HIRDLS comes through
Even though the orbiting instrument is damaged, ACD staffers and their colleagues have figured out how to achieve most of their science objectives.
The instrument. Researchers are unsure why HIRDLS has experienced problems, but they believe a piece of plastic became lodged by the outer scan mirror just inside the viewing aperture. In this file photo, the aperture, shaped somewhat like a hot dog, is toward the top of the instrument. As it orbits, HIRDLS (just more than a cubic yard in size) hangs from the bottom of the Aura satellite, flipped from the orientation shown here. (Photo courtesy HIRDLS.)
For a brief time last July, ACD's John Gille and the rest of the HIRDLS team felt like they were on top of the world.
The much-anticipated HIRDLS instrument (HIRDLS stands for High Resolution Dynamics Limb Sounder) took off aboard NASA's Aura satellite in a perfect launch on July 15—exactly 16 years after the initial proposal for the breakthrough instrument. Everything seemed on track for the instrument to measure atmospheric chemicals and temperatures in unprecedented detail, advancing scientific insights on such critical issues as ozone loss and the impact of greenhouse gases on climate.
But when team members began to activate the instrument in August, they discovered something had gone wrong. The signal, instead of revealing the expected range of atmospheric radiation detected by infrared technology, was almost entirely flat. Slowly a horrifying realization sunk in: the technology-laden orbiting instrument to which they had devoted years of their professional lives was not able to observe the atmosphere because its view was blocked.
"I felt like I had been kicked in the stomach," recalls John, the U.S. principal investigator on the project.
Rather than accede to recommendations that the project be scrapped, the team devoted the following year to a tenacious struggle to extract vital scientific data from their damaged machine. Their efforts have paid off dramatically. It now appears that HIRDLS, over the next few years, will be able to carry out 85–90% of its original science studies. NASA managers, at a September meeting with the HIRDLS principal investigators, accordingly signed off on continued funding.
Here's a look at how the HIRDLS project was saved.
Bright red from top to bottom
After getting their proposal accepted in 1989, John and colleagues in ACD, CU–Boulder, and Oxford University worked intensively to design and build an instrument that would provide a detailed picture of chemistry from the upper troposphere to well above the stratosphere.
Members of the HIRDLS team pose next to a model of the orbiting instrument in Center Green 2.
Seated (left to right): Vince Dean, Dan Packman, Cheryl Craig, and Hyunah Lee. Standing in back row (left to right): Linda Henderson, Charlie Krinsky, Brent Petersen, Doug Kinnison, Tom Eden, Joe McInerney, Gene Francis, Brendan Torpey, Bruno Nardi, Chris Halvorson, Rashid Khosravi, Greg Young, and Charles Cavanaugh. Standing to the right of the model: John Gille (left) and Joanne Loh.
Not pictured, include Mike Coffey, Jim Craft, Steve Massie, and Barb Tunison.
(Photo by Carlye Calvin, UCAR.)
The result, HIRDLS—little more than a cubic yard in size—was laden with technical wizardry. An outer scan mirror would reflect light from the atmosphere onto other mirrors and eventually to the instrument's core. There, infrared detectors, designed to sense wavelengths just longer than those that humans can see, would use 21 channels to measure signals indicating the amounts of numerous chemical compounds that influence ozone levels, climate, and air quality. The data would help scientists learn more about such issues as the role the stratosphere plays in climate change and whether the ozone layer is recovering as predicted.
John and other team members went to Vandenberg Air Force Base in California last year to watch NASA launch HIRDLS and three companion instruments aboard Aura. Once the satellite settled into orbit, they activated onboard coolers so the instrument's detectors would work. Then, on August 10, they opened the outer doors so light would be reflected onto the detectors.
That's when they got their first view from HIRDLS and realized there was a problem. "We weren't seeing what we were supposed to see," ACD's Tom Eden says of the digitized plots that were unexpectedly uniform.
Once visualized, the data should have revealed a two-dimensional image, indicating altitude and longitude, with colors ranging from yellow at the bottom (lower atmosphere) to black at the top (outer space). Instead, the team saw a bright red from top to bottom and left to right, except for a small area of orange at the lower left.
The researchers suspected something was blocking the instrument's view. They swung into action with a team of engineers from NASA and Lockheed Martin, which had built the instrument. While the researchers, working with NASA, gingerly repositioned the instrument's mirrors, Lockheed conducted a series of simulations. The company determined that a piece of plastic film, which had been designed to protect the instrument's optics, might have torn when the external pressure dropped during launch and become lodged over the outer scan mirror.
The next step was for NASA to try to shake off the plastic. For about two months, the team attempted a series of increasingly aggressive maneuvers, sharply moving the 8.5-inch (22-centimeter) diameter mirror in hopes that the plastic would fall away.
In January, a NASA review board concluded that the situation was hopeless. "They were on the verge of writing off the experiment," John says. But NASA agreed to John's request that funding be continued through the September 30 end of the fiscal year to see whether HIRDLS could still produce any usable data.
Subtracting the signal
Despite the setback, John and a few of his colleagues remained hopeful for a couple of reasons. First, the impact of the plastic appeared to be stable and predictable, which meant that perhaps it could be defined. Second, when the mirror was at an angle far from the Sun, the team could measure the atmosphere through a small gap in the plastic.
Perhaps, the researchers hypothesized, they could calculate the signal coming from the plastic and subtract it from the total signal. Whatever remained would be coming from the unobstructed portion of the mirror. They could then scale up the accurate piece of the signal to recreate the entire picture as it would exist without any obstruction.
"We had come back from the precipice."
As ACD's Gene Francis explains, "It became evident that the thermal signal from the obstruction is relatively stable and repeatable. That gave us confidence we should be able to mathematically model its behavior and subtract it from the measured signal to obtain the atmospheric signal of interest."
The team scored an important breakthrough when NASA pitched up the Aura spacecraft to point HIRDLS toward space, where the temperature, and thus the infrared signal through the gap, would be close to absolute zero. This meant the entire remaining signal from HIRDLS could be attributed to the plastic, and it enabled the team to produce a first estimate of the plastic's impact.
But the challenges remained enormously complex. Even though the plastic's signal was stable, it varied regularly as Aura orbited from the day to the night side of Earth, and as the seasons changed. Moreover, it blocked different amounts of the view as the mirror was moved up and down to gather data from various heights in the atmosphere.
As the researchers chipped away at the problem, they struggled to keep up their morale. Some left for other jobs, concerned that funding for the project would ultimately be cut. "We were on a roller coaster," Tom says. But he credited John's optimism for helping to keep the team going.
By summer, the team was beginning to see important results. The researchers used HIRDLS to estimate amounts of relatively abundant chemicals, such as ozone, and to determine temperatures in various regions of the atmosphere. By comparing their readings to data from other instruments, they verified that HIRDLS was, in fact, providing an accurate picture of the atmosphere.
"We had come back from the precipice," said ACD's Steve Massie. "To be able to work with the data and to do the reality checks—graph the data and see that it all made sense—was enormously gratifying. This was a triumph of the human spirit."
The progress also gave the team hope that it could home in on less abundant chemicals, including water vapor and various nitrogen compounds. To accurately measure such chemicals, the team will need to further refine its equations because even a small error in the process could greatly affect the estimate of a relatively rare chemical.
Armed with the findings, John went to NASA headquarters to ask that funding for the project be continued. On September 23, NASA managers agreed that the five-year project could go forward as originally conceived.
John and his colleagues were elated. As John puts it, "We were given up for dead and we've come back."
Thanks to the team's work, scientists are seeing aspects of the atmosphere in more detail than ever before.
For example, HIRDLS can detect the locations of polar stratospheric clouds over the Arctic, which play a central role in chemical processes involving chlorine that deplete stratospheric ozone. Steve believes the instrument will ultimately show the clouds on a daily basis, helping scientists better understand changes in the ozone layer that protects Earth's surface from dangerous levels of solar radiation.
"These are unique measurements," Steve says. "They will allow us to better estimate ozone loss."
Along with ozone, the HIRDLS team expects to gather data on water vapor, another important greenhouse gas, and on pollutants such as nitrogen oxides. The instrument will also enable researchers to learn more about the behavior of high-forming cirrus clouds that influence climate by affecting the amount of solar radiation that reaches Earth's surface.
The view from HIRDLS. Researchers have verified that information from HIRDLS is providing an accurate picture of important chemicals in the atmosphere. The HIRDLS image (above left) corresponds to data from another orbiting instrument, the Microwave Limb Sounder (above right). At right, HIRDLS is generating data that enable researchers to map the locations of polar stratospheric clouds in the Arctic (shown in dark gray), which play an important role in ozone depletion. (Illustrations courtesy HIRDLS.)
But the plastic obstruction is forcing team members to scale back some of their scientific goals. They cannot turn the outer mirror as much as originally planned, which means the instrument cannot gather data from the extreme southern regions of the planet, including Antarctica. The limits on the mirror's movements will also curtail the instrument's horizontal resolution, which means researchers will not learn as much about longitudinal variations in chemical composition, clouds, and temperatures as they had hoped.
Still, John believes the team can compensate for some of these shortcomings. For example, HIRDLS is taking more vertical readings along its scan track than originally planned, and these narrow views of the atmosphere can be sewn together into a broader picture with the help of computer modeling. "We think we can reconstruct a lot of the longitudinal detail that we don't see directly," John explains.
As frustrating as the last year has been at times, team members say they've enjoyed the challenge of working with a compromised instrument. "It's problem solving in the real sense that you don't know what the answer is, you don't even know if there is an answer, but you try to move ahead in a systematic way," Gene says. "So far we haven't run out of ideas, and we are making progress. The collective expertise of the HIRDLS team is making this work."
• by David Hosansky
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