Low polarization sensitivity is critical for a number of Earth science applications, including the measurement
of ocean color and ozone from satellite sensors. Polarization control places serious constraints on the optical
design of these sensors, particularly for the wide field of view imaging spectrometers contemplated for future
Decadal Survey applications. The PolZero Time Domain Polarization Scrambler (TDPS) is an optical
component that provides significant polarization sensitivity reduction for spectrometric instruments without
producing beam replication, image distortion or requiring a specialized optical path. The PolZero is
implemented using a pair of photo-elastic modulators to induce a particular high frequency polarization
structure onto the transmitted beam. Averaging this modulated beam over many temporal polarization cycles,
that is, over a few milliseconds, produces results approximating transmission through an ideal depolarizer.
Measurements have shown reduction in the transmitted degree of polarization by factors of over 30. Mueller
matrices of the TDPS, measured using a Mueller matrix imaging polarimeter, are presented as a function of
wavelength in the region 450-700 nm.
Polarimetry has been shown to aid in target detection by improving target contrast against both background and clutter. Further, Stokes vector sensors can provide surface orientation and material data, and permit extraction of targets obscured by scattering in the observer to object atmospheric column. This report presents a discussion of the construction of, and data from, the BATC imaging Stokes polarimeter. The polarimeter employs electro-optic modulators to acquire full Stokes vector image sets at a cadence sufficient to freeze target and background motion. These images are co-added to produce the polarimetric SNR necessary to fulfill mission requirements. We discuss the relationship of image timing, integration time, and SNR in the context of specific objectives for the polarimetric products. The relative advantages of implementing different electro-optic technologies will be illustrated by test and field image data.
Polarimetric measurements in the VIS/NIR spectral region improve aerosol microphysical and compositional retrievals. The retrieval approaches exploit the unique polarimetric signatures of aerosols as function of scattering angle, thereby driving the requirement for data collection over a large range of scattering angles. The scattering angle coverage is a function both of the instrument/sun/target geometry and the instrument architectural approach toward acquiring multi-angular data. These two functions are important aspects of a spaceborne, multi-angular polarimetric mission. The instrument design must also consider the impact of retrieval error arising from aerosol spatial variability. For a single-pixel scanning architecture, both the pixel separation as a function of earth rotation beneath the spacecraft and the pixel growth with increasing scan angle can result in significant retrieval errors due to aerosol spatial variability. We have investigated the impact of aerosol spatial inhomogeneity on the performance of a single-pixel, along-track scanning, multi-angular polarimetric instrument operating in a low-earth orbit (LEO) such as the EOS Aqua orbit of 705 km. Possible mitigation strategies to reduce the impact of the spatial inhomogeneity on aerosol property retrieval performance are also reviewed.
SPoRTMap is a system simulation tool for satellite-based polarimetric aerosol measurements. It integrates a large number of the tasks needed to simulate polarimetric Earth observations from satellite sensors: phenomenology model setup and run, sensor geometry setup, integration of sensor radiometric models, interpolation from model grid to sensor field of view, Stokes parameter SNR computations, etc. The architecture of the simulation system is modular to enable replacement of radiative transfer and sensor noise models. Operation of SPoRTMap is illustrated through creation of an orbital simulation using a specific aerosol model. Integration of diverse aerosol models into orbital mosaics is shown.
The multianode microchannel array (MAMA) has been chosen as the detector for two instruments on the ESA/NASA Solar Heliospheric Observatory. The response of the MAMA to the two extreme types of solar spectra, disk and corona, have been modeled with a view toward evaluating dynamic range effects present. The method of MAMA operation is discussed, with emphasis given to modeling the effect of electron cloud charge spreading to several detector anodes and amplifiers (n-fold events). Representative synthetic EUV spectra have been created. The detector response to these spectra is modeled by dissecting the input photon radiation field across the detector array into contributions to the various amplifier channels. The results of this dissection are shown for spectral regions across the entire wavelength region of interest. These results are used to identify regions in which total array photon counting rate or individual amplifier rate may exceed the design limits. This allows the design or operational modes to be tailored to eliminate the problem areas.