Proceedings Article | 5 October 2007
KEYWORDS: Sensors, Landsat, Calibration, Contamination, Data modeling, Short wave infrared radiation, Thin films, Satellites, Electro optical modeling, Earth observing sensors
The Thematic Mapper (TM) is a multi-spectral electro-optical sensor featured onboard both the Landsat 4 (L4) and
Landsat 5 (L5) satellites. TM sensors have seven spectral bands with center wavelengths of approximately 0.49, 0.56,
0.66, 0.83, 1.65, 11.5 and 2.21 μm, respectively. The visible near-infrared (VNIR) bands are located on the primary
focal plane (PFP), and two short-wave infrared (SWIR) bands and the thermal infrared (TIR) band are located on the
cold focal plane (CFP). The CFP bands are maintained at cryogenic temperatures of about 91 K, to reduce thermal noise
effects. Due to the cold temperature, an ice film accumulates on the CFP dewar window, which introduces oscillations
in SWIR and an exponential decay in TIR band responses. This process is usually monitored and characterized by the
detector responses to the internal calibrator (IC) lamps and the blackbody. The ice contamination on the dewar window
is an effect of the sensor outgassing in a vacuum of the space environment. Outgassing models have been developed,
which are based on the thin-film optical interference phenomenon. They provide the coefficients for correction for
outgassing effects for the entire mission's lifetime. While the L4 TM ceased imaging in August 1993, the L5 TM
continues to operate even after more than 23 years in orbit. The process of outgassing in L5 TM is still occurring,
though at a much lower rate than during early years of mission. Although the L4 and L5 TM sensors are essentially
identical, they exhibit slightly different responses to the outgassing effects. The work presented in the paper summarizes
the results of modeling outgassing effects in each of the sensors and provides a detailed analysis of differences among
the estimated modeling parameters. For both sensors, water ice was confirmed as a reasonable candidate for
contaminant material, the contaminant growth rate was found to be gradually decreasing with the time since launch, and
the indications exist that some film may remain after the CFP warm-up procedures, which are periodically initiated to
remove accumulated contamination. The observed difference between the models could be contributed to differences in
the operational history for the sensors, the content and amount of contaminant impurities, the sensor spectral filter
responses, and the internal calibrator systems.
