746
4. CONCLUSIONS
It is clear that following the first field deployment of the ATLAS instrument and use under fairly severe
conditions in Niger that a number of modifications and enhancements are required. In particular the alignment of
the solar tracker and the 1” FOV head needs to be addressed before further work on the validation of the
atmospheric optical parameter retrievals, namely the aerosol optical depth and size distribution, can commence.
Despite these limitations the instrument has provided the data for further analysis of the sky radiance distribution
modelling and fulfilled a major requirement of the surface radiation budget experiment by acquisition of the
Bidirectional Reflectance Distribution Function for many of the cover-types encountered in the Sahelian region.
Further work within the TIGER research programme will concentrate on the validation of joint retrievals of
surface directional properties and atmospheric loading from multi-look remotely sensed data, when this becomes
available, and on the incorporation of such retrieved data into soil/vegetation/atmosphere transfers (SVATs) to
improve global climate and macrohydrological models.
This prototype instrument, whose initial design and construction was constrained by the availability of fast
detector array spectroradiometers, has lead to a number of designs for 2nd generation instruments that can
provide contiguous high spectral resolution data for retrieval of atmospheric optical parameters and surface
directional reflectances. It is also clear that the availability of such data to the research community is essential
for the successful development of a number of applications using remotely sensed data for monitoring the global
environment. ATLAS will continue to support these activities in the UK under a number of NERC supported
programmes including the validation of data products from the ATSR-2 satellite instrument and for the
development of atmospheric correction strategies for the SeaWiFS ocean colour sensor.
ACKNOWLEDGEMENTS
This work was carried out under a research grant from the Natural Environment Research Council (NERC)
under its Terrestrial Initiative in Global Environmental Research (TIGER) Programme (TIGER III.1D). I would
like to acknowledge the contributions of the Royal Greenwich Observatory (RGO) Cambridge for their
assistance in developing the ATLAS levelling platform and gantry system, the NERC Equipment Pool for Field
Spectroscopy (EPFS) for providing laboratory calibration facilities, and to members of the NERC Unit for
Thematic Information Systems (NUTIS) for help in the field deployment of ATLAS in Niger.
REFERENCES
Gourtourbe, J.P., Lebel, T„ Tinga, A., Bessemoulin, P., Brouwer, J„ Dolman, AJ., Engman, E.T., Gash, J.H.C.,
Hoeppfner, M„ Kabat, P., Kerr, Y„ Monteny, B., Prince, S., Said, F., Sellers, P., and Wallace, J.S., 1993.
A large scale study of land-atmosphere interactions in the semi-arid tropics (HAPEX-Sahel). In Bolle,
H.J., Feddes, R.A., and Kalma, J. (Eds) Exchange Processes at the Land Surface for a Range of Space
and Tune Scales. IAHS Publ. 212, 357-361.
Gourtourbe, J.P., and above authors, 1993. HAPEX-Sahel: A large scale study of land-atmosphere interactions
in the semi-arid tropics. Annales Geophysicae, in press.
Hooper, F. C. and Brunger, A. P„ 1980, A model for the angular distribution of sky radiance, Transactions of
the ASME: Journal of Solar Energy Engineering, 102,196-202.
King, M. D., Byrne, D. M., Herman, B. M. and Reagan, J. A., 1978, Aerosol size distributions obtained by
inversion of spectral optical depth measurements, Journal of the Atmospheric Sciences, 35,2153-2167.
Wilson, A.K., 1991. ATLAS: An Auto-Tracking Land and Atmosphere £snsor for retrieval of atmospheric
optical parameters and acquisition of bidirectional surface reflectance, Proceedings of the 5th
International Colloquium - "Physical Measurements and Signatures in Remote Sensing", Courcheval,
France, (ESA: Noordwijk), Vol 1. p 463-466.
