James C. Storey

Achieving sub-pixel geolocation accuracy in support of MODIS land science

The Moderate Resolution Imaging Spectroradiometer (MODIS) was launched in December 1999 on the polar orbiting Terra spacecraft and since February 2000 has been acquiring daily global data in 36 spectral bands—29 with 1 km, five with 500 m, and two with 250 m nadir pixel dimensions. The Terra satellite has on-board exterior orientation (position and attitude) measurement systems designed to enable geolocation of MODIS data to approximately 150 m (1r) at nadir. A global network of ground control points is being used to determine biases and trends in the sensor orientation. Biases have been removed by updating models of the spacecraft and instrument orientation in the MODIS geolocation software several times since launch and have improved the MODIS geolocation to approximately 50 m (1r) at nadir. This paper overviews the geolocation approach, summarizes the first year of geolocation analysis, and overviews future work. The approach allows an operational characterization of the MODIS geolocation errors and enables individual MODIS observations to be geolocated to the sub-pixel accuracies required for terrestrial global change applications. D 2002 Elsevier Science Inc. All rights reserved.

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.

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.

Four years of Landsat-7 on-orbit geometric calibration and performance

Unlike its predecessors, Landsat-7 has undergone regular geometric and radiometric performance monitoring and calibration since launch in April 1999. This ongoing activity, which includes issuing quarterly updates to calibration parameters, has generated a wealth of geometric performance data over the four-year on-orbit period of operations. A suite of geometric characterization (measurement and evaluation procedures) and calibration (procedures to derive improved estimates of instrument parameters) methods are employed by the Landsat-7 Image Assessment System to maintain the geometric calibration and to track specific aspects of geometric performance. These include geodetic accuracy, band-to-band registration accuracy, and image-to-image registration accuracy. These characterization and calibration activities maintain image product geometric accuracy at a high level-by monitoring performance to determine when calibration is necessary, generating new calibration parameters, and verifying that new parameters achieve desired improvements in accuracy. Landsat-7 continues to meet and exceed all geometric accuracy requirements, although aging components have begun to affect performance.

Landsat-8 Sensor Characterization and Calibration

Landsat-8 was launched on 11 February 2013 with two new Earth Imaging sensors to provide a continued data record with the previous Landsats. For Landsat-8, pushbroom technology was adopted, and the reflective bands and thermal bands were split into two instruments. The Operational Land Imager (OLI) is the reflective band sensor and the Thermal Infrared Sensor (TIRS), the thermal. In addition to these fundamental changes, bands were added, spectral bandpasses were refined, dynamic range and data quantization were improved, and numerous other enhancements were implemented. As in previous Landsat missions, the National Aeronautics and Space Administration (NASA) and United States Geological Survey (USGS) cooperated in the development, launch and operation of the Landsat-8 mission. One key aspect of this cooperation was in the characterization and calibration of the instruments and their data. This Special Issue documents the efforts of the joint USGS and NASA calibration team and affiliates to characterize the new sensors and their data for the benefit of the scientific and application users of the Landsat archive. A key scientific use of Landsat data is to assess changes in the land-use and land cover of the Earth’s surface over the now 43-year record. [...]

Landsat Data Continuity Mission, now Landsat-8: six months on-orbit

The Landsat Data Continuity Mission (LDCM) with two pushbroom Earth-imaging sensors, the Operational Land Imager (OLI) and the Thermal InfraRed Sensor (TIRS), was launched on February 11, 2013. Its on-orbit check out period or commissioning phase lasted about 90 days. During this phase the spacecraft and its instruments were activated, operationally tested and their performance verified. In addition, during this period, the spacecraft was temporarily placed in an intermediary orbit where it drifted relative to the Landsat-7 spacecraft, providing near simultaneous imaging for about 3 days, allowing data comparison and cross calibration. After this tandem-imaging period, LDCM was raised to its final altitude and placed in the position formerly occupied by Landsat-5, i.e., 8 days out of phase with Landsat-7, with about a 10:10 AM equatorial crossing time. At the end of commissioning, the satellite was transferred to the United States Geological Survey (USGS), officially renamed Landsat-8 and declared operational. Data were made available to the public beginning May 31, 2013. The performance of the satellite and two instruments has generally been excellent as evidenced in the quality of the distributed data products.

Landsat sensor cross-calibration using nearly coincidental matching scenes

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 and international ground stations, the Landsat-5 TM as well. The paper presents a methodology for Landsat-7 ETM+ and Landsat-5 TM cross-calibration and results based on analysis of three tandem image pairs. The approach incorporates adjustments for spectral band differences between the two sensors. With the well- calibrated ETM+ as a reference, the tandem-based cross-calibrations for the three image pairs yield TM responsivities that are consistent to each other to within a few percent 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 in close agreement with the independent results in spectral bands 1-3 but compares less favourably in the other bands.

Foreword to the Special Issue on Landsat Sensor Performance Characterization

THE primary sensors on the recent Landsat satellites are Thematic Mapper (TM) class instruments. The TMs on Landsat-4 and -5 and the Enhanced TM Plus (ETM+) on Landsat-7 have successfully operated on-orbit. Landsat-4 TM operated from 1982 to 1993, Landsat-5 TM from 1984 to present, and Landsat-7 ETM+ from 1999 to present. Landsat-4 and -5 have gone through several transitions in operational oversight from the National Aeronautics and Space Administration to the National Oceanic and Atmospheric Administration to EOSAT (now Space Imaging, Inc.) to the U.S. Geological Survey (USGS), with varying degrees of interest in calibration and performance monitoring. With the possible exception of the early period of Landsat-4 TM operations, the past approximately five years have shown the most interest in characterizing and calibrating the geometric, spatial, and radiometric regimes of these systems. This interest was spurred primarily by the return of the Landsat systems to U.S. Government operation, and the increasing role the data from these sensors plays in the assessment of changes in the earth’s land cover. One of the key enhancements to the Landsat-7 system was the addition of an Image Assessment System (IAS) to the ground system. This IAS has the responsibility of characterizing the performance of the ETM+ instrument geometrically, radiometrically, and spatially and updating the calibrations as necessary. This capability within the Landsat-7 system has enabled a significantly improved understanding of the Landsat-7 sensor data. The USGS is currently in the process of extending the IAS concept to Landsat-4 and -5 TM data. The papers contained within this special issue document the recent work on the characterization and calibration of Landsat-5 TM and Landsat-7 ETM+ data. Topics include geometric characterization of the Landsat-7 ETM+ data and geometric performance of the Landsat-5 TM instrument in its bumper mode of operation: “Four Years of Landsat 7 On-Orbit Geometric Calibration and Performance” by S. Lee et al. and “Landsat-5 Bumper-Mode Geometric Correction” by M. J. Choate and J. C. Storey. Landsat-5 TM was commanded into its backup bumper mode of operation in 2002, as its scan mirror bumpers had worn to the point that the instrument could no longer operate correctly in the primary scan mirror mode. Several papers are included on aspects of the Landsat-5 TM radiometric characterization and calibration: “Landsat Thematic Mapper Radiometric Artifacts” by D. L. Helder et al., “Landsat-5 Thematic Mapper Outgassing Effects” by D. L. Helder et al., Landsat-5 Thematic Mapper Reflective-Band Radiometric Stability” by D. L. Helder et al.,