An integrated multiangle, multispectral, and polarimetric imaging concept for aerosol remote sensing from space

Abstract

Techniques for passive remote sensing of aerosol optical and microphysical properties from space include visible, nearand shortwave-infrared imaging (e.g., from MODIS), multiangle intensity imaging (e.g., ATSR-2, AATSR, MISR), near-ultraviolet mapping (e.g., TOMS/OMI), and polarimetry (e.g., POLDER, APS). Each of these methods has unique strengths. In this paper, we present a concept for integrating these approaches into a unified sensor. Design goals include spectral coverage from the near-UV to the shortwave infrared; intensity and polarimetric imaging simultaneously at multiple view angles; global coverage within a few days; kilometer to sub-kilometer spatial resolution; and measurement of the degree of linear polarization (DOLP) for a subset of the spectral complement with an uncertainty of 0.5% or less. This high polarimetric accuracy is the most challenging aspect of the design, and is specified in order to achieve climate-quality uncertainties in optical depth, refractive index, and other microphysical properties. Based upon MISR heritage, a pushbroom multi-camera architecture is envisioned, using separate line arrays to collect imagery within each camera in the different spectral bands and in different polarization orientations. For the polarimetric data, accurate crosscalibration of the individual line arrays is essential. An electro-optic polarization "scrambler", activated periodically during calibration sequences, is proposed as a means of providing this cross-calibration. The enabling component is a rapid retardance modulator. Candidate technologies include liquid crystals, rotating waveplates, and photoelastic modulators (PEMs). The PEM, which uses a piezoelectric transducer to induce rapid time-varying stress birefringence in a glass bar, appears to be the most suitable approach. An alternative measurement approach, also making use of a PEM, involves synchronous demodulation of the oscillating signal to reconstruct the polarization state. The latter method is potentially more accurate, but requires a significantly more complex detector architecture.

Tolerancing and alignment of a three-mirror off-axis telescope

“…1) has a three-mirror reflective off-axis anastigmatic design suitable for dual photoelastic modulator (PEM) based spectropolarimetric remote sensing of aerosols from space. The ASPC optical design meets the following specifications [6]:…”

“…1. The effective focal length (EFL) of 29-mm and 10 µm pitch focal plane pixel size allow a nadir resolution of 7 m from a 20-km altitude and 225 m from 650 km [6]. Spot sizes across the FOV were kept below 10 µm in diameter by careful optimization of the mirror asphere parameters.…”

“…An f/5.6 focal ratio is chosen to balance light throughput requirements and the need to limit the angle of the cone of light passing through the filters and polarizers mounted above the focal plane line arrays. The system is telecentric in image space, so chief rays from different points in the field traverse the interference filters at nearly the same angle [6].…”

“…The PEMs are included to achieve cross-calibration of detectors [6]. Glass experiences stress-birefringence, or the photoelastic effect, when compressed along one axis.…”

Future Mission Concept for 3-D Remote Sensing of Aerosols from Low Earth Orbit

“…MSPI uses new technology to combine measurement characteristics of multiple sensors into a unified instrument, capitalizing on the complementary sensitivities of different techniques to particle characteristics [1] and using them synergistically to improve accuracy by reducing non-uniqueness. MSPI also incorporates an innovative "self-calibrating" approach to polarimetric imaging [2], providing size distribution variance and size-resolved real refractive index [3,4] over the aerosol column. Multiangle intensities constrain particle size and shape by sampling the particle scattering phase function [5,6], and are able to retrieve aerosol properties over bright source regions (desert and urban areas) [7,8].…”