SPEX airborne is a multi-angle spectro-polarimeter payload designed to fly onboard NASA’s high altitude research aircraft ER-2 where it performs remote sensing measurements of aerosol and cloud particles.
Aerosol refers to small solid or liquid particles suspended in the Earth’s atmosphere, like fine dust, smoke, pollen or soot. These particles scatter or absorb (sun)light. Analysis of scattered light provides information about properties like size, shape, and chemical composition of these aerosols. This information is needed to study the impact of aerosols on climate and air quality. See remote sensing of aerosols.
Spectral modulation for polarimetric accuracy
For the characterization of aerosols, it is of utmost importance to measure the degree of polarization with very high accuracy (far below the 1% level). In order to achieve this high polarimetric accuracy, SPEX airborne employs the method of spectral modulation to measure the degree of linear polarization (DoLP) of sunlight scattered by the Earth’s atmosphere and the particles therein.
The degree of linear polarization is encoded onto the 400-800nm spectrum in the form of sinusoidal variation of the intensity with wavelength. As a consequence radiance and polarization are captured without moving parts in a single measurement with a single aperture without parallax allowing for high polarimetric accuracy. From calibration measurements performed at NASA-JPL facilities it is found that SPEX airborne has an DoLP error of less than 0.2% over the entire wavelength range.
The first SPEX prototype instrument was developed in 2010 by a Dutch public-private consortium led by SRON, showing excellent results in the lab. Subsequently, the prototype participated in ground-based aerosol campaigns at the Cabauw meteorological site, in collaboration with the RIVM (the Dutch National Institute for Public Health and the Environment). These first good real measurements raised the interest of NASA to fly the airborne version of SPEX on their ER-2 high altitude research aircraft, together with other (US) aerosol instruments. Only satellite missions allow for a global and consistent view of the Earth. Therefore, this is the next step on the SPEX roadmap. See instrument development of the Earth programme for more information.
|Performance Specifications SPEX airborne instrument|
|Spectral range||400-800 nm|
|Spectral resolution polarization||20 nm|
|Spectral resolution radiance||2 nm|
|Viewing directions||0°, ±14°, ±28°, ±42°, ±56°|
|Field of view per viewing direction||7°×1.0° (cross × along track)|
|Accuracy Degree of Linear Polarization||< 0.002+0.005*DoLP|
|Accuracy Spectral radiance||< 2%|
|Mass SPEX airborne (total)||65 kg|
|Mass optical subsystem||1 kg|
|Dimensions||1320 x 470 x 718 mm3|
|Dimensions optical subsystem||145 x 115 x 40 mm3|