MISR: In-Flight Radiometric Calibration and Characterization Plan

Climatology Project (ISLSCP) is sponsored by the World Meteorological Organization and theInternational Commission of Scientific Unions).Sunphotometers available to the MISR vicarious calibration team include Reagan manualand autotracking sunphotometers, as well as CIMEL sunphotometers. These instruments aredescribed in the MISR [Val Plan]. As the Reagan instrument and data reduction techniques havebeen key to VCs for over a decade, these data will be baseline unless the other measurements areproven to be of greater accuracy. Sunphotometer calibration will include a trending analysis ofclear-atmosphere data, taken over the vicarious test sites.These atmospheric measurements will be used as input to a RTC. The code MISR willused is termed the West RTC, and is based on the adding-doubling method 48 . It will be used tosimulate TOA radiances as would be observed by the MISR instrument, for a given choice ofaerosol type and amount, and surface reflectance (or upwelling radiance) properties. Eventuallythe aerosol model to be used for VC computations will be based upon the retrieval work of theMISR aerosol team over our test sites. Other RTC tools, such as use of MODTRAN, will proveinvaluable at computing radiance distributions with wavelength. MODTRAN results, if utilized,will be scaled to agree with the monochromatic computations of the West RTC, at wavelengthswhere the West code is run.In order to generate TOA radiances, averaged over the MISR spectral band profile, the insitumeasurement approach will be to:• measure radiances and reflectances in bands simulating the MISR passband;• characterize the scene spectral distribution of the measured radiances or reflectances;and• measure atmospheric parameters in narrow bands (~ 10 nm).Portions of the filter plates constructed for the MISR instrument procurement have beenset aside for use in MISR in-situ instruments. These will be used in the field instruments used tomeasure radiance and reflectance.The rationale for this is that radiances should be measured using sensors with spectralresponse profiles matching MISR, where possible (yielding a product simulating the MISRretrieved radiances). When propagating these radiances using a RTC, however, monochromaticradiance, reflectance, and atmospheric parameters are used. To generate RTC input, therefore, aspectral model of the radiances are assumed. The RTC propagates radiance components atdiscrete wavelength sample points (e.g., every 5 nm throughout the MISR in-band response range,and every 100 nm elsewhere between 365 and 1100 nm). These TOA radiances then need to beinterpolated with wavelength, to deliver MISR band-averaged radiances. Radiances representativeof all MISR bands and field angles will be computed. The Validation Team will deliver theseradiances to the IFRCC team, who will regress them against the corresponding camera DN valuesto provide the VC coefficients.5. In-flight calibration46