The study of Bi-directional Reflectance Distribution
Function ( BRDF) for the Grassland
4. Observation
We carried out a observation using above system for the 6th to the 20th of August 1996 in Mongolia . measurement areas are the three points with different vegetation conditions ( see the figure .2) targets vegetation is a flat and homogenous grass, ex. The grass height is about 15 cm on average and vegetation coverage is about 20% at the area (3). The measurement conditions show the Table .2. One cycle of measurement is every 30 degree for 0 degree to 330 degrees for the sensor azimuth of sensor zenith angle. Time of one cycle require about 25 minutes .
Figure 2: Observation Areas
Table 2.measurement conditions
| Location |
N47°41, E108°03 (3) |
| Date |
1997/08/20 |
| Time |
9:00-16:00 |
| Solar zenith angle |
35-70 deg |
| Solar Azimuth Angle |
94-250 deg |
| Vegetation Coverage |
About 20% |
| Vegetation height |
3-20 cm |
| Sensor Height |
5 m |
| Sensor Azimuth Angle |
0-330 deg ( every 30 deg ) |
| Sensor zenith Angle |
0-60 deg ( every 10 deg ) |
5. Model Simulation
our developed model is a simple layer model based on flour-flux theory like the suits model(1) and the SAIL model(2). The four-flux theory is written by :
| Speclar Flux : |
dEs
-----------
dx |
= kEs |
| unward Diffuse Flux : |
dE-
-----------
dx |
=sEs + aE--sE+ |
| Downword Diffuse Flux : |
dE+
-----------
dx |
= s'Es +sE--aE+ |
| Radiation Flux : |
dE0
----------
dx |
= wEs+vE-+uE+-KE0 |
each coefficient in the different equations are calculated form vegetation parameters. This model purpose is for estimation and correction of radiance energy from the target, therefore we should not use that a measurement is difficult parameter such as the leaf area index ( LAI) or the leaf inclination density functions ( LIDF), then we used the vegetation height and the vegetation coverage calculated from the target image.
we need scattering parameters in the vegetation layer for lead coefficients. Then we define a model consisting of simple geometric figures using the vegetation height and the vegetation coverage. Coefficient is calculated by using the scattering characteristic of figures instead of the vegetation scattering characteristic in the layer.
