SPECTRAL EMISSIVITY VARIATIONS OBSERVED IN AIRBORNE SURFACE-TEMPERATURE MEASUREMENTS

The Thermal Infrared Multispectral Scanner (TIMS) was flown on the NASA C-130 aircraft for series of a 12 flights during the HAPEX experiment in the Southwest of France during 1986. TIMS provides coverage of the 8-12-mu-m thermal infrared band in six contiguous channels so that it is possible to observe the spectral behaviour of the surface emissivity over this wavelength interval. The surfaces observed ranged from bare soils to coniferous forest. As expected the fully vegetated fields, a small lake, and a coniferous forest exhibited little or no spectral variation. However, the bare soil surfaces had a 5-10-degrees-C difference in radiant temperature over the TIMS channels with the three shorter wavelength channels (8.0-mu-m < lambda < 9.6-mu-m) having lower temperatures than the three longest wavelength channels. This qualitatively agrees with laboratory measurements done by a group from the Universite Louis Pasteur in Strasbourg on soil samples from this area. The quantitative agreement of the relative variation after correcting for atmospheric effects is good, but does depend on the day and location of the sample in the field. This spectral variation arises from the absorption of infrared radiation due to the stretching vibrations of the silicon - oxygen bonds of silicates in the soil. These results indicate that the longer wavelength channels in the 8-12-mu-m band may be preferable for surface temperature sensing for soils rich in silicates.