Race for More Spectral Resolution for Earth Observation Satellites Is On

The race in space for commercial earth observation satellites was primarily for ever increasing spatial resolution. From the early days of Landsat’s 80 meter spatial resolution till today’s sub-meter accuracy on GeoEye and DigitalGlobe satellites, the demand for seeing objects on the ground more clearly was tantamount. Higher spectral resolution, however, took a back seat. But the next generation of satellites could change the perspective of users. While sub-meter spatial resolution will help you identify the class of ground-based objects, sub-classification can only be achieved by adding more spectral bands with narrower band widths.

For example, let’s say you are looking to find that rare copper mine nobody’s found yet. With copper prices fetching a premium price on commodity exchanges, you’re really looking to strike it rich. But while it’s easy to find mineralization in non-vegetated areas, it’s a different story altogether in the middle of a lodge pole pine forest. What you’ll need is a high spectral resolution imager that can discern the impact of chlorosis on pine needles exposed to high percentages of copper in the soil. The naked eye just will not see the change in reflected solar radiance values. The “digital prospector” will need a fairly sophisticated instrument.

All this might change with the upcoming launch of government and private sector satellites. DigitalGlobe announced yesterday that they were including Shortwave Infrared (SWIR) bands on WorldView-3. In addition to the more typical 8-band multispectral imager included on the satellite, there will be eight SWIR bands, each 30-40nm wide from the 1195-2365 nm range of the spectrum. Within this spectral range certain characteristics of plant material can be much more easily discerned.1

Walter Scott, DigitalGlobe’s CTO, was interviewed by Space News in which he commented that, “Shortwave infrared is really good at detecting differences between materials,” but he wouldn’t elaborate beyond that except to say that the additional bands would be good for “mineral exploration, vegetation moisture monitoring and water-resource management.”

On the upcoming Landsat Data Continuity Mission (LDCM), scheduled for launch in 2013, a thermal infrared sensor (TIRS) will be included on the mission. According to NASA, “TIRS was added to the satellite mission when it became clear that state water resource managers rely on the highly accurate measurements of Earth's thermal energy obtained by NASA satellites like LDCM's predecessors, Landsat 5 and Landsat 7, to track how land and water are being used.” Landsat has always had near IR bands included to support the detection of vegetation but thermal bands are new.

Now, this is all obviously good news for remote sensing scientists but it does introduce an entirely different part of remote sensing that GIS users may not be as familiar. More education about the value of higher spectral sensitivity will need to be conducted. While satellite born sensors will now carry 8 to 16 different spectral bands, airborne multispectral imagers have carried hundreds of bands of very narrow width. These platforms have usually been very expensive to operate. Whether UAVs will now be tasked for short missions with higher spectral imagers remains to be seen but you can anticipate what could happen.

It’s a welcome sign to see the advances in spectral imaging come to pass.

1Spectral Reflectance of Wheat Residue during Decomposition and Remotely Sensed Estimates of Residue Cover, Craig S.T. Daughtry, et. al., Remote Sensing, 2, 2010

Published Thursday, April 19th, 2012

Written by Joe Francica

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