The Landsat archive provides more than 35 years of uninterrupted multispectral remotely sensed data of
Earth observations. Since 1972, Landsat missions have carried different types of sensors, from the Return
Beam Vidicon (RBV) camera to the Enhanced Thematic Mapper Plus (ETM+). However, the Thematic
Mapper (TM) sensors on Landsat 4 (L4) and Landsat 5 (L5), launched in 1982 and 1984 respectively, are the
backbone of an extensive archive.
Effective April 2, 2007, the radiometric calibration of L5 TM data processed and distributed by the U.S.
Geological Survey (USGS) Center for Earth Resources Observation and Science (EROS) was updated to use
an improved lifetime gain model, based on the instrument's detector response to pseudo-invariant desert site
data and cross-calibration with the L7 ETM+. However, no modifications were ever made to the radiometric
calibration procedure of the Landsat 4 (L4) TM data. The L4 TM radiometric calibration procedure has
continued to use the Internal Calibrator (IC) based calibration algorithms and the post calibration dynamic
ranges, as previously defined.
To evaluate the "current" absolute accuracy of these two sensors, image pairs from the L5 TM and L4 TM
sensors were compared. The number of coincident image pairs in the USGS EROS archive is limited, so the
scene selection for the cross-calibration studies proved to be a challenge. Additionally, because of the lack of
near-simultaneous images available over well-characterized and traditionally used calibration sites, alternate
sites that have high reflectance, large dynamic range, high spatial uniformity, high sun elevation, and minimal
cloud cover were investigated. The alternate sites were identified in Yuma, Iraq, Egypt, Libya, and Algeria.
The cross-calibration approach involved comparing image statistics derived from large common areas
observed eight days apart by the two sensors. This paper summarizes the average percent differences in
reflectance estimates obtained between the two sensors. The work presented in this paper is a first step in
understanding the current performance of L4 TM absolute calibration and potentially serves as a platform to
revise and improve the radiometric calibration procedures implemented for the processing of L4 TM data.
The Landsat Thematic Mappers have obtained imagery of the Earth's surface since 1982 with the launch of Landsat 4. However, the absolute calibration of this first instrument, as well as it's cross-calibration to the other two thematic mappers on Landsat 5 and 7, remains in question. The objective for this work was to provide an absolute radiometric calibration of the Landsat 4 instrument. Landsat 4's internal calibrator, while still useful, does not provide an absolute calibration; it does provide a relative calibration of the instrument's responsivity over the lifetime of the mission. The same is true for the Landsat 5 internal calibrator; however, Landsat 5 has been cross-calibrated to Landsat 7's Enhanced Thematic Mapper Plus, which is believed to be absolutely calibrated to within 5%. Therefore, by cross-calibrating Landsat 4 to Landsat 7 through Landsat 5, an absolute calibration for Landsat 4 can be determined. This study provides only the Landsat 4 and 5 cross-calibration models. To determine these models, Landsat 4/Landsat 5 scene pairs were studied. Within each pair, 8 400x400-pixel sub-regions were selected from the image. The exact geo-located sub-region was located from both instruments and an assumption was made that the ground and the atmosphere did not change between image dates. Therefore, any difference between the images may be attributed to the difference in the instruments. Results of this cross-calibration using multiple dates were consistent to within 2%. Once the cross-calibration points were determined, they were used to correct the relative lifetime-calibration model from the internal calibrator, hence producing an absolute lifetime-calibration model.