Brian L. Markham

An emerging ground‐based aerosol climatology: Aerosol optical depth from AERONET

Long-term measurements by the AERONET program of spectral aerosol optical depth, precipitable water, and derived Angstrom exponent were analyzed and compiled into an aerosol optical properties climatology. Quality assured monthly means are presented and described for 9 primary sites and 21 additional multiyear sites with distinct aerosol regimes representing tropical biomass burning, boreal forests, midlatitude humid climates, midlatitude dry climates, oceanic sites, desert sites, and background sites. Seasonal trends for each of these nine sites are discussed and climatic averages presented.

Revised Landsat-5 TM radiometric calibration procedures and postcalibration dynamic ranges

Effective May 5, 2003, Landsat-5 (L5) Thematic Mapper (TM) data processed and distributed by the U.S. Geological Survey (USGS) Earth Resources Observation System (EROS) Data Center (EDC) will be radiometrically calibrated using a new procedure and revised calibration parameters. This change will improve absolute calibration accuracy, consistency over time, and consistency with Landsat-7 (L7) Enhanced Thematic Mapper Plus (ETM+) data. Users will need to use new parameters to convert the calibrated data products to radiance. The new procedure for the reflective bands (1-5,7) is based on a lifetime radiometric calibration curve for the instrument derived from the instruments internal calibrator, cross-calibration with the ETM+, and vicarious measurements. The thermal band will continue to be calibrated using the internal calibrator. Further updates to improve the relative detector-to-detector calibration and thermal band calibration are being investigated, as is the calibration of the Landsat-4 (L4) TM.

Normalized difference vegetation index measurements from the advanced very high resolution radiometer

Abstract The potential of computing normalized difference vegetation index (NDVI) measurements from the global, daily observations collected by the NOAA Advanced Very High Resolution Radiometer (AVHRR) has created great interest in the use of these data sets to study global biospheric dynamics. Initial qualitative studies demonstrated this potential but the move to quantitative assessments is hindered by limited understanding of the performance characteristics of the AVHRR observations relative to surface vegetation conditions. Factors related to instrument precision and calibration, atmospheric attenuation and off-nadir viewing create deviations of the NDVI observations unrelated to vegetation dynamics. Deviations in excess of 50% between the satellite and equivalent ground observations are possible if no effort is made to account for these effects. In addition, off-nadir viewing causes spatio-temporal variations in the measurements and cloud occurrence reduces temporal resolution below the AVHRRs daily repeat cycle. It appears possible to reduce these errors to approximately ± 10% (± 0.1 NDVI) with at least a monthly time resolution if all of the observation attributes are addressed adequately. Much of the remaining error resides in atmospheric variability, uncertainties in off-nadir views and loss of sensor precision at large solar zenith angles. A global measure of vegetation green foliage dynamics with a measurement precision of ± 10% and a monthly time resolution may not yet meet the exacting needs of some biospheric modelers, but it is, in fact, remarkable, considering that this possibility was not even conceived when the AVHRR was designed. Further refinement of global remotely sensed vegetation foliage measurement precision appears possible and should be the primary focus for terrestrial remote sensing research in the next decade.

Radiometric cross-calibration of the Landsat-7 ETM+ and Landsat-5 TM sensors based on tandem data sets

Abstract Early in its mission, the Landsat-7 spacecraft was temporarily placed in a “tandem” orbit very close to that of the Landsat-5 spacecraft in order to facilitate the establishment of sensor calibration continuity between the Landsat-7 Enhanced Thematic Mapper Plus (ETM+) and Landsat-5 Thematic Mapper (TM) sensors. The key period for the tandem configuration was June 1–4, 1999, during which hundreds of nearly coincident matching scenes were recorded by both the Landsat-7 ETM+ and, in cooperation with Space Imaging/EOSAT and international ground stations, the Landsat-5 TM as well. The paper presents a methodology for radiometric cross-calibration of the solar reflective spectral bands of the Landsat-7 ETM+ and Landsat-5 TM sensors and results based on analysis of two different tandem image pairs for which ground reference data are available. With the well-calibrated ETM+ as a reference, the tandem-based cross-calibrations for the two image pairs yield TM responsivities that are consistent to each other to within 2% or better depending on the spectral band. Comparisons with independent methods and results obtained by other groups indicate that the tandem-based cross-calibration is within 3% of the independent results on average in spectral bands 1–4 but compares less favorably in bands 5 and 7. The present study indicates that the tandem cross-calibration approach can provide a valuable “contemporary” calibration update for Landsat-5 TM in the visible and near-infrared spectral bands based on the excellent radiometric performance of Landsat-7 ETM+. The methodology also incorporates adjustments for spectral band differences between the two Landsat sensors. Spectral band difference effects are shown to be more dependent on the surface reflectance spectrum than on atmospheric and illumination conditions. A variety of terrestrial surfaces are assessed regarding their suitability for Landsat radiometric cross-calibration in the absence of surface reflectance spectra.

Revised Landsat-5 Thematic Mapper Radiometric Calibration

Effective April 2, 2007, the radiometric calibration of Landsat-5 (L5) Thematic Mapper (TM) data that are processed and distributed by the U.S. Geological Survey (USGS) Center for Earth Resources Observation and Science (EROS) will be updated. The lifetime gain model that was implemented on May 5, 2003, for the reflective bands (1-5, 7) will be replaced by a new lifetime radiometric-calibration curve that is derived from the instruments response to pseudoinvariant desert sites and from cross calibration with the Landsat-7 (L7) Enhanced TM Plus (ETM+). Although this calibration update applies to all archived and future L5 TM data, the principal improvements in the calibration are for the data acquired during the first eight years of the mission (1984-1991), where the changes in the instrument-gain values are as much as 15%. The radiometric scaling coefficients for bands 1 and 2 for approximately the first eight years of the mission have also been changed. Users will need to apply these new coefficients to convert the calibrated data product digital numbers to radiance. The scaling coefficients for the other bands have not changed.

Sun photometric measurements of atmospheric water vapor column abundance in the 940‐nm band

Sun photometers operating in the strong water vapor absorption 940-nm combination-vibrational band have been used to determine water column abundance in the atmosphere when the path to the Sun is clear of clouds. We describe a procedure to perform a rapid determination of the water column abundance, using sun photometers with an accuracy that is easily comparable to that of the radiosondes. The effect of parameters, such as filter bandwidth, atmospheric lapse rate, and the water vapor amount, on the accuracy of the retrieved abundance is examined. It is seen that a narrow filter band of approximately 10-nm bandwidth, positioned at the peak of absorption, is quite insensitive to the type of atmosphere present during calibration or measurement with less than 1% variability under extreme atmospheric conditions. A comparison of the retrieved water column abundance using sun photometers with contemporaneously measured values using radiosondes and microwave radiometers shows that the latest version of the radiative transfer algorithm used in this procedure, MODTRAN-3, gives far superior results in comparison with earlier versions because of the use of improved absorption coefficients in the 940-nm bands. Results from a network of sun photometers spread throughout the globe will be discussed.

Landsat sensor performance: history and current status

The current Thematic Mapper (TM) class of Landsat sensors began with Landsat-4, which was launched in 1982. This series continued with the nearly identical sensor on Landsat-5, launched in 1984. The final sensor in the series was the Landsat-7 Enhanced Thematic Mapper Plus (ETM+), which was carried into orbit in 1999. Varying degrees of effort have been devoted to the characterization of these instruments and data over the past 22 years. Extensive short-lived efforts early in the history, very limited efforts in the middle years, and now a systematic program for continuing characterization of all three systems are apparent. Currently, both the Landsat-5 TM and the Landsat-7 ETM+ are operational and providing data. Despite 20+ years of operation, the TM on Landsat-5 is fully functional, although downlinks for the data are limited. Landsat-7 ETM+ experienced a failure of its Scan Line Corrector mechanism in May 2003. Although there are gaps in the data coverage, the data remain of equivalent quality to prefailure data. Data products have been developed to fill these gaps using other ETM+ scenes.

Thematic Mapper bandpass solar exoatmospheric irradiances

Abstract Based on solar irradiance data published by Neckel and Labs (1984) and Iqbal (1983), the solar exoatmospheric irradiances for Thematic Mapper (TM) bands 1, 2, 3 and 4 have been calculated. Results vary by up to 1 per cent from our previously published values which were based on earlier data of Neckel and Labs. For TM bands 5 and 7, integrated solar exoatmospheric irradiances have also been recalculated using solar irradiance data published by Labs and Neckel (1968), Arvesen et al. (1969) and Iqbal (1983). These irradiances vary by up to 6 per cent from our previously published results, which were based on data published by Thekaekara (l972).

Algorithm for atmospheric corrections of aircraft and satellite imagery

Abstract An algorithm is described for making fast atmospheric corrections. The required radiation parameters are stored in a lookup table. The procedure is to enter the lookup table with the measured radiance, wavelength, view and illumination directions, heights of observation and surface, and the aerosol and gaseous absorption optical thicknesses. The surface radiance, the irradiance incident on a surface, and surface reflectance are computed then. Alternately, the program will compute the upward radiances at specific altitudes for a given surface reflctance, view and illumination directions, and aerosol and gaseous absorption optical thicknesses.

The effects of spatial resolution on the classification of Thematic Mapper data

Abstract Actual and degraded LANDSAT-4 Thematic Mapper (TM) data were analysed to examine the effect of spatial resolution on the performance of a per pixel, maximum-likelihood classification algorithm. Analysis of variance (ANOVA) and a balanced, three-factor, eight-treatment, fixed-effects model were used to investigate the interactions between spatial resolution and two other TM refinements, spectral band configuration and data quantization. The goal was to evaluate quantitatively the effects of these attributes on classification accuracies obtained with all pixels (pure pixels plus mixed pixels) and on accuracies obtained with pure pixels alone. A comparison of results from these separate analyses supported previous explanations of the effects of increasing spatial resolution. First, the difficulty in classifying mixed pixels was demonstrated by an average 21 per cent decrease in percentage accuracy from the pure-pixel case to the pure-plus-mixed-pixel case for the eight ANOVA treatments. In the pure-...