Prototyping for the Spectropolarimeter for Planetary EXploration (SPEX): calibration and sky measurements

Abstract

We present the Spectropolarimeter for Planetary EXploration (SPEX), a high-accuracy linear spectropolarimeter measuring from 400 to 800 nm (with 2 nm intensity resolution), that is compact (∼ 1 liter), robust and lightweight. This is achieved by employing the unconventional spectral polarization modulation technique, optimized for linear polarimetry. The polarization modulator consists of an achromatic quarter-wave retarder and a multiple-order retarder, followed by a polarizing beamsplitter, such that the incoming polarization state is encoded as a sinusoidal modulation in the intensity spectrum, where the amplitude scales with the degree of linear polarization, and the phase is determined by the angle of linear polarization. An optimized combination of birefringent crystals creates an athermal multiple-order retarder, with a uniform retardance across the field of view. Based on these specifications, SPEX is an ideal, passive remote sensing instrument for characterizing planetary atmospheres from an orbiting, air-borne or ground-based platform. By measuring the intensity and polarization spectra of sunlight that is scattered in the planetary atmosphere as a function of the single scattering angle, aerosol microphysical properties (size, shape, composition), vertical distribution and optical thickness can be derived. Such information is essential to fully understand the climate of a planet. A functional SPEX prototype has been developed and calibrated, showing excellent agreement with end-to-end performance simulations. Calibration tests show that the precision of the polarization measurements is at least 2 · 10 −4 . We performed multi-angle spectropolarimetric measurements of the Earth's atmosphere from the ground in conjunction with one of AERONET's sun photometers. Several applications exist for SPEX throughout the solar system, a.o. in orbit around Mars, Jupiter and the Earth, and SPEX can also be part of a ground-based aerosol monitoring network.

Remote sensing of ice crystal asymmetry parameter using multi-directional polarization measurements – Part 1: Methodology and evaluation with simulated measurements

“…5b). However, the bias in scaled optical thickness τ * = τ (1 − g) (van de Hulst, 1957;King, 1987) is much smaller, as expected (Zhang et al, 2009). Figure 5c shows that mean differences between aspect ratios of the individual bullets and retrieved values are about 20 ± 40 % for τ cloud > 5, but increase to 60 ± 50 % for optically thinner clouds.…”

“…where µ 0 is the cosine of the solar zenith angle, F is the solar irradiance, and Q and U are the second and third Stokes parameters, respectively (van de Hulst, 1957;Chepfer, 1998;Baran and C.-Labonnote, 2006). To produce a look-up table (LUT) containing R p values for clouds consisting of hexagonal plates and columns with varying aspect ratios and roughness parameters, forward calculations are made using a plane-parallel doubling-adding code (Hansen and Travis, 1974;De Haan et al, 1987).…”

“…Vogelmann and Ackerman (1995) estimated that asymmetry parameters must be known to within 2-5 % to constrain computed shortwave fluxes to within about 5 %. Furthermore, a remote sensing technique to obtain ice asymmetry parameter could help to improve operational retrievals of ice cloud optical thickness and effective radius, which substantially depend on the scattering properties of the ice crystal model used in the retrieval algorithms (Rolland et al, 2000;Yang et al, 2008a;Zhang et al, 2009;Baum et al, 2011;van Diedenhoven et al, 2012).…”

“…Moreover, while previous results were difficult to relate to physical and radiative properties of ice crystals, here we directly relate the retrieved particle aspect ratio and roughness parameter to the ice crystal asymmetry parameter. A similar approach was recently applied to a subvisual cirrus cloud (Ottaviani et al, 2012a) and thick tropical clouds (van Diedenhoven et al, 2012), as well as over a snow surface (Ottaviani et al, 2012b).…”

The 3MI instrument on the Metop second generation

“…The Multi-angle SpectroPolarimeter Imager (MSPI) [6] as well as the SPEX instrument concept [7], offer equally powerful measurement techniques for the retrieval of polarimetric products, albeit both using a different modulation technique than that of POLDRER and 3MI (the first implementing a temporal modulation approach, while the later an innovative spectral modulation approach).…”