Aerosol Instrument SPEX
(Texts, images courtesy of SRON)
Sunlight that is incident on a planet can be assumed to be unpolarised. Sunlight that has been scattered by gas molecules and/or particles (aerosol and/or cloud particles) in the atmosphere of a planet is usually linearly polarized. Polarimetry has proven to be an excellent tool to study the composition and structure of planetary atmospheres, because the degree, and to a lesser extent, the direction of polarization of the scattered sunlight is very sensitive to the microphysical properties of the scatterers (their size, shape, and composition) and to their vertical distribution. Accurate knowledge about these microphysical properties and the vertical distribution is essential to better understand their impact on climate, air quality and health on Earth.
The SPEX measurement concept
SPEX is an innovative, mult-angle single channel, high precision polarimeter for the characterization of planetary atmospheres. It can measure the intensity and the degree and angle of linear polarization of sunlight that has been scattered within a planetary atmosphere from 350 to 800 nm with 2 nm (intensity) to 20 nm (polarization) spectral resolution.
SPEX’ spectropolarimetry works by encoding the degree and angle of linear polarization of the incoming light in a sinusoidal modulation of the flux spectrum. A single modulated flux spectrum thus provides the degree and angle of linear polarization as functions of the wavelength. SPEX measures two flux spectra with different modulation phases, and from these two spectra, the total flux spectrum itself can be obtained. The modulation of the flux spectra is done with birefringent crystals. With its 1-liter volume of the optical prototype, SPEX is capable of full linear spectropolarimetry, without moving parts. The SPEX prototype collects light instantaneously from nine viewing directions (looking “downward” along the flight path). It measures the degree of linear polarization (DoLP), the angle of linear polarization (AoLP) and the flux simultaneously as a function of wavelength. Each viewing direction provides a seven degree swath with a 1×1 degree instantaneous field of view.
(Figure 1: The SPEX prototype (with commercial camera attached) in the lab during characterisation)
Although SPEX can be used to study any planetary atmosphere, including the Earth’s, the current design of SPEX is tailored to study Martian dust and clouds from an orbiting platform: a compact module with 9 fixed entrance pupils to simultaneously measure spectra from 350 to 800 nm in 9 different directions along the flight direction (including two limb viewing directions). With this design, the flux and polarization phase functions of dust and cloud particles at a given location are sampled when SPEX passes over the location.
A prototype of SPEX, employing the novel polarimetry method of spectral modulation has been fully characterized in laboratory conditions (ESA program). The absolute polarimetric accuracy – the key performance parameter of SPEX – has been determined with a specially designed optical polarisation stimulus. The results show the capabilities of the spectral modulation technology: already the prototype fulfils the stringent requirements for an aerosol characterisation space mission (see figure 2). In addition outside ground-based measurements have been performed at the Cabauw (NL) meteorological site looking at the Earth’ sky (van Harten et al, Atmospheric Measurement Techniques, 2014) and aerosol properties deduced using a dedicated retrieval algorithm (di Noia et al, AMT, 2015). Following the successful ground-based measurements the prototype is currently made suitable for high altitude airborne flights.
(Figure 2: Calibration results airborne SPEX instrument)
Difference between DoLP as measured by SPEX and DoLP generated by the absolute polarization stimulus, for four wavelengths (see legend). The dashed lines mark the required DoLP accuracy that result from the climate science requirements (deduced from Mishchenko, 2004; Loeb and Su, 2010). The calibration results of the SPEX prototype instrument indicate that the SPEX technology is suited to meet the challenging requirements on polarimetric accuracy.
The prototype model of SPEX has been constructed by a Dutch team comprising the University of Leiden, SRON Netherlands Institute for Space Research, TNO, Dutch Space, the optics group of NOVA-ASTRON, MECON, and Cosine with national funding from the Netherlands Space Office (NSO) and SRON.
Towards SPEX satellite instrument for Earth observation
For Earth science and monitoring applications, the spatial coverage (swath) must be extended compared to the present prototype. Therefore a dedicated modular medium-swath design, fully employing the proven spectral modulation technology of SPEX, has been established by TNO in collaboration with SRON and funded by NSO and SRON.
An instrument in the UVIS-NIR spectral range with very high polarimetric accuracy and medium swath is an excellent starting point as a socio-scientific Earth Science/monitoring mission. Given the modular design of SPEX, another promising option is to form a constellation of small satellites (distributed satellite system) to enhance the capabilities of a single instrument to form a synthetic instrument with larger swath (and/or larger revisit times), spectral range and number of viewing angles.