Inhalation of airborne particulate matter (PM) is associated with a variety of adverse health outcomes. However, the relative toxicity of specific PM types—mixtures of particles of varying sizes, shapes, and chemical compositions—is not well understood. A major impediment has been the sparse distribution of surface sensors, especially those measuring speciated PM. Aerosol remote sensing from Earth orbit offers the opportunity to improve our understanding of the health risks associated with different particle types and sources. The Multi-angle Imaging SpectroRadiometer (MISR) instrument aboard NASA’s Terra satellite has demonstrated the value of near-simultaneous observations of backscattered sunlight from multiple view angles for remote sensing of aerosol abundances and particle properties over land. The Multi-Angle Imager for Aerosols (MAIA) instrument, currently in development, improves on MISR’s sensitivity to airborne particle composition by incorporating polarimetry and expanded spectral range. Spatiotemporal regression relationships generated using collocated surface monitor and chemical transport model data will be used to convert fractional aerosol optical depths retrieved from MAIA observations to near-surface PM10, PM2.5, and speciated PM2.5. Health scientists on the MAIA team will use the resulting exposure estimates over globally distributed target areas to investigate the association of particle species with population health effects.
The Multi-angle Imaging SpectroRadiometer (MISR) has successfully operated on the EOS/ Terra spacecraft since 1999. It consists of nine cameras pointing from nadir to 70.5° view angle with four spectral channels per camera. Specifications call for a radiometric uncertainty of 3% absolute and 1% relative to the other cameras. To accomplish this, MISR utilizes an on-board calibrator (OBC) to measure camera response changes. Once every two months the two Spectralon panels are deployed to direct solar-light into the cameras. Six photodiode sets measure the illumination level that are compared to MISR raw digital numbers, thus determining the radiometric gain coefficients used in Level 1 data processing. Although panel stability is not required, there has been little detectable change in panel reflectance, attributed to careful preflight handling techniques. The cameras themselves have degraded in radiometric response by 10% since launch, but calibration updates using the detector-based scheme has compensated for these drifts and allowed the radiance products to meet accuracy requirements. Validation using Sahara desert observations show that there has been a drift of ~1% in the reported nadir-view radiance over a decade, common to all spectral bands.
The Airborne Multiangle SpectroPolarimetric Imager (AirMSPI) is an ultraviolet/visible/near-infrared pushbroom camera mounted on a single-axis gimbal to acquire multiangle imagery over a ±67° along-track range. The instrument flies aboard NASA’s high-altitude ER-2 aircraft, and acquires Earth imagery with ~10 m spatial resolution across an 11- km wide swath. Radiance data are obtained in eight spectral bands (355, 380, 445, 470, 555, 660, 865, 935 nm). Dual photoelastic modulators (PEMs), achromatic quarter-wave plates, and wire-grid polarizers also enable imagery of the linear polarization Stokes components Q and U at 470, 660, and 865 nm. During January-February 2013, AirMSPI data were acquired over California as part of NASA’s Polarimeter Definition Experiment (PODEX), a field campaign designed to refine requirements for the future Aerosol-Cloud-Ecosystem (ACE) satellite mission. Observations of aerosols, low- and mid-level cloud fields, cirrus, aircraft contrails, and clear skies were obtained over the San Joaquin Valley and the Pacific Ocean during PODEX. Example radiance and polarization images are presented to illustrate some of the instrument’s capabilities.
KEYWORDS: Cameras, Calibration, Data modeling, Error analysis, Clouds, Space operations, Image processing, Data processing, Data acquisition, Image registration
The Multi-angle Imaging SpectroRadiometer (MISR) is a part of the payload for NASA's Terra spacecraft launched in December 1999. The MISR instrument continuously acquires a systematic, global, multi-angle imagery in reflected sunlight in order to support and improve studies of the Earth ecology and climate. This paper focuses on the photogrammetric aspect of the data production and discusses quality of the global mapping as evaluated during the first two years of the mission. Usually, remote sensing image data has been only radiometrically and spectrally corrected, as a part of standard processing, prior to being distributed to investigators. In the case of the spaceborne MISR instrument with its unique configuration of nine fixed pushbroom cameras, continuous and autonomous coregistration and geolocation of image data are required prior to application of scientific retrieval algorithm. In order to address this problem, the MISR ground data processing system includes photogrammetric processing. From the entire MISR production system, three segments can be singled out as photogrammetric in nature. These are 1) in-flight geometric calibration, 2) georectification, and 3) cloud height retrieval. The data obtained through in-flight geometric calibration significantly simplify georectification part of the standard processing. Georectification gives fundamental input to scientific retrieval including cloud-top height retrieval.
The multi-angle imaging spectro-radiometer (MISR) instrument, which is scheduled to fly on the EOS AM1 platform, contains nine refractive cameras (four different lens designs) at preselected view angles which image in the push broom mode. Each focal plane contains four charge coupled device (CCD) line arrays consisting of 1504 active pixels; each array is preceded by one of the MISR spectral filters. In order to facilitate registration of the data generated by the 36 arrays during the initial phase of the mission, the crosstrack pointing angle of each pixel in each array was measured in the laboratory at the camera subsystem level. These measurements were particularly challenging because the pixels had to be calibrated under flight conditions (in a vacuum over the temperature range 0 to 10 degrees Celsius) to an accuracy of 1/8 pixel or 2.6 micrometer. Given the first order properties of the various lenses, this requirement implies that the distortion had to be calibrated to better than 10 arcsec. This paper will discusses the hardware and software techniques utilized to accomplish this stringent calibration.
During the standard geo-rectification processing of MISR imagery, all four spectral bands belonging to each of the nine MISR cameras are required to be geolocated and co- registered automatically to approximately one pixel accuracy. Two steps of processing are designed to accomplish this goal: (1) a complex multi-camera geolocation and co- registration of the red spectral band data for all nine cameras, and (2) the co-registration of the other three spectral bands of MISR imagery of each camera using their relationship with the already geolocated red band imagery. This paper addresses the second processing step. The geometry of the satellite orbit, the ellipsoid rotating earth, and the separation of the view angles between different spectral bands are combined in a mathematical model which describes the band-to-band line and sample parallaxes. The sensitivity study of this model to numerous error sources, such as variations in the orbit and earth radius, orbit perturbation, and navigation errors, leads to a practical polynomial band-to-band transform solution, and the decision on the usage of either a static or dynamic band-to-band transform as well as the application range of the transform.
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