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The Advanced Space-borne Thermal Emission and Reflection Radiometer (ASTER) is one of the five sensors on the
NASA’s Terra satellite on orbit since December 1999. ASTER consists of three radiometers, the Visible and Near
InfraRed (VNIR), the Short-Wave InfraRed (SWIR) and Thermal InfraRed (TIR) whose spatial resolutions are 15 m, 30
m and 90 m, respectively. Unfortunately the SWIR image data are saturated since April 2008 due to the offset rise
caused by the cooler temperature rise, but the VNIR and the TIR are taking Earth images of good quality. The VNIR and
the TIR experienced responsivity degradation while the SWIR showed little change. From the lamp calibration, Band 1
decreased the most among three VNIR bands and 31% in thirteen years. The VNIR has the electrical calibration mode to
check the healthiness of the electrical circuits through the charge coupled device (CCD). Four voltage levels from Line 1
to Line 4, which are from 2.78 V to 3.10 V, are input to the CCD in the onboard calibration sequence and the output
digital numbers (DNs) are detected in the images. These input voltages are monitored as telemetry data and have been
stable up to now. From the electrical calibration we can check stabilities of the offset, gain ratio and gain stability of the
electric circuit. The output level of the Line1 input is close to the offset level which is measured while observing the
earth at night. The trend of the Line 1 output is compared to the offset level. They are similar but are not exactly the
same. The trend of the even pixel and odd pixel is the same so the saturated offset levels of the odd pixel is corrected by
using the even pixel trend. The gain ratio trend shows that the ratio is stable. But the ratio values are different from those
measured before launch. The difference comes up to 10% for the Band 2. The correct gain ratio should be applied to the
vicarious calibration result because the onboard calibration is measured with the Normal gain whereas the vicarious
calibration often measures with the High gain. The cause of the VNIR responsivity degradation is not known but one of
the causes might be the change of the electric circuit. The band 3 gain shows 16 % decrease whereas the gain changes of
the band 1 and band 2 are 5% to 8%. The responsivity decrease after 1000 days since launch might be controlled by the
electric circuit change.
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