BRDF CORRECTION ON AVHRR IMAGERY FOR SPAIN 
H. Heisig 
Institut für Photogrammetrie und Fernerkundung, Universität Karlsruhe, D-76137 Karlsruhe heisig@ipf.uni-karlsruhe.de 
TS 3 YF Youth Forum - Remote Sensing 
KEY WORDS: Remote Sensing, Radiometry, Correction, Model, Land Cover 
ABSTRACT 
The free reception of Advanced Very High Resolution Radiometer (AVHRR) data keeps a growing interest for the remote sensing 
community concerned with vegetation studies on regional and global scales. However, the use of AVHRR data for long-term 
quantitative monitoring requires consistent surface reflectance data. This implies correcting the effects from varying sun sensor target 
geometries in multitemporal AVHRR data sets described by the Bidirectional Reflectance Distribution Function (BRDF). In a joint 
research undertaken by the Geography Department of the University of Alcalá (Spain) and the Canadian Centre for Remote Sensing 
(CCRS) a new semi-empirical model, the Non-linear Temporal Angular Model (NTAM), has been applied for the BRDF correction 
of AVHRR data for Spain. Reflectance data derived from AVHRR channels I and 2 were corrected for BRDF effects by 
normalization to a standard viewing geometry. The study period was between May and September 2002. The required correction 
parameters were derived by model inversion. In order to obtain a good representation of the temporal and spatial dimensions of the 
data to be corrected, a sampling scheme was implemented. The land cover based approach of the NTAM was accounted for by the 
use of CORINE Land Cover data from Spain. A cloud mask algorithm was used to exclude cloud contaminated observations in the 
sampling data. The evaluation of the statistical parameters obtained from model inversion showed good results for practically all land 
cover classes in the two channels. Derived parameters allow for visualization of angular dependence of reflectance for different land 
cover classes. 
  
1. INTRODUCTION 
Current research work at the Geography Department at the E 
University of Alcalä is strongly focussed on the field of fire risk dE; dL, 
assessment (Chuvieco 2002). Since 1998 the department T 
‘receives AVHRR data in the High Resolution Picture " T 
Transmission (HRPT) format through an own receiver. These 
data are used for estimation of plant water content with the | 
purpose of incorporating these estimations in fire risk indices | | 
(Aguado et al., 2003; Chuvieco et al., 2003). 
  
  
Sun-synchronous polar-orbiting satellites like the National 
Oceanic and Atmospheric Administration (NOAA) AVHRR 
sensors are operated on must have such an orbital inclination 
that the rate of the resulting precession compensates for the 
motion of the Earth around the sun (Cracknell 1997). The 
  
  
  
  
satellites orbit tracks projected on the Earth's surface reveal à Figure 1: Concepts and parameters of the BRDF (Sandmeier & 
parallel shift of about one degree east per day. This results in Itten, 1999; modified) 
changing view angles for the same target within consecutive 
day passes. where: 
0, 2 zenith incidence angle 
BRDF 0, = zenith reflection angle 
¢; = azimuth incident angle 
Vegetation, as most other surfaces, does not scatter irradiance 0, = azimuth reflection angle 
in equal quantities in all directions. In fact, it shows a behaviour ¢ = phase angle 
far from being Lambertian (Dymond et al, 2001). Its 
reflectance depends on the angle of observation as well as on Parallel to the enhancement of BRDF measuring methods 
the angle of the incidence of the solar radiation. This bi- (goniometers, measurements from helicopters), considerable 
directional dependency lead to the idea of the BRDF effort has been spent on development and testing of analytical 
(Bidirectional Reflectance Distribution Function). The BRDF and empirical models to describe the BRDF (Chen & Cihlar, 
describes the ratio of radiance dLr [W m” sr! nm'!] reflected in 1997: Chopping, 2000; Hu et al., 1997; Ni et al., 1999; Ni & Li, 
one direction (8, $) to the irradiance dEi [W m^ nm"] from — 2900; O'Brien et al., 1998; Roujean et al., 1992; Wanner ct al.. 
direction (8; $) (Sandmeier & Itten, 1999). Figure 1 displays 1995), BRDF models potentially allow for prediction of 
the angles that the BRDF is dependent on. reflectance for any desired sun sensor target geometry. 
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