A method for monitoring the temporal variation of surface spectral emissivity: Application to thermal infrared multispectral scanner (TIMS) data in HAPEX-Sahel
Zl. Li et al., A method for monitoring the temporal variation of surface spectral emissivity: Application to thermal infrared multispectral scanner (TIMS) data in HAPEX-Sahel, J GEO RES-A, 104(D24), 1999, pp. 31217-31224
During the Hydrologic Atmospheric Pilot Experiment (HAPEX)-Sahel experiment
in 1992, thermal infrared multispectral scanner (TIMS) data were acquired
from aircraft at an altitude of 600 m. The main steps to retrieve the surfa
ce relative spectral emissivities E from TIMS data using the emissivity nor
malization method were recalled. Several TIMS data on September 2 and 4 wer
e processed, and the retrieved E for different types of surfaces showed tha
t E in channels 1-3 (8.2-9.4 mu m) are less than E in channels 4-6 (9.6-12.
0 mu m) for bare soils, whereas for vegetation the E profile is not as flat
as it should be. In order to correct for the effect of uncertainties in at
mospheric profiles given by radiosonde data and to get the emissivity profi
le flat for vegetation the temperature and humidity profiles were modified
by considering the vegetation to be a gray body. Although E obtained with t
he modified atmospheric profiles seemed to be improved significantly for ea
ch day, E obtained for the same type of surface at two days are still very
different, showing that it is difficult to compare E derived from different
images with the method proposed. Through the comparison of E derived with
radiosonde data and E derived with the modified atmospheric profiles the li
near relationship found by Li et al. [1999] between E calculated with inacc
urate atmospheric profiles and its actual value was confirmed. On the basis
of this linear relationship property a method called temporal atmospheric
normalization was proposed to correct for the errors caused by the imperfec
t atmospheric corrections under horizontally invariant atmospheric conditio
ns. This method relates linearly the measurement of relative emissivity mad
e at a time t(2), E-c(t(2)), to the measurement E-c(t(2) --> t(1)) that wou
ld have been made if the atmospheric condition of lime t(2) was that of a r
eference time tl, In order to calculate the coefficients of the linear rela
tionship between E-c(t(2) --> t(1)) and E-c(t(2)) it is necessary to select
in the image some samples whose relative emissivity are known to be time i
nvariant. This method was validated using TIMS data. It may now be used to
monitor the temporal variation of surface relative emissivity correcting fo
r possible temporal variation of the atmosphere particularly using Advanced
Spaceborn Thermal Emission Reflection Radiometer data.