Validation Field Instruments
The CIMEL sun/sky photometer system is capable of long-term independent operation for measurements of direct solar irradiance and sky radiances used to determine optical depth and aerosol scattering and absorption properties. The sensor head is attached to a fully automated mechanism that carries out direct beam solar measurements as well as sky radiance measurements in the solar almucantar and in the solar principal plane. Two collimators feed detectors for measurement of direct sun, aureole and sky radiance. There are 8 interference filters at various wavelengths and 3 polarizing filters at 870 nm.
This battery powered automated-tracking solar radiometer was developed at the University of Arizona. It contains 10 sensors at wavelengths from 380 nm to 1028 nm with bandwidths of 7 to 17 nm. It is used to gather data that will determine atmospheric aerosol optical depth, estimate ozone optical depth and the column abundance of atmospheric precipitable water. Tracking accuracy is ± 0.05° and the system maintains solar tracking in the presence of occasional small clouds. Data collection is possible for up to an entire day at intervals adjustable from 10 seconds to several minutes. The MISR Validation Team has two essentially identical instruments, giving them the opportunity of comparing results.
The Analytical Spectral Devices Field Spectrometer (here abbreviated ASD) is a portable, light weight, battery powered instrument that uses a fiber-optic cable to sample the light field being measured. Inside the instrument the fiber-optic bundle within the cable is divided into three bundles. Each bundle delivers the collected light to the entrance slit of one of three spectrometers covering the spectral range 350-2500 nm. The first spectrometer covers the wavelength region from 350-1000 nm with a sampling interval of 1.4 nm and a spectral resolution of approximately 3 nm. The second spectrometer covers the interval 900 nm to 1800 nm and the third from 1700 nm to 2500 nm. The spectral sampling interval of the second and third spectrometers is 2 nm and spectral resolution is 10-11 nm. The fiber optic cable has a full view angle of 25°, with special attachments allowing restriction to 8°. The total spectral acquisiton time is on the order of a few seconds per spectrum depending on integration time chosen, making the instrument very useful for rapid acquisition of large numbers of ground spectra which can be used to form average reflectances of large natural target areas.
The Multi-Filter Rotating Shadowband Radiometer (MFRSR) is a portable instrument that determines nearly simultaneous global and diffuse components of the solar spectral irradiance. From these an estimate of the imaginary part of the aerosol refractive index and an average surface Lambertian reflectance can be found. It uses independent interference filter-photodiode combinations that are mounted in a temperature-controlled enclosure for measurement of spectral irradiance at six wavelengths and one broadband channel. A rotating shadowband moves to four positions, blocking the instrument entrance aperture occulting the direct solar beam plus adjacent aureole components, observations on either side to correct for excess blocked sky during the sun occultation, and a rest position.
The Portable Apparatus for Rapid Acquisition of Bidirectional Observation of the Land and Atmosphere (PARABOLA) is a sphere-scanning 8 channel radiometer system that generates radiance measurements from a transportable platform mount over both ground and sky. Observations will be utilized to calculate bidirectional reflectance factors of the surface, and by spatial integration, the hemispherical directional and bihemispherical reflectances (HDR and BHR, respectively), plus the downwelling total and diffuse irradiances at the surface. During its 3 minute cycle it takes measurements at 72 azimuthal and 37 elevation positions.
The Airborne Multi-angle Imagery SpectroRadiometer (AirMISR) is an important tool in the MISR validation program. Whereas our other field instrument can only take samples of data at one location at a time, AirMISR can fly over a region, and better allow us to gather enough information to compare to the MISR observations. This is particulary important because MISR samples Earth in no smaller than 250 meter chunks at a time. This area is large compared to the fraction of a meter seen by the other validation field instrument. AirMISR is thus an important bridge between the extremes of the field instruments, and MISR.