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first deployment of a ground-based instrument to retrieve 
ATMOSPHERIC OPTICAL PARAMETERS AND SURFACE BRDF DURING THE 
HAPEX-SAHEL EXPERIMENT, NIGER 1992 
A.K. WILSON 
British National Space Centre, 
Remote Sensing Applications Development Unit, 
Monks Wood, Abbots Ripton, Huntingdon 
Cambridgeshire. PE 17 2LS (UK) 
ABSTRACT: 
This paper reports on the first field deployment of a ground-based instrument: The Auto-Tracking Land and 
Atmosphere Sensor (ATLAS) in support of surface radiation budget estimation during the second Hydrological- 
Atmospheric Pilot Experiment (HAPEX-Sahel). A part of the experiment required validation data for joint 
retrievals of surface albedo and atmospheric parameters from multi-angle remotely sensed data. These were 
based on co-ordinated ground, aircraft and satellite measurements that were carried out in Niger, W. Africa 
during the Intensive Observing Period GOP) in August and September 1992. The design criteria for a single 
instrument capable of retrieving a range of atmospheric optical parameters and acquiring multispectral 
directional surface reflectance from the local cover types is outlined and examples of the datasets presented. The 
current status of database construction, radiometric and spectral calibration, and preliminary modelling is 
discussed with emphasis on the potential for retrieval of atmospheric optical parameters. Further enhancements 
to the instrument are being made in the light of this initial field deployment and future research plans are 
¡resented. 
KEYWORDS: HAPEX-Sahel, Surface Radiation Budget, Atmospheric Optical Parameters, Surface BRDF 
1. INTRODUCTION 
HAPEX-Sahel is one of a series of international experiments under the WCRP-IGBP Joint Working Group on 
Land Surface Experiments and is the second in the Hydrological Atmospheric Pilot Experiment (HAPEX) 
series on large scale energy, water and carbon balance. The experiment took place during the summer of 1992 
in the Sahelian region of Niger, W. Africa (Goutourbe et al, 1993a and 1993b). The overall objective of the 
experiment is to improve the parameterization of semi-arid land surface processes in General Circulation Models 
(GCMs) by a combination of low intensity long term monitoring of a large GCM scale area and intensive 
observations at a key period of the year at a few representative locations. An Intensive Observation Period GOP) 
took place between 17 August and 10 October 1992 with three 10 km by 10 km super-sites situated within the 
100km by 100km study area. The entire programme consisted of work on ground water recharge, unsaturated 
zone soil moisture, vegetation water use, micrometeorology, atmospheric boundary layer sounding and both 
aircraft and satellite remote sensing. One particular research project carried out at the southern (UK) super-site, 
Estimation of the Surface Radiation Budget using Multi-Look Remotely Sensed Data" is based on the retrieval 
of surface albedo for representative surface cover-types and the use of remotely sensed airborne and satellite 
data to provide aggregation of local measurements to sub-site, super-site and GCM scales. To validate the joint 
retrieval of the surface albedo and the atmospheric loading from the remotely sensed data, both the directional 
reflectance properties of the characteristic cover-types and the atmospheric optical parameters, defining the 
irradiance field incident at the surface, are required. 1 
1 INSTRUMENTATION AND METHODS 
2.1 The Auto-Tracking Land and Atmosphere Sensor (ATLAS) 
An instrument being developed by the Remote Sensing Applications Development Unit (RSADU) in support of 
research on retrieval of atmospheric optical parameters and based on the only instantaneous recording