John L. Barker

Terrestrial remote sensing science and algorithms planned for EOS/MODIS

Abstract The Moderate Resolution Imaging Spectroradiometer (MODIS) will be the primary daily global monitoring sensor on the NASA Earth Observing System (EOS) satellites, scheduled for launch on the EOS-AM platform in June 1998 and the EOS-PM platform in December 2000. MODIS is a 36 channel radiometer covering 0·415-14·235 μm wavelengths, with spatial resolution from 250 m to 1 km at nadir. MODIS will be the primary EOS sensor for providing data on terrestrial biospheric dynamics and process activity. This paper presents the suite of global land products currently planned for EOSDIS implementation, to be developed by the authors of this paper, the MODIS land team (MODLAND). These include spectral albedo, land cover, spectral vegetation indices, snow and ice cover, surface temperature and fire, and a number of biophysical variables that will allow computation of global carbon cycles, hydrologic balances and biogeochemistry of critical greenhouse gases. Additionally, the regular global coverage of these var...

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.

Characterization of the Landsat-7 ETM Automated Cloud-Cover Assessment (ACCA) Algorithm

A scene-average automated cloud-cover assessment (ACCA) algorithm has been used for the Landsat-7 Enhanced Thematic Mapper Plus (ETM� ) mission since its launch by NASA in 1999. ACCA assists in scheduling and confirming the acquisition of global “cloud-free” imagery for the U.S. archive. This paper documents the operational ACCA algorithm and validates its performance to a standard error of � 5 percent. Visual assessment of clouds in three-band browse imagery were used for comparison to the five-band ACCA scores from a stratified sample of 212 ETM� 2001 scenes. This comparison of independent cloud-cover estimators produced a 1:1 correlation with no offset. The largest commission errors were at high altitudes or at low solar illumination where snow was misclassified as clouds. The largest omission errors were associated with undetected optically thin cirrus clouds over water. There were no statistically significant systematic errors in ACCA scores analyzed by latitude, seasonality, or solar elevation angle. Enhancements for additional spectral bands, per-pixel masks, land/water boundaries, topography, shadows, multidate and multi-sensor imagery were identified for possible use in future ACCA algorithms.

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).

A definitive calibration record for the Landsat-5 thematic mapper anchored to the Landsat-7 radiometric scale

A coordinated effort on the part of several agencies has led to the specification of a definitive radiometric calibration record for the Landsat-5 thematic mapper (TM) for its lifetime since launch in 1984. The time-dependent calibration record for Landsat-5 TM has been placed on the same radiometric scale as the Landsat-7 enhanced thematic mapper plus (ETM+). It has been implemented in the National Landsat Archive Production Systems (NLAPS) in use in North America. This paper documents the results of this collaborative effort and the specifications for the related calibration processing algorithms. The specifications include (i) anchoring of the Landsat-5 TM calibration record to the Landsat-7 ETM+ absolute radiometric calibration, (ii) new time-dependent calibration processing equations and procedures applicable to raw Landsat-5 TM data, and (iii) algorithms for recalibration computations applicable to some of the existing processed datasets in the North American context. The cross-calibration between Landsat-5 TM and Landsat-7 ETM+ was achieved using image pairs from the tandem-orbit configuration period that was programmed early in the Landsat-7 mission. The time-dependent calibration for Landsat-5 TM is based on a detailed trend analysis of data from the on-board internal calibrator. The new lifetime radiometric calibration record for Landsat-5 will overcome problems with earlier product generation owing to inadequate maintenance and documentation of the calibration over time and will facilitate the quantitative examination of a continuous, near-global dataset at 30-m scale that spans almost two decades.

Lifetime responsivity behavior of the Landsat-5 thematic mapper

The responsivities of the Landsat-5 Thematic Mapper (TM) reflective bands are characterized over the lifetime of the instrument using its internal calibration system. This system illuminates only the focal planes and aft optics of the TM so that it does not capture changes in the telescope. The observed changes are quantified and categorized as to whether they are likely to be true instrument responsivity changes or changes in the internal calibrator system itself. Changes observed that are likely to be true instrument changes are: (1) 7 percent, 5 percent, 8 percent and 7 percent exponential-like decreases in responsivity with decay half lives of 250, 180, 60 and 110 days in bands 1 to 4, respectively, during the initial on-orbit period and (2) an oscillation in response of about 5 percent peak-to-peak in bands 5 and 7. The first effect is believed to originate in the TM spectral bandpass filters and the second effect is believed to be due to an icing build up in the cold focal plane window. Two rapid apparent responsivity changes, one a decrease and one an increase, which are peculiar to particular internal calibrator lamps are believed to be due to changes in the lamp assemblies themselves as is a gradual increase in all detectors responsivities with time. An annual oscillation of up to 2 percent peak-to-peak in all bands is likely the product of both a temperature sensitivity of the IC and the TM primary focal plane.

Landsat-7 mission and early results

The goal of the current Landsat mission is to acquire annual data sets of optical band digital imagery of the landmass of the Earth. Ground spatial resolutions for the panchromatic, reflective and emissive bands are 15, 30 and 60 meters, respectively. The design life for the Enhanced Thematic Mapper Plus (ETM+) imager on the Landsat-7 satellite is five years. The satellite was launched on April 15, 1999. The mission builds on the 27-year continuous archive of thematic images of the Earth from previous Landsat satellites. Early results from the ETM+ instrument, the spacecraft, and the ground processing indicate that the image quality is as good as expected and all systems are working. Partial Aperture Solar Calibrator (PASC) 100-day radiometric background stability is approximately plus or minus 1.0%. Full Aperture Solar Calibrator (FASC) 2-day stability is approximately plus or minus 0.2%. Mid-scale per pixel noise is approximately plus or minus 1.0%. Operational collection of Landsats Long Term Acquisition Plan (LTAP) started June 29th. NASA Goddard Space Flight Center (GSFC) is responsible for the instrument, spacecraft, launch, flight operations and science team investigations. On October 1, 2000 USGS EROS Data Center (EDC) takes over flight operations while continuing archiving, monitoring quality, and distributing the imagery without restrictions on reprocessing and redistribution.

Short term calibration of Landsat TM: recent findings and suggested techniques

Landsat Thematic Mapper (TM) data has traditionally been radiometrically calibrated on an individual scene basis. While this may be adequate for many applications, particularly single date applications, there are many cases where more advanced techniques are necessary. With emphasis today on global change, multi-temporal analysis is common place. Thus, forms of radiometric calibration are required that discriminate temporal change from changes in instrument response. To address this issue, a project is underway to evaluate TM calibration at several different time scales: short term, intermediate term, and lifetime. This paper presents some of the short term results. Short term is defined as within a single orbit. Continuous swaths of night data have been collected and the individual scenes, along with calibration files, stitched together. Analysis of the calibration files has revealed a number of interesting characteristics about detector gain and bias as well as instrument anomalies such as memory effect and scan correlated shift. Detector bias is a rapidly varying parameter that requires evaluation and correction as frequently as every scan. Detector gain varies significantly within an orbit in the primary focal plane in a predictable manner. Scan correlated shift has been accurately quantified and its effects may easily be removed from the data. Memory effect is in the process of being characterized and an algorithm is being developed to remove it. Line droop has been shown to simply be due to memory effect. Results from these investigations are being used in the development of radiometric correction algorithms for the Landsat 7 Image Assessment System.

Landsat Thematic Mapper band-to-band registration

This paper on band-to-band (B-B) registration of Landsat Thematic Mapper (TM) imagery has three objectives. Firstly, it documents a one dimensional (1D) Z-score analytical fast Fourier transform (FFT) procedure for measuring along-scan misregistration in raw uncalibrated and unresampled TM imagery to a precision better than /spl plusmn/0.002 pixels. Secondly, it demonstrates that, with the TDRSS antenna quiet, the largest within-scene coherent changes in BB sample misregistration (SM) occur at the same 2 Hz frequency as the most significant changes in measured on-orbit scan mirror errors in the instrument for Landsat-4 (L4), and at an additional common 1.2 Hz peak for Landsat-5 (L5). Thirdly, it demonstrates that the cluster of frequency peaks in the FFTs of spacecraft x-axis gyro and angular displacement sensor (ADS) telemetry below 3 Hz are not seen in FFTs of normalized plots of SM versus forward scan number. The 1984 142.7 msec scan time on L5 was observed to increase by 1.5 msec in 8 years.

Evaluation of the Landsat-5 TM radiometric calibration history using desert test sites

The U.S. radiometric calibration procedure for the reflective bands of the Landsat-5 Thematic Mapper was updated in May 2003. This update was based on a model of the performance of the instrument developed from its response to the best-behaved internal calibration lamp and from a cross calibration with Landsat-7 ETM+ that occurred in June 1999. Since this update was performed, there have been continued attempts to validate the model. These validations have relied primarily upon data acquired over deserts of the world. These studies have been limited by the amount of data available over any one site for the 22-year life of the mission. Initial attempts over the desert Southwest of the United States were inconclusive, though they were suggestive of additional degradation occurring in the shorter wavelength channels. More recently, significant holdings from European Space Agency of data over North Africa have been made available for analysis. The North Africa test area results to date for one site in Libya are considerably less noisy than the North American datasets. They indicate an exponential-like decay of about 19%, 16%, 8% and 4% for TM bands 1, 2, 3 and 4, with the degradation, at least in bands 1 and 2 occurring throughout the mission. The current model shows changes of roughly the same magnitude, but with the change occurring more rapidly so that nearly all the change is completed in 4 years. These results are generally consistent with independent work going on outside of this effort. Additional sites are being analyzed as data become available.