SPEX airborne

NASA’s atmospheric research aircraft ER-2 taking off. Instrumentation for atmospheric research can be mounted onto the aircraft at various locations; in the nose section, the fuselage, and in the two wingpods. SPEX airborne is located in the tailcone section of the right wingpod.
SPEX airborne payload inside the tailcone section of the wingpod of the ER-2 aircraft.
Artist impression of SPEX mounted onto the alignment structure of SPEX airborne. Scattered sunlight is captured from nine viewing directions. The degree of linear polarization is determined using spectral modulation of the state of polarization onto the flux spectrum. Each viewport has a 1×7 degree swath, which corresponds with 0.4×2.5 km2 instantaneous footprint at a cruise altitude of 20 km (60.000 ft). Nine viewports, nine times poloptics, nine times modulation spectra. Each viewport has 7 degrees swath, 2.5 km from ER-2.
SPEX airborne peeking through the ER-2 aircraft, from an opening in the tailcone section of the right wingpod.
Pilot Tim Williams about to get seated in the ER-2 for a flight with the SRON SPEX airborne and NASA-JPL AirMSPI polarimeters (2016). Operating at an altitude of 20km (60.000 ft)

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.

SPEX development

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
Power 75 Watt