A neural network approach
was used to develop accurate algorithms for inverting a complex forest backscatter
model. The model combines a forest growth model with a radar backscatter model.
The forest growth model captures natural variations of forest stands (e.g.
growth, regeneration, death, multiple species, and competition for light).
This model was used to produce vegetation structure data typical of transitional/northern
boreal hardwood forests in Maine. These data supplied inputs to the radar
backscatter model which simulated the polarimetric radar backscatter (C, L,
P, X bands) above the forests. Using these simulated data, various neural
networks were trained with inputs of different backscatter bands and output
parameters of above ground biomass, total number of trees, mean tree height,
and mean tree age. These trained neural networks act as efficient algorithms
for inverting the complex forest backscatter model.
The accuracies (RMS and R 2 values) for inferring various parameters from
radar backscatter were above ground biomass (1.6 kg m -2 , 0.94), number of
trees (48. ha -1 , 0.94), tree height (0.47 m, 0.88), and tree age (24.0 yrs.,
0.83). The networks that used only AIRSAR bands (C, L, P) had a high degree
of accuracy. The inclusion of the X band with the AIRSAR bands did not seem
to significantly increase the accuracy of the networks. The networks that
used only the C and L bands still had a relatively high degree of accuracy
for all forest parameter ( R 2 values from 0.75 to 0.91). Modest accuracies
( R 2 values from 0.65 to 0.84) were obtained with networks that used only
the L band and poor accuracies (R 2 values from 0.36 to 0.46) were obtained
with networks that used only the C band. Several networks were shown to be
relatively insensitive to the addition of random noise to radar backscatter.
The results demonstrate that complex, forest backscatter models can be efficiently
inverted using neural networks that use only radar backscatter data.