(a) (b; (c)
(d)
(e)
(Í)
Figure 9. The relative contributions, according to the assumptions of the multi-level model, from lay
ers in the forest canopy and the ground. Each bar represents a 50% confidence interval for the corres
ponding estimates. In each figure 2, 3 or 4 results are grouped for each layer.
rences between relevant scatterers are not accounted
for is hard to predict. The error becomes negligible
when all contributions originate from a small
layer. This situation might occur when (a) the
forest is very low or (b) the forest has a closed
and smooth canopy and attenuation is very strong (at
the higher frequencies).
But when the canopy is not closed, or has a rough
surface (emergent trees), or the attenuation is
not strong (lower frequencies) the effect is far from
predictable. In these situations only a worst case
error can be indicated for a given height of flight,
canopy height and incidence angle.
7. CONCLUSIONS
By means of large corner reflectors the Dutch
X-band SLAR can be used for the acquisition of data
on the attenuation properties of forest canopies. The
attenuation was found to be primarily determined by
canopy architecture (i.e. factors like crown closure,
crown shape etc.) and aspect of measurement. The
attenuation of crowns of deciduous trees is strong.
For crowns of coniferous trees it may be lower but
still is in the order of 20 dB (two-way).
The multi-band scatterometer DUTSCAT was used to
conduct the same experiment in the L-band but the
acquired data have not been analysed yet.
The attenuation properties are of direct impor
tance when the influence of the soil, soil moisture
content, standing water or undergrowth under a fo
rest canopy has to be modelled.
The DUTSCAT when operated from relatively low
altitudes can provide information on the vertical
distribution of backscattering. This may be done
through inversion of the multi-level model introdu
ced in section 4. The y value of a forest stand
could be divided in contributions from a number of
layers (3 or 4) of the forest. In C-band at low
incidence angles, it was found that a substantial
amount of the backscattering from poplar stands
originates from the ground and understory. In L-band
at low incidence angles ~75% of the backscattering
originates from the ground. Apparently the poplar
crowns are very transparent at this frequency. As
expected the relative contributions from the lower
layers decreased with increasing incidence angle.
The accuracy of these measurements can be influenced
by measurement geometry. When more detailed infor
mation is desired the DUTSCAT has to be modified.
The range resolution has to be increased and the
beam width in range direction decreased.
Measurements of additional physical parameters
of forest canopies, other then y or 0°, can simpli
fy the model-making effort. This was illustrated
by the cloud model. Some parameters of the cloud
model could be determined directly through inversion
of the appropriate form of the multi-level model.
The multi-level model is also a useful tool in the
pre-processing of scatterometer data of forests. In
this model the radar equation is applied for each
level individually and as a result the calculation
of y is more accurate. In fact when the model is not
used for the calculation of y or o° gross errors may
occur.
8. ACKNOWLEDGEMENTS
The author would like to acknowledge the support of
the working group ROVE-Forestry. Financial support
for the radar flights was provided by the Netherlands
Remote Sensing Board (BCRS).
9. REFERENCES
Attema, E.P.W. and F.T. Ulaby, 1978, Vegetation
modelled as a water cloud, Radio Science, Vol. 13,
Figure 10. :
as a fune tii
incidence ai
in the secoi
of 1800 metí
No. 2, pp
Attema, E.P
a 6-frequ(
EARSeL Woi
to vegetai
ESA SP-22;
Hoekman, D.l
A multila)
vegetatior
Remote Ser
Voi. 2, T/
Hoekman, D.E
backscatte
classificò
Colloquiun
remote sen
SP-247, pp
Ulaby, F.T.,
Microwave
Addison-We