Sensors and Systems
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Greenwood GeorgeIn order to make use of multispectral remote sensing, fieldwork called ground truthing is required to calibrate the spectral returns and match them to a library of known profiles in order to understand and classify the materials. ASD Inc., recently acquired by PANalytical, is a Boulder-based company that has a line of field spectrometers and spectroradiometers that are used widely in research and natural resource settings. Sensors & Systems (S&S) editor Matt Ball spoke recently with George Greenwood, senior market manager at ASD, about the many practical applications of this technology, and some of the exciting user applications of the company’s tools, particularly tied to measurements of change.

S&S: With spectroscopy, does it start with ground truthing and the calibration of a remote sensing image?

Greenwood: ASD started with the work of Dr. Alexander Goetz on ground truthing for NASA and JPL, and the calibration and validation of what were then newly launched sensors. Prior to that, spectroscopy was generally restricted to the lab with very large instruments. Alex’s breakthrough came with a radical new spectrometer design that reduced the size of the instrumentation, adding the element of field portability to spectral analysis. The science of field-based spectroscopy was born, providing scientists the opportunity to interact directly with sensor overflights to analyze other targets in their native environments.

Since those early days, the science of earth observation has turned into a multi-billion dollar imagery-based industry. Ground truthing is where the company came from, but since then the imaging industry has come a long way. Through smart phones, portable GPS, and other consumer-oriented mapping solutions, access to geospatial information is commonplace. Imagery is a big part of the geospatial information that people depend on, but there’s so much more to the image than just the pretty picture. Without spectral analysis, you lose all that additional information.

In a sense, remote sensing is the science involved with analyzing how light energy interacts with an object’s surface. Whether you are analyzing a leaf in your hand or a spot on the ground from a satellite image, it’s remote sensing if you are analyzing a discrete reflective spectral signature. It’s a non-destructive way to characterize things, with no drilling, no grinding.

Satellites are doing the same thing we do in the field, looking at the reflective energy, but it may be 400 kilometers away. We help to validate and characterize the elements of an image. Satellite and aerial imagery is getting more resolute information packed with higher resolution and more spectral bands for each pixel. Unless you visit that pixel on the ground, you’re just guessing what is. Software can quickly locate pixels with a similar signature, but unless it’s ground truthed and validated, it is an unsupervised classification.

To maximize the value of the analysis, you have to ground truth that pixel. With GPS, you can locate the pixel. With a spectroradiometer, you can assess that pixel spectrally, and conduct a supervised classification of the entire image and identify not just similar pixels, but characterize specific materials and their locations throughout the image.

There is also vicarious calibration, where you construct targets of known reflectance on the ground and remotely image them with an aerial or satellite sensor. Since you know the reflective characteristics of the target, you can assess the in-flight performance of the sensor. It’s a reliable method of verifying and calibrating performance. There are calibration sites all over the world, where known targets on the ground are imaged to make sure that the sensors are still calibrated.

S&S: It’s really exciting to think about the capacity of “seeing the unseen.” These spectral bands of imagery have been around for a while, right?

Greenwood: The early imaging satellites produced regional images, with low resolution and limited spectral bands. Successful programs like Landsat raised the bar with both increased resolution and spectral bands, but it was still really difficult to discern finite targets on the ground. Finally, with the launch of QuickBird and Ikonos over 10 years ago, the commercial marketplace was treated with one meter and eventually sub-meter imagery, and we wanted to characterize what was in those pixels.

As sensors became more resolute over time, they also added spectral bandwidth. They quickly pushed the envelope of the visible range. We are now pulling so much information out of a given image that is unseen to the eye – UV wavelengths on one side of the spectrum, and NIR, SWIR, and thermal (and beyond) on the other. And this is valuable information.

Today, we have hyperspectral aerial sensors with more than 200 spectral bands becoming more and more commonplace. We anticipate scheduled launches like Worldview-3 that can open high-resolution analysis capability to shortwave infrared, thermal, and UV studies. We’re learning more and more about the value that those wavelengths carry, and they come at a time when we have to get smarter about our understanding about what is going on with the rapidly changing environment.

S&S: Are there new discoveries about material signatures all the time?

Greenwood: With full-range spectrometers, when you are doing material recognition or classification, there’s what you see in the visible range and there is all that information that is available spectrally that goes into the near infrared and shortwave infrared range. So much information is out there that can help classify and characterize material, and there are many applications for that.

Let’s face it – it’s a smart and innovative community of researchers and academics that buy these instruments. I think the real value comes when we, as the manufacturer, step out of the way and let real application innovation run its course. It is amazing what people are discovering, validating and understanding. Imagination is really the only limitation when it comes to innovative applications of spectroscopy. I think it’s safe to say that new discoveries through this technology are happening all the time.

S&S: I notice that we’ve talked about spectrometers and spectroradiometers, what’s the difference?

Greenwood: It can be a little confusing. There are two basic flavors but at the core, both spectrometers and spectroradiometers take in light, break it into discrete spectral components, digitize the signal as a function of wavelength, and display the results.  We typically think of spectrometers and reflectance measurements. Based on its molecular make-up, everything interacts with light differently and produces unique spectral signatures. Spectrometers let us measure, display and store this information. They typically use internal or other artificial light sources.

A spectroradiometer can be used as a spectrometer for reflectance measurements but has the added capacity to directly measure emitted irradiance and radiance. This measurement basically characterizes the intensity and other parameters of the light energy source. It is essentially a calibrated spectrometer. Our spectroradiometers are factory calibrated to a NIST-traceable standard to accurately measure absolute energy levels of light sources. We typically think of using spectroradiometers in outdoor settings where solar and atmospheric variability play a critical role in field measurement accuracy. They are also used in the lighting industry to analyze artificial light sources.

S&S: One of the exciting aspects of this work is the highly calibrated scientific method, and as you alluded, our ability to understand global change.

Greenwood: We have to get smarter in understanding what’s going on, and with the potential impacts that are happening. The more we understand, the more effective our mitigation strategies will be. The research that ASD’s technology supports facilitates that understanding and includes impacts on ecosystems like agriculture, forestry, snow and ice, and coastal environments.

Globally, agencies and companies launching new satellites to better understand these changes, which include weather patterns, atmosphere, ecologies, and many other things. The industry is building more resolute satellite and aerial instruments, with more bandwidth, and resulting in more information. They are also getting innovative with smaller instruments and same path orbit insertion strategies to increase revisit times. The concept of a satellite train, where instruments follow each other in the same orbit is being used more and more by commercial companies The NASA A-Train project is another example of an innovative orbital insertion strategy with multiple satellites

With the implications of inaction so huge, and it’s never been more important to understand what’s going on. We recently attended a conference in India that was entirely focused on food, the environment and security. If we look at the implications of sea-level rise, the concentration of population in less stable areas, and the destabilizing potential forced population shifts hold, it’s easy to see how security issues become a key strategic planning piece.

We’re concerned here in Colorado, because it really is a different climate than 30 years ago, and a different forest. Wildfires have always been a seasonal event here but the season is lengthening, the fire intensity we are now seeing is literally off the charts, and it seems we just don’t have a local climate that sustains forest regrowth anymore. We just don’t have the moisture.

I like to think we are right in the thick of it with trying to better understand these changing ecosystems

S&S: It’s interesting that we seem to be moving in the path of Australia, where climate change is recognized and there is a great deal of legislation, because they felt it first.

Greenwood: There are certain areas in the world, including here in Colorado, where we’re on the edge of semi-arid and alpine ecosystems. Here, that edge is 6,000 to 8,000 feet. It’s an ecosystem boundary that is so sensitive to the one to two degree shift in temperature that has happened in the last 20 or 30 years. Coupled with the lack of moisture, it’s primed for what we see every year. It just happens to be where people love to live. And indeed, Australia is experiencing the same wildfire impacts in their ecosystem boundary regions. And while legislation may have been lagging in the U.S., we are catching up.

Where do we fit in? Spectroscopy is used to better understand these ecosystems, including agriculture – crop health, potential crop yield, and moisture and organic content in soil. It’s not just a matter of putting seeds in the ground and waiting for it to rain anymore. Spectrographic analysis is a big part of crop studies, from plant health to soils assessment and grain analysis.

S&S: One area that interests me is the ability to do real-time measurements. What is the real-time advantage in regard to spectral analysis?

Greenwood: I think a good example of the value of real-time data analysis is taking a look at the workflow process involved with mineral exploration. Back in the day, an exploration geologist had a topo map, a hand lens, a pocket knife and a bottle of acid. That was your toolkit. Mapping alteration zones was, at best, pretty subjective. You’d weight your pack down with samples destined for the assay lab when your field shift was done. You waited on those results. The whole process took weeks.

Now with portable field spectroscopy, spectral characterization is instantaneous. If spectral libraries are available, accurate mineral ID is on the spot.

Real-time analysis brings a whole new level of efficiencies to just about any operation that traditionally relied on sending samples off to a lab for analysis. You’ve removed the subjective element from your fieldwork. You save the lab cost, and now have an instantaneous answer. It saves money, time and lets you make informed decisions in the field.

S&S: We’ve touched on mining, agriculture, and snow and ice. Are there a great many more potential applications?

Greenwood: There are ongoing studies on many fronts and new applications presented all the time. Remote sensing really focuses on gaining a better understanding about what’s going on with the surface of the earth and that’s a pretty big topic. We touched on a couple of critical uses of spectroscopy relating to better understanding our dynamic environment, but there are so many more potential applications that have enormous commercial and industrial value. It’s really a science with unlimited potential.

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