Proceedings of the Symposium on Global and Environmental Monitoring: Proceedings of the Symposium on Global and Environmental Monitoring

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Persistent identifier:: 856665355

Title:: Proceedings of the Symposium on Global and Environmental Monitoring

Sub title:: techniques and impacts ; September 17 - 21, 1990, Victoria Conference Centre, Victoria, British Columbia, Canada

Year of publication:: 1990

Place of publication:: Victoria, BC

Publisher of the original:: [Verlag nicht ermittelbar]

Identifier (digital):: 856665355

Language:: English

Document type:: Multivolume work

Volume

Persistent identifier:: 856669164

Title:: Proceedings of the Symposium on Global and Environmental Monitoring

Sub title:: techniques and impacts; September 17 - 21, 1990, Victoria Conference Centre, Victoria, British Columbia, Canada

Scope:: XIV, 912 Seiten

Year of publication:: 1990

Place of publication:: Victoria, BC

Publisher of the original:: [Verlag nicht ermittelbar]

Identifier (digital):: 856669164

Illustration:: Illustrationen, Diagramme, Karten

Signature of the source:: ZS 312(28,7,1)

Language:: English

Usage licence:: Attribution 4.0 International (CC BY 4.0)

Editor:: International Society for Photogrammetry and Remote Sensing, Commission of Photographic and Remote Sensing Data

Publisher of the digital copy:: Technische Informationsbibliothek Hannover

Place of publication of the digital copy:: Hannover

Year of publication of the original:: 2016

Document type:: Volume

Collection:: Earth sciences

Chapter

Title:: [THP-1 HIGH SPECTRAL RESOLUTION MEASUREMENT]

Document type:: Multivolume work

Structure type:: Chapter

Chapter

Title:: SPECTRAL VARIABILITY EFFECTS ON THE ATMOSPHERIC CORRECTION OF IMAGING SPECTROMETER DATA FOR SURFACE REFLECTANCE RETRIEVAL. P. M. Teillet and J. R. Irons

Document type:: Multivolume work

Structure type:: Chapter

the measured spectral response profiles of the ASAS channels. Thus, an attempt was made to define an arbitrary but perhaps more realistic bandpass profile. The profile spans 0.045 micrometers and is 0.015 micrometers across at the half-maximum points (Figure 1). The same profile was applied to all 9 ASAS channels used in this study. The results of the 5S runs are given in Table 2 under the heading "5S BAND (PROF)". Discussion of Results The results given in Table 2 are portrayed in Figure 2. In addition to the 5S results, the values obtained after atmospheric correction using the code of Fraser et al. (1989) monochromatically at NASA are also indicated for comparison. The general trend of all four of the surface reflectance spectra is characteristic of vegetation, increasing from the chlorophyll absorption minimum at lower wavelengths up to the infrared plateau. However, the monochromatic calculations from 5S (LAMBDA GRID) and from the code of Fraser et al. yield anomalously high values of surface reflectance in the vicinity of the narrow oxygen absorption feature at 0.762 micrometers. The bandpass results should be more representative of the situation that actually obtains during a measurement in the ASAS channels, and this is borne out by the improved results in the region of the oxygen band. There also appears to be a slight over-prediction of surface reflectance around 0.722 micrometers where there is some absorption due to water vapour. If a moister atmosphere were to be used in the 5S runs (a mid-latitude summer profile, for example), the bump in the spectrum at 0.722 micrometers would be considerably higher, especially in the monochromatic case but also in the bandpass cases. The bandpass cases are also affected because the water absorption feature is less narrow than the oxygen feature and occupies a greater proportion of the ASAS bandpass. The point to be made here is that the proper atmospheric correction of high spectral resolution data in absorption regions depends on a good knowledge of the relevant gas content at the time of data acquisition. This reguirement is more critical for the more variable gases such as water vapour, and less critical for other gases such as oxygen which is less variable. (Note that some gas profile data were obtained on the ASAS over flight day, but the US62 standard atmosphere profile was used for convenience in this study.) Finally, it should be noted that the surface reflectance retrieval necessarily assumes that the wavelength calibration of the sensor is good. The effects of spectral shifts on sensor response have been addressed by Suits et al. (1988) and Teillet (1990). FURTHER INSIGHT Of all the parameters involved in the atmospheric correction process used in surface reflectance retrieval, the exoatmospheric solar irradiance, E«,(A), and the two-way gas transmittance, x„(A), have the greatest spectral variability. More specifically, the formulation used to obtain surface reflectance from apparent radiance at the sensor includes a factor of [E„( A)x a (A) ]~ x in the multiplicative term (Teillet, 1989). In spectral regions where there is significant gas absorption, x„(A) can become very small and hence the multi plicative atmospheric correction coefficient can become very large. This is necessary in order to retrieve the surface reflectance which will have been severely attenuated in such absorption regions. However, the predicted surface reflectance will be rather sensitive to uncertain ties in x a (A). Thus, if there is no information available on gaseous absorption at the time of image data acguisition and standard values are used in the atmospheric correction, the retrieved surface reflectances can potentially depart significantly from normal values in spectral regions affected by absorption. The guantity [E„( A)x„( A) ]"' was plotted as a function of wavelength in order to illustrate where problems might occur (Figure 4). The values were obtained from special runs of the 5S atmospheric code. The gas transmittance includes absorption by water vapour, ozone, and oxygen. Most of the spikes of higher value are due to water vapour and the narrow spike at 0.762 micrometers is due to oxygen. Figure 4 shows the effect of degrading the spectral resolution from 0.005 micrometers to 0.01, 0.02, 0.03, and 0.04 micrometers, using the filter profile described in Figure 3. Apart from the general smoothing of the spectrum, it is interesting to note how the narrow oxygen feature is greatly diminished with decreas ing resolution. This explains the ASAS results described earlier in that a monochromatic calcula tion near the oxygen feature will use too large a value of [E„( A)x a (A) ]" 1 compared to the value that would obtain in a 0.015-micrometer bandpass. CONCLUDING REMARKS The narrowness of imaging spectrometer bands implies greater sensitivity to spectrally- selective atmospheric absorption features. To study this effect, surface reflectances were retrieved from ASAS data using monochromatic and bandpass atmospheric computations. It was found that anomalous results for surface reflectance can be obtained in the vicinity of absorption features in the monochromatic case whereas bandpass cal culations yield better results. Even with bandpass computations, the retrieved surface reflectances can depart significantly from normal values in spectral regions affected by absorption if there is no information available on atmospheric conditions at the time of image acguisition. ACKNOWI.EDGEMENTS The author wishes to thank G. Fedosejevs and A. Kalil for assistance with the preparation of the manuscript. REFERENCES Fraser, R.S., Ferrare, R.A., Kaufman, Y.J., and Mattoo, S. (1989), Algorithm for Atmospheric Corrections of Aircraft and Satellite Imagery, NASA Technical Memorandum 100751, NASA Goddard Space Flight Center, Greenbelt, Maryland. Irons, J.R., Ranson, K.J., Williams, D.L. and Irish, R.R. (1989), Forest and Grassland Ecosystem Studies Using the Advanced Solid-state Array Spectroradiometer, Proceedings of the 1989 International Geoscience and Remote Sensing Symposium (IGARSS'89) and the Twelfth Canadian Symposium on Remote Sensing, Vancouver, B. C., pp.1761-1764. 580

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