Novel airborne imaging polarimeter undergoes flight testing
Tiny airborne particles—forming haze layers, pollution plumes, and water and ice clouds—are major players in Earth's climate system. Many of these particles efficiently reflect incoming sunlight back to space, cooling our planet. Others have a warming effect. The diversity of particle types and their high spatial and temporal variability make accurate determination of their effects on the global climate a challenging task.
Satellite remote sensing is the most effective means of rapidly monitoring large areas of Earth. To determine the properties and distributions of airborne particles by remote sensing requires accurate measurements of the intensity and polarization of light across a range of wavelengths. Climate scientists are therefore interested in polarimetric imaging with a spatial resolution of one kilometer or finer over a broad swath, and with an uncertainty of 0.5% or better in degree of linear polarization (DOLP).
Along with our colleagues at the Jet Propulsion Laboratory and the University of Arizona, we have developed a novel imaging polarimeter to meet this challenge. We envision a future satellite instrument—the Multiangle SpectroPolarimetric Imager (MSPI)—containing several polarimetric cameras pointed at different view angles (forward and backward along the swath being mapped out). An airborne prototype, AirMSPI, flies aboard NASA's high-altitude Earth Resources 2 (ER-2) aircraft. AirMSPI is a single camera mounted on a pointable gimbal, and includes eight spectral channels between 355 and 935nm. The 470, 660, and 865nm channels are polarimetric. The camera functions in push-broom mode: the channels have linear detector arrays that sweep along the aircraft's flight path without scanning from side to side. Figure 1 shows example imagery acquired during AirMSPI's maiden flight on 7 October 2010.
At the heart of the AirMSPI camera is an optical assembly that modulates the polarization of incoming light. This assembly rotates the light's plane of polarization at the camera's frame rate (about 25 frames per second). At each pixel in a polarimetric detector array, the modulation of light arriving during one frame encodes the original light's intensity I and either the Q or U Stokes parameter of its polarization, depending on the orientation of a polarizer overlying the array. The light's DOLP is computed from the ratios Q/I and U/I, which are determined very accurately.1, 2
The components performing this modulation are a pair of photoelastic modulators (PEMs) sandwiched between two achromatic quarter wave plates. The PEMs oscillate at slightly different frequencies near 40,000Hz, so that the beat frequency of the combination matches the camera's frame rate. This approach to measuring polarization required development of several technologies, including high-reflectance mirror coatings that would have minimal impact on the light's polarization,3 detector readout circuitry synchronized to the PEM oscillations,2 and methods to fabricate broadband quarter wave retarders4 and integrated filter/polarizer assemblies.2
This approach also requires processing large amounts of raw data. To generate the DOLP image in Figure 1, the sampled modulation patterns were recorded in flight and processed later on the ground. More recently, we developed the technology to process the modulated data on the fly in a field programmable gate array.5 Such a system on the satellite-based MSPI will greatly reduce the volume of data needing to be downlinked for each image. We have also developed an optical probe to monitor the PEMs during flight. These two systems were tested successfully on AirMSPI during an ER-2 flight on 31 August 2011.
These flight tests, which acquired images at visible and near-infrared wavelengths, have advanced AirMSPI's readiness for scientific field campaigns focused on aerosols and clouds. As a next step on the pathway to space, we are currently developing a new prototype, AirMSPI-2, with a spectral range that extends into the shortwave infrared (i.e., above 1400nm), which is very important for sensing coarse aerosol and cloud particles.
This research is being carried out at the Jet Propulsion Laboratory (JPL), California Institute of Technology, and the University of Arizona. Sponsorship by the NASA Earth Science Technology Office and Earth Science Research Program is acknowledged. We are grateful to the MSPI team at JPL and the University of Arizona and to many people at NASA's Dryden Flight Research Center. We owe special thanks to Sven Geier, Brian Rheingans, and Thomas Werne of JPL for overseeing the AirMSPI deployment.
David J. Diner is acting supervisor of the Aerosol and Cloud Science Group in the Science Division at JPL. He received a BS in physics from the State University of New York at Stony Brook and an MS and PhD in planetary science from the California Institute of Technology.
Paula J. Pingree is supervisor of the Flight Instruments Electronics and Small Satellite Technology Group in the Instruments and Science Data Systems Division at JPL. She received a BE in electrical engineering from Stevens Institute of Technology and an MS in electrical engineering from California State University, Northridge.
Russell A. Chipman is professor of optical sciences at the University of Arizona and heads the Polarization Lab in the College of Optical Sciences. He holds a BS in physics from the Massachusetts Institute of Technology and an MS and PhD in optical sciences from the University of Arizona.