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SPEXone Polarimeter

PACE's SPEXone instrument is a multi-angle polarimeter. It measures the intensity, Degree of Linear Polarization (DoLP) and Angle of Linear Polarization (AoLP) of sunlight reflected back from Earth's atmosphere, land surface, and ocean. The focus of the SPEXone development is to achieve a very high accuracy of DoLP measurements, which facilitates accurate characterization of aerosols in the atmosphere.

Aerosols are small solid or liquid particles suspended in the air that affect climate directly through interaction with solar radiation. Aerosols affect climate indirectly by changing the micro- and macro-physical properties of clouds. According to the Intergovernmental Panel on Climate Change, aerosols are the largest source of error in quantifying the radiative forcing of climate change. SPEXone will enable measurements of optical and micro-physical properties of aerosols with unprecedented detail and accuracy.

Artist's impression of SPEXone instrument in space - ©Airbus Defence and Space Netherlands & SRON Netherlands Institute for Space Research
Artist's impression of SPEXone instrument in space - ©Airbus Defence and Space Netherlands & SRON Netherlands Institute for Space Research
SPEXone instrument in SRON clean room - ©Airbus Defence and Space Netherlands & SRON Netherlands Institute for Space Research
SPEXone instrument in SRON clean room - ©Airbus Defence and Space Netherlands & SRON Netherlands Institute for Space Research.

SPEXone is being developed by a Dutch consortium consisting of SRON Netherlands Institute for Space Research (SRON) and Airbus Defence and Space Netherlands (Airbus DS NL), supported by optical expertise from the Netherlands Organisation for Applied Scientific Research. SRON and Airbus DS NL are responsible for the design, manufacturing and testing of the instrument. The scientific lead is in the hands of SRON. SPEXone is a public-private initiative funded by the Netherlands Space Office (NSO), the Netherlands Organization of Scientific Research (NWO), SRON, and Airbus DS NL.

SPEXone will feature:
  • A compact three-mirror segmented telescope assembly (patented by the consortium) to gather light from 0°, ±20° and ±50° (at satellite level) and direct the light towards a common entrance slit of a spectrometer.
  • Polarization Modulation Optics (PMO) to encode the state of linear polarization in the intensity spectrum as a sinusoidal modulation.
  • A compact and lightweight all-reflective imaging grating spectrometer.
SPEXone multi-angle polarimeter instrument. First image: Artistic rendition of the polarimeter operating in space. Second image: Building blocks along the signal path: 1) five-viewing angle telescope assembly receiving light from the Earth atmosphere under angles (50°, 25°, 0°, -25°, -50°), 2) polarization modulation optics module projecting the light on two orthogonal polarization directions (horizontal and vertical dash), 3) spectrometer module that creates a spectrum for each viewing direction and polarization direction 4) detector module that records the ten spectrally modulated spectra. Credit: SRON, the Netherlands Institute for Space Research.

SPEXone Heritage

Optical unit of SPEX airborne
Optical unit of SPEX airborne.
For the polarization modulation technique, SPEXone is based on heritage in ground-based and airborne applications. The spectral modulation technique was invented by Frans Snik and Christoph Keller at Leiden University and has been further developed in the Netherlands through several national programs.

Major steps in this development have been the development, characterization, and field-testing (ground-based) of a SPEX Prototype (originally designed for a Mars orbiter) and the "upgrade" of the SPEX prototype into a stand-alone instrument, SPEX airborne, for operating on the high-altitude (21 km or 13 mi) NASA ER-2 research aircraft.

The spectrometer of SPEXone is based on Dutch heritage with the Sentinel-5 precursor Tropomi instrument, its predecessor OMI, and the derived compact version Spectrolite.

SPEXone Overview
Less than 15 kg (33 lb) and 6 cubic decimeters (366 in3).
Less than 20 W.
2.7 x 2.3 km2 (1.7 x 1.4 mi2)
5.4 x 4.6 km2 (3.4 x 2.8 mi2).
100 km (62 mi) swath. Global coverage in 1 month.
385-770 nm.
2-5 nm for radiance, 10-40 nm for DoLP.
0°, ±20° and ±58° (on ground).
0.003 polarimetric, 2% radiometric.
SPEXone Details

SPEXone is a spectro-polarimeter that provides continuous wavelengths coverage in the range 385-770 nm. Spectral resolution is 2-5 nm for radiance and 10-40 nm for DoLP. It observes a ground pixel under 5 viewing angles (0°, ±20° and ±58° on ground), where the ±20° viewports will be used for cross calibration with OCI.

A compact three-mirror segmented telescope assembly (patented by the consortium) gathers light from 0°, ±20° and ±50° (at satellite level) and directs the light towards a common entrance slit of a spectrometer. There are several optical components before and after the slit that together form the polarization modulation optics (PMO). The PMO encode the state of linear polarization in the intensity spectrum as a sinusoidal modulation. The polarizing beam splitter in the PMO provides two complementary light beams where I is the intensity of the incoming light on the instrument, P the DoLP, β the Angle of Linear Polarization (AoLP), λ is wavelength, and δ is the instrumental retardance.
Equation
By combining I+ and I−, I can be determined at spectrometer resolution and the DoLP and AoLP at a resolution determined by the modulation width. I+ and I− both enter the spectrometer and are focused onto the detector as two pairs of five spectral images. SPEXone has optimized telescopes for each viewing angle, such that for all viewing angles a 100 km swath (ACT direction) is imaged onto the detector with a spatial resolution close to 5.4 km (3.4 mi) at a Spatial Sampling Distance (SSD) of 2.7 km (1.7 km). In the ALT direction, Nyquist sampling is achieved by performing two image acquisitions per 4.6 km (2.9 mi) sub-satellite point movement.
SPEXone Resources

SPEXone Science Requirements Document PDF (1.1 MB)

Aerosol measurements by SPEXone on the NASA PACE mission: expected retrieval capabilities, Hasekamp et al. (2019), Journal of Quantitative Spectroscopy and Radiative Transfer Science Direct

SPEXone: A compact multi-angle spectro-polarimeter, van Amerongen et al. (2018), International Conference on Space Optics PDF (932 KB)

Spectral modulation for full linear polarimetry, Snik, F., Karalidi, T., and Keller, C.U. (2009), Applied Optics 48 (7), 1337-1346 OSA Publishing