Kurt Thome

Variability of aerosol and spectral lidar and backscatter and extinction ratios of key aerosol types derived from selected Aerosol Robotic Network locations

Received 13 June 2004; revised 1 February 2005; accepted 3 March 2005; published 3 May 2005. [1] The lidar (extinction-to-backscatter) ratios at 0.55 and 1.02 mm and the spectral lidar, extinction, and backscatter ratios of climatically relevant aerosol species are computed on the basis of selected retrievals of aerosol properties from 26 Aerosol Robotic Network (AERONET) sites across the globe. The values, obtained indirectly from sky radiance and solar transmittance measurements, agree very well with values from direct observations. Low mean values of the lidar ratio, Sa, at 0.55 mm for maritime (27 sr) aerosols and desert dust (42 sr) are clearly distinguishable from biomass burning (60 sr) and urban/industrial pollution (71 sr). The effects of nonsphericity of mineral dust are shown, demonstrating that particle shape must be taken into account in any spaceborne lidar inversion scheme. A new aerosol model representing pollution over Southeast Asia is introduced since lidar (58 sr), color lidar, and extinction ratios in this region are distinct from those over other urban/industrial centers, owing to a greater number of large particles relative to fine particles. This discrimination promises improved estimates of regional climate forcing by aerosols containing black carbon and is expected to be of utility to climate modeling and remote sensing communities. The observed variability of the lidar parameters, combined with current validated aerosol data products from Moderate Resolution Imaging Spectroradiometer (MODIS), will afford improved accuracy in the inversion of spaceborne lidar data over both land and ocean.

Prime candidate Earth targets for the post-launch radiometric calibration of space-based optical imaging instruments

This paper provides a comprehensive list of prime candidate terrestrial targets for consideration as benchmark sites for the post-launch radiometric calibration of space-based instruments. The key characteristics of suitable sites are outlined primarily with respect to selection criteria, spatial uniformity, and temporal stability. The establishment and utilization of such benchmark sites is considered to be an important element of the radiometric traceability of satellite image data products to SI standards for use in the accurate monitoring of environmental change.

Characterization and field use of a CCD camera system for retrieval of bidirectional reflectance distribution function

Vicarious calibration and field validation is a critical aspect of NASAs Earth Observing System program. As part of calibration and validation research related to this project, the Remote Sensing Group (RSG) of the Optical Science Center at the University of Arizona has developed an imaging radiometer for ground-based measurements of directional reflectance. The system relies on a commercially available 1024×1024 pixel, silicon CCD array. Angular measurements are accomplished using a fish-eye lens that has a full 180° field of view with each pixel on the CCD array having a nominal 0.2° field of view. Spectral selection is through four interference filters centered at 470, 575, 660, and 835 nm. The system is designed such that the entire 180° field is collected at one time with a complete multispectral data set collected in under 2 min. The results of laboratory experiments have been used to determine the gain and offset of each detector element as well as the effects of the lens on the system response. Measurements of a stable source using multiple integration times and at multiple distances for a set integration time indicate the system is linear to better than 0.5% over the upper 88% of the dynamic range of the system. The point spread function (PSF) of the lens system was measured for several field angles, and the signal level was found to fall to less than 1% of the peak signal within 1.5° for the on-axis case. The effect of this PSF on the retrieval of modeled BRDFs is shown to be less than 0.2% out to view angles of 70°. The degree of polarization of the system is shown to be negligible for on-axis imaging but to have up to a 20% effect at a field angle of 70°. The effect of the system polarization on the retrieval of modeled BRDFs is shown to be up to 3% for field angles of 70° off nadir and with a solar zenith angle of 70°. Field measurements are made by mounting the camera to a boom mounted to a large tripod that is aligned toward south. This tripod obstructs sampling of the surface reflectance past 25° off nadir northward. The system is typically operated at a height of 1.5 m to view over a large sampling of surface features, such as cracks. To evaluate the surface BRDF, measurements are collected throughout the morning as a function of Sun angle. A single measurement consists of all four bands and a dark-current measurement. Data sets have been collected over several vicarious calibration sites and calibration tarpaulins. Comparisons with measurements made by a simple goniometer-based system indicate that the camera system is as accurate as the goniometer. Scattering phase function values derived from the camera system are fit to a modified Pinty-Verstraete equation. This function is shown to fit the data to better than 0.3% for data collected during an example RSG vicarious calibration experiment. Bidirectional reflectance data derived from the camera system also compare well to those predicted from the Walthall model. These BRDF models are critical for determining the applicability of measurements taken over small areas to represent the BRDF properties of an entire site, which in some cases is of the order of several kilometers in size.

Dust transport model validation using satellite- and ground-based methods in the southwestern United States

Dust is known to aggravate respiratory diseases. This is an issue in the desert southwestern United States, where windblown dust events are common. The Public Health Applications in Remote Sensing (PHAiRS) project aims to address this problem by using remote-sensing products to assist in public health decision support. As part of PHAiRS, a model for simulating desert dust cycles, the Dust Regional Atmospheric Modeling (DREAM) system is employed to forecast dust events in the southwestern US. Thus far, DREAM has been validated in the southwestern US only in the lower part of the atmosphere by comparison with measurement and analysis products from surface synoptic, surface Meteorological Aerodrome Report (METAR), and upper-air radiosonde. This study examines the validity of the DREAM algorithm dust load prediction in the desert southwestern United States by comparison with satellite-based MODIS level 2 and MODIS Deep Blue aerosol products, and ground-based observations from the AERONET network of sunphotometers. Results indicate that there are difficulties obtaining MODIS L2 aerosol optical thickness (AOT) data in the desert southwest due to low AOT algorithm performance over areas with high surface reflectances. MODIS Deep Blue aerosol products show improvement, but the temporal and vertical resolution of MODIS data limit its utility for DREAM evaluation. AERONET AOT data show low correlation to DREAM dust load predictions. The potential contribution of space- or ground-based lidar to the PHAiRS project is also examined.

Absolute-radiometric calibration of Landsat-5 Thematic Mapper and the proposed calibration of the Advanced Spaceborne Thermal Emission and Reflection Radiometer

The reflectance-based method is used to determine an absolute-radiometric calibration of Landsat-5 Thematic Mapper for the solar reflective portion of the spectrum. Results are given for data collected at White Sands Missile Range in New Mexico on 1993-08-15 and 1993-10-21. These results are compared to those obtained from applying an identical approach to data collected in 1984, 1985, and 1988. Results indicate sensor degradation in the first three bands with greatest degradation at the shortest wavelengths. The reflectance-based method is also to be used for the in-flight, absolute-radiometric calibration of the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) to be launched on the Earth Observing Systems AM1 platform. The authors describe the anticipated improvements to the reflectance-based method and their application and impact on the calibration of ASTER. These improvements should allow the absolute calibration of ASTER at the 3% level.<<ETX>>

Solar radiation-based calibration of laboratory grade radiometers

The Remote Sensing Group (RSG) at the University of Arizona Optical Sciences Center has been performing high accuracy laboratory calibration for over 20 years. This work has been done both in support of our work in vicarious calibration of space-borne and airborne imaging sensors and as a standalone means of achieving NIST-traceable radiometric calibration. The solar radiation-based calibration (SRBC) has in the past been a way for the RSG to verify calibration results and to achieve continued calibration of field-grade instruments. This paper presents multiple SRBC results for multiple laboratory-grade radiometers. These results are compared with laboratory calibrations and studied for their merit as a sole means of high-accuracy calibration.

Comparison of ground-reference calibration results for Landsat-7 ETM+ for large and small test sites

Recent results of the vicarious calibration of the Landsat-7 ETM+ sensor are presented based on the reflectance-based vicarious method using results from a smaller test site in close proximity to the University of Arizona. The typical, larger test site is brighter and more spatially uniform then the smaller site. However, the location of the small test site allows for more frequent data collections and a more complete understanding of the calibration coefficients of the sensor as a function of time. The Remote Sensing Group previously reported results based on data collected from the smaller test site on seven dates. Here we report calibration values for additional dates as well as a comparison of the calibration values for the large and smaller test sites over recent years. The most recent data shows the calibration values using the smaller sites continue to agree within 3% of the large test sites in all bands.

Proposed atmospheric correction for the solar-reflective bands of the Advanced Spaceborne Thermal emission and Reflection Radiometer

The reflectance-based method of absolute radiometric calibration has been used successfully in the solar reflective portion of the spectrum. This method corrects for atmospheric effects by using a plane-parallel radiative transfer code. This code is intended to be modified for use in the atmospheric correction of data from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) due to be launched on the Earth Observing Systems AM1 platform. The radiative transfer code numerically integrates the radiative transfer equation for a plane-parallel, horizontally-homogeneous atmosphere using Gauss-Seidel iteration. This code will be used to generate a look-up table of top-of-the-atmosphere (TOA) and surface radiance for several values of surface reflectance using given atmospheric optical depths. The atmospheric correction relies on the fact that surface reflectance and TOA radiance are linearly related for limited ranges of surface reflectance. Then surface reflectance and radiance are determined by linearly interpolating the TOA radiances measured by ASTER from the look-up-table results. Planned tests of this method are presented as well as anticipated accuracies.<<ETX>>

Accuracy assessment for the radiometric calibration of imaging sensors using preflight techniques relying on the sun as a source

The Remote Sensing Group (RSG) at the University of Arizona has performed high-accuracy radiometric calibration in the laboratory for more than 20 years in support of vicarious calibration of space-borne and airborne imaging sensors. Typical laboratory calibration relies on lamp-based sources which, while convenient to operate and control, do not simulate the solar spectrum that is the basic energy source for many of the imaging systems. Using the sun as a source for preflight radiometric calibration reduces uncertainties caused by the spectral mismatch between the preflight and inflight calibration, especially in the case in which a solar diffuser is the inflight calibration method. Difficulties in using the sun include varying atmospheric conditions, changing solar angle during the day and with season, and ensuring traceability to national standards. This paper presents several approaches using the sun as a radiometric calibration source coupled with the expected traceable accuracies for each method. The methods include direct viewing of the solar disk with the sensor of interest, illumination of the sensors inflight solar diffuser by the sun, and illumination of an external diffuser that is imaged by the sensor. The results of the error analysis show that it is feasible to achieve preflight calibration using the sun as a source at the same level of uncertainty as those of lamp-based approaches. The error analysis is evaluated and compared to solar-radiation-based calibrations of one of RSGs laboratory-grade radiometers.

Instrument for retrieval of BRDF data for vicarious calibration

The University of Arizonas Remote Sensing Group has performed vicarious calibrations of satellite and airborne sensors since the mid-1980s. Improvements of the accuracy of these techniques requires that the bi-directional reflectance of the test sites be characterized. The Remote Sensing Group has developed a four-band, imaging radiometer based on a two-dimensional CCD array and 8-mm fisheye lens for the retrieval of bidirectional reflectance. This paper describes the design of this radiometer and the methods used to calibrate the system. The calibration is based upon measurements of a 40-inch spherical integrating source and we describe a method to separate the spatial inhomogeneity of the source from that of the CCD array. Early data collected with this instrument of test sites at White Sands Missile Range and Lunar Lake Playa are presented and show good agreement with previous data collected at these or similar sites.