spectroradiometer, then currently available, was enhanced to enable the additional retrieval of surface 
Bidirectional Reflectance Distribution Function (BRDF) (Wilson 1991). The Auto-Tracking Land and 
Atmosphere Sensor (ATLAS) consists of three Spectre« SE-590 spectroradiometers, mounted on a computer 
controlled altazimuth platform, which can record directional radiances from the surface and sky hemispheres 
through 1' and 15' FOV heads and simultaneous global spectral irradiance using a cosine corrected head (Figure 
1). Each spectroradiometer head provides a contiguous spectral profile from 400-1 lOOnm in 252 (~llnm wide) 
channels using a purpose built interface to connect to the laptop computer for automatic integration control and 
data recording. The spectroradiometers and the altazimuth mount are controlled from a virtual instrument panel 
developed on an Apple Powerbook that enables manual or automatic control of all functionality. 
Altazimuth Platform 
Figure 1. Schematic diagram of ATLAS instrument. 
The instrument itself sits on a levelling platform which can be attached to the end of a 7 metre long boom and 
pivoted on a tripod to raise the spectroradiometer heads to over 5 metres above ground and so provide sufficient 
clearance above the typical Sahelian vegetation of Guiera bushes and Combretum canopy or millet crop (Figure 
2 and 3). When levelled at the appropriate height above the surface, measurements can be automatically 
retrieved from pre-programmed view zenith and azimuth directions to match the observation directions of the 
airborne sensors or as required in the joint retrieval procedures. The instrument was originally designed to 
retrieve a number of atmospheric optical parameters which are used in the calculation of the directional 
reflectance properties of the cover-types, the radiative transfer modelling and the radiometric correction of the 
remotely sensed data. These include the retrieval of direct beam solar irradiance combining the use of a solar 
tracker to lock on to the solar position from an initial estimate using ephemeris and the 1" FOV 
spectroradiometer with automatic insertion of Neutral Density (ND) filters to avoid saturation. Having located 
the solar disk the instrument can either track the sun and make periodic measurements of direct beam irradiance 
or perform a series of measurements at off solar angles in the almucantar (a horizontal line at the solar zenith 
angle) or vertically through the solar position. Automatic sampling of the diffuse sky radiance distribution is also 
possible using up to 73 pre-defined sample locations. For the surface directional reflectance the 15’ FOV head is 
typically used to either sample along the Solar Principal Plane (SPP), at some inclined azimuth to the sun, or for 
a complete BRDF at up to 85 pre-defined measurement angles. Near-simultaneous global irradiance can be 
collected with each 1' or 15' FOV measurement to monitor the stability of the solar irradiance and for the 
calculation of the surface reflectance. 
2.2 Field Deployment 
During the IOP the ATLAS instrument was deployed on those days suitable for optical measurements at the 
southern super-site at one or other of the three chosen sub-sites. These three sub-sites were chosen to cover the 
main characteristic cover-types of the area and include: (1) millet, agricultural land which is used for growing 
traditional crops, (2) Guiera fallow, agricultural land which has not been cropped for a number of years and is