Dr. Bruce Wielicki, Senior Earth Scientist, Langley Research Center, Hampton, VA
- Wednesday, 01 February 2012
NTB: What are these “risks in technology?”
Dr. Wielicki: All NASA missions are trying to do something new, so the risk is really “Can you take from the laboratory an actual instrument that’s going to go into space, through launch, through contamination in orbit, through all of the other things that it might go through, and then achieve what it is you’re doing?” In our case, it’s pushing accuracy to levels to a factor of 5 beyond what current instruments do, and that really requires more care, detail, and independent verification systems. It’s a lot more like what the metrology world does actually — people like the National Institute of Standards and Technology (NIST), and like the National Physics Lab in the UK. If you think about it, no one accepts an international standard for the meter or the kilogram or watt until multiple independent groups have verified at high accuracy, within certainty determination of every component, what we know and don’t know about the observation.
CLARREO achieves traceability at every possible error source, how it can come unglued in different ways, and then we build up a really rigorous error budget of what’s normally called SI or International Standard traceability, to the accuracy levels we need. And that’s why we engaged people like NIST, who have been doing this in the laboratory for a long time. NASA Goddard [Space Flight Center], JPL [NASA Jet Propulsion Laboratory], and we at NASA Langley are involved, and we even have some of the other national physics labs around the world involved, the UK in particular.
NTB: What are the different instruments that you’re building in the laboratory, and how are they each used in the CLARREO mission?
Dr. Wielicki: One of them is a reflected solar spectrometer, and that one will have about a ½ kilometer field of view. It’s a 2D detecting array, so it’s kind of a push-broom in orbit. As you sweep along the ground, one linear dimension is mapping the earth in a 100 kilometer swath. The other dimension is doing the spectrum of reflected solar radiation. We’re getting that direction from about 350 nanometers out to 2300 nanometers. That includes about 98 percent of the reflected solar energy that the earth puts back out to space. We need it to achieve that high accuracy and the total earth’s reflectance of sunlight, which is a critical climate component. A normal imager, like a Moderate Resolution Imaging Spectroradiometer (MODIS), may have sensitivity to polarization of about 1 or two percent, or as much as 4 percent with some slow drifts in orbit. The CLARREO instrument is going to have a polarization sensitivity that’s closer to a quarter percent.
One of the other big differences is that we're able to look directly at the sun and the moon. Using precision apertures, filters, or other instruments to bring the solar radiance down into an earth radiance dynamic range, we are able to test, for example, the subtle nonlinearities that you might get in an instrument viewing the Earth. These nonlinearities might only be a few percent, but knowing their magnitude is critical for achieving the accuracy we need to observe climate change over decades. Again, there are things that aren’t critical for a weather observation, but we’re going for accuracy for that solar instrument in CLARREO.
NTB: How are the work and the responsibilities divided up? What’s it like to work in that coordinated approach?
Dr. Wielicki: That has really been a fantastic collaboration for us, because NIST, of course, is really interested in pushing metrology, in terms of accurate standards that really can benefit the world. When we brought CLARREO to them, the real advantage was we needed improved standards out into some wavelength ranges that they had done in some original explorations. They were slowly moving into that direction, but we gave them a lot more impetus for why it was important to get there now.
So, in particular, the far infrared wavelengths beyond about 15 or 20 microns out to 100 microns, and then in the near infrared. They had things really well understood out to 1100 or 1200 nanometers, but as you started to get out to 2300 or 2400 nanometers, they still needed some work to do. We have regular meetings with them. We have a formal agreement with NIST, we also have formal agreements with the United Kingdom with their National Physical Lab, with their climate modeling center, and also with some of their universities that are involved as well.
NTB: What is your specific role in the program? What is your day-to-day work?
Dr. Wielicki: I’m the cat herder. The scientists in the room are all very bright, determined, strongly motivated people, so you could imagine a room of 50 of those people arguing about the relative importance of different science topics.
I’m a science team lead on a science team for CLARREO that is especially diverse, where we have teams of scientists involved in reflected solar, in the infrared, and teams in radio occultation. You have to take those diverse teams and get them to agree on science requirements across the whole mission, so we’ve really worked hard on not just saying that we need really accurate data, but defining when do you reach the point of diminishing returns.
For example, you don’t need perfect data. Somewhere you have to define the real level of accuracy that you need, and what we were really able to achieve was to very rigorously define that what really was needed was to get under the natural variability or noise of the climate system itself, because we only have one earth. We use natural variability – things like El Niño – as a floor, and then get our accuracy by the factor of 2 under that, and then once you’re below that, then you’re at the point of diminishing return.