique type of fiber amplifier developed by the team to increase the power of the laser by a factor of 100. The ultimate design of the sounder instrument will feature six of these laser amplifiers which will be employed in parallel, and laboratory breadboard testing will take place by next year, said Stephen.

“The advancement of technology has been amazing. I wouldn’t have thought this was possible just a few years ago,” Ramanathan said.