An imaging polarimeter for sensing of aerosol scattering and other atmospheric phenomena has been constructed and tested. The instrument is a testbed for a multispectral system architecture, in which spectral channels are added in a modular fashion using dichroic beamspltters and dedicated detectors. The testbed operates in a pushbroom scanning mode, with two co-boresighted optical trains. Each optical train features a narrow-band filter, an intermediate image at a slit, collimating optics, an appropriately oriented Wollaston prism, and two linear detector arrays. Consequently, the testbed is capable of determining the first three Stoke components (linear polarization) at a single wavelength. We describe calibration and field testing and present preliminary data analysis results.
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.
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.
Satellite-based remote sensing offers the best opportunity for studying the properties of atmospheric aerosols and their radiative effects on the global scale over extended periods Tropospheric aerosols represent the largest uncertainty in predicting the radiative forcing of climate (IPCC, 2001). A key question is whether atmospheric aerosols contribute to warming or cool the climate system. Since the sign of direct radiative forcing is controlled by the ability of aerosol particles to absorb light, the information on the single scattering albedo of different aerosol types and its variations with aerosol lifecycles is clearly very desirable. Furthermore, the wavelength-dependent values of single scattering albedo from the UV to IR are important in retrieving aerosol optical depth from satellites passive remote sensing and for climate modeling. We describe the design of a space-borne polarimeter that was derived from the key science requirements. The performance of this candidate polarimeter, operating in the UV/VIS/NIR spectral regions, is described. The optical design form and various optical performance parameters, including transmittance calculations of the polarimeter’s spectral channels showing the “leakage” of polarization states, as well as the degree of polarization as function of wavelength and field of view are presented for the various wavelength regions. An overview of the instrument performance and driving operational parameters is presented.
Spectralon has been proposed as a material for diffuse calibration panels for several satellite-based earth-observing instruments. This paper is an interim report on the ongoing flight qualification testing of this material, including tests of the stability of the material under exposure to atomic oxygen, proton fluences, and UV/VUV radiation which match those of the polar orbit environment. No measurable degradation of optical properties were observed following atomic oxygen exposure or proton bombardment. Under initial UV/VUV exposure tests, some degradation of the optical properties of the material were observed; this optical degradation has been linked to degradation of organic contaminants. Correlative tests (Gas Chromatography/Mass Spectrometry) indicate that such contamination can be prevented by the adoption of a more rigid production protocol. It is believed that elimination of these contaminants will result in a significant improvement in the optical stability of Spectralon under UV/VUV exposure. The validation of this new production protocol will occur during a second phase of UV/VUV testing.
In recent years data have been collected on conductive thermal-control paints, such as PCBZ and NS43G, in order to evaluate their stability to the space environment. In addition to being considered for spacecraft thermal control, the paints have been considered as an alternate material for use within on- orbit calibration systems. This will provide both an absolute calibration, by knowing the magnitude of the reflected light, and flat-field pixel-to-pixel comparisons within an instrument. Data are summarized here, as collected by the Cassini, Multi-angel Imaging SpectroRadiometer, and the Medium Resolution Imaging Spectrometer projects. Properties evaluated include absorptance, reflectance factor, electrical conductivity, thermal cycling, resistance, and environmental exposure stability.