Calibration and validation of Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) polarization measurements

Gerard van Harten; David J. Diner; Brian J. S. Daugherty; Brian E. Rheingans; Michael A. Bull; Felix C. Seidel; Russell A. Chipman; Brian Cairns; Andrzej P. Wasilewski; Kirk D. Knobelspiesse

doi:10.1364/AO.57.004499

Applied Optics
Vol. 57,
Issue 16,
pp. 4499-4513
(2018)
•https://doi.org/10.1364/AO.57.004499

Calibration and validation of Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) polarization measurements

Gerard van Harten, David J. Diner, Brian J. S. Daugherty, Brian E. Rheingans, Michael A. Bull, Felix C. Seidel, Russell A. Chipman, Brian Cairns, Andrzej P. Wasilewski, and Kirk D. Knobelspiesse

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Gerard van Harten, David J. Diner, Brian J. S. Daugherty, Brian E. Rheingans, Michael A. Bull, Felix C. Seidel, Russell A. Chipman, Brian Cairns, Andrzej P. Wasilewski, and Kirk D. Knobelspiesse, "Calibration and validation of Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) polarization measurements," Appl. Opt. 57, 4499-4513 (2018)

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Abstract

The Airborne Multiangle SpectroPolarimetric Imager (AirMSPI), a precursor to the future Multi-Angle Imager for Aerosols satellite instrument, is a remote-sensing instrument for the characterization of atmospheric aerosols and clouds. To help discriminate between different aerosol particle types, which is crucial to improve our understanding of their impact on climate and air quality, AirMSPI acquires imagery over multiple view angles in the ultraviolet, visible, and near-infrared, and it employs dual photoelastic modulators (PEMs) to target an uncertainty requirement of $\pm 0.005$ in the degree of linear polarization (DoLP) at selected wavelengths. Laboratory polarimetric calibrations using a second-generation Polarization State Generator-2 (PSG-2) and validation measurements at $0 < DoLP < 1$ demonstrate a systematic uncertainty of $< 0.002$ . In-flight calibrations of the temperature sensitivity of the PEMs, which could otherwise introduce DoLP errors up to 0.02, are extracted from onboard “validator” measurements as well as from the AirMSPI imagery of the Earth. After this in-flight calibration, the stability of measurements of the validator’s DoLP throughout the POlarimeter Definition EXperiment and Studies of Emissions and Atmospheric Composition, Clouds and Climate Coupling by Regional Surveys field campaigns is $\pm 0.001$ . Comparisons of DoLP data from these campaigns with the Research Scanning Polarimeter show root-mean-square differences ranging between 0.003 and 0.006, while the regression slopes deviate from unity by currently unexplained values up to 0.024. The observed differences are the result of measurement errors in both instruments, as well as imperfections in the intercomparison. A complete polarimetric uncertainty model for AirMSPI is presented, including the effects of absolute calibration uncertainty, in-flight modulation uncertainty, and random noise.

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Instrument	POLDER-3	APS	HARP	3MI	MAIA
Year	2004–2013	2011 (launch failed)	2018-	2019-	$\sim 2021$ -
Platform	PARASOL	Glory	3U CubeSat	MetOp-SG-A	To be determined
Along-track angles	16	250	60 (670 nm) 20 (otherwise)	14 (VNIR) 6 (SWIR)	$\sim 5$ (selectable)
Along-track range (° at instrument)	$\pm 51$	$- 63.0 - 50.5$	$\pm 58$	$\pm 50.2$ (VNIR) $\pm 30.0$ (SWIR)	$\pm 60$
Across-track range (° at instrument)	$\pm 43$	0 (single pixel)	$\pm 47$	$\pm 50.2$ (VNIR) $\pm 50.2$ (SWIR)	$\pm 45$
Spatial resolution at nadir ( ${km}^{2}$ )	$6 \times 6$	$10 \times 20$ ( $across - \times along - track$ )	$2.5 \times 2.5$	$4 \times 4$	$\sim 0.2 \times 0.2$
Spectral bands (polarized)	10 (3)	9 (9)	4 (4)	12 (9)	14 (3)
Spectral range (nm)	443–1020	410–2250	440–870	410–2130	367–2125
DoLP uncertainty	0.01–0.02	0.002–0.004	$> 0.001$	0.01–0.02	0.003–0.005
Polarimetric technique	Rotating polarizers at 0/60/120°	Polarizing beam splitters for 0/90° and 45/135°	Three-way beam splitter with polarizers at 0/45/90°	Rotating polarizers at 0/60/120°	Photoelastic modulators
Reference	[5,6]	[9,10]	[11–13]	[14]	[15]

Deployments	$\sim 110$ flights since 2010. Participated in several field campaigns: PODEX (2013), ${SEAC}^{4} RS$ (2013), Pre-HyspIRI (2014), Pre-PACE (2014), CalWater (2015), RADEX/OLYMPEX (2015), ImPACT-PM (2016), ORACLES (2016), ACEPOL (2017).
Platform	NASA ER-2 research aircraft, flying at 20,000 m altitude at 210 m/s cruise speed.
Observables	Radiance	Degree and angle of linear polarization
Spectral bands (widths)	355 (48), 377 (40), 443 (46), 469 (46), 553 (39), 659 (45), 863 (44), 931 (53)	469 (45), 659 (46), 864 (47)
	Different spectral bands and polarization directions (Stokes $Q$ and $U$ ) are recorded in different across-track detector rows, overlaid with spectropolarimetric filters.
Imaging	Pushbroom imager on programmable gimbal for variable along-track angle.
Field of view across-track	$\pm 15 °$ for 10 km swath at nadir.
View angle range along-track	$\pm 67 °$
Observing modes	Step and stare: pushbroom imaging of a target from typically nine discrete view angles during overflight. This mode samples the aerosol scattering phase matrix and polarized surface bidirectional reflectance for aerosol retrieval.	Sweep: continuously changing view angle across the full $\pm 67 °$ range. This mode samples the primary cloudbow and supernumerary bows for the retrieval of liquid cloud droplet size distribution.	Step and pseudostare: step and stare with slow forward slew at each view angle for increased along-track spatial coverage.
Image size: along-track × across-track	$10 \times 10 {km}^{2}$	$100 \times 10 {km}^{2}$ forward sweep $70 \times 10 {km}^{2}$ backward sweep	$12 \times 10 {km}^{2}$
Spatial resolution of geo-rectified image products: along-track × across-track	$10 \times 10 m^{2}$	$25 \times 25 m^{2}$	$10 \times 10 m^{2}$
Polarimetric technique	Photoelastic modulators, measuring Stokes $I$ , $Q$ and $U$ at 23 Hz.

	$δ_{0} (470 nm)$	$δ_{0} (660 nm)$	$δ_{0} (865 nm)$	$r$	$η$
On-ground calibration	4.472	3.081	2.284	0.000	0.009
In-flight calibration PODEX	$4.502 \pm 0.003$	$3.123 \pm 0.003$	$2.330 \pm 0.004$	$0.000 \pm 0.000$	$- 0.002 \pm 0.001$
In-flight calibration ${SEAC}^{4} RS$	$4.428 \pm 0.101$	$3.068 \pm 0.072$	$2.286 \pm 0.054$	$0.014 \pm 0.008$	$0.009 \pm 0.005$

	PSG	PSG-2	Improvement
Light source	Exchangeable LEDs at AirMSPI’s three polarized wavelengths.	150 W Xenon arc lamp solar simulator.	Increased brightness and spectral coverage; natural spectral illumination of AirMSPI bands.
Illumination	Light pipe and lens for Köhler illumination over 4° field of view.	Integrating sphere directly illuminating 6° field of view; no lens.	Improved uniformity of intensity and polarization; reduced sensitivity of DoLP to pupil alignment; no diattenuation or stress birefringence due to lens.
Depolarization	Light pipe and polarization scrambler.	Integrating sphere.	Achromatic.
Partial polarizers	Single glass plate, tiltable to adjust DoLP.	Fixed pairs of glass plates with opposite tilt angles; four pairs for $DoLP = 0.01$ , 0.05, 0.10, 0.20.	No beam shift; higher DoLP uniformity across field of view.
Polarization angles	Orientation of partial polarizers fixed, followed by rotatable half-wave plate to adjust AoLP.	Polarizers rotatable.	No half-wave plate chromaticity, diattenuation, or stress birefringence.

$λ$ (nm)	470	660	865
$Δ λ$ (nm)	45	46	47
$ξ$	0.516	0.605	0.602
$QE$	0.40	0.35	0.13
$s_{min} - s_{max}$	$4.11 - 4.99$	$3.54 - 4.17$	$2.93 - 3.27$

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Applied Optics