123
rest arrive
eiver (section
rpreter to
étions 4 and 5)
' pre-
reas question-
eight of flight
ound range dis-
idth and low
(DUTSCAT) ma-
from different
easured in dif-
em in every
ally) repre-
ENTS USING
easurements of
ted with the use
iment of this
ite over stands
R. In 1985 these
X-band
e have not been
ectors with an ,
pace) of 45 dBm
and 3 of 38 dBm 2 were especially manufactured for this
purpose. They are dismountable and were assembled in
situ under the canopy at the forest floor. A
detailed description of the experiment design, the
method used to acquire the attenuation factors and
the detection probabilities of covered corners are
given by Hoekman (1986). In this section only the
results of the campaigns and its importance in model
research will be discussed.
3.2 Discussion of X-band results
Table 3 shows the overall results of 2 X-band HH-
polarized SLAR flights. The other flights have not
been analysed yet. In each flight the run over the
6 corners is repeated once. Substantial between
run variations were noticed and could be explained
from changing sensor aspect (Hoekman 1986) . For some
corners no signal could be perceived therefore only
lower limits could be extracted. The signal from
corner 3 in the poplar stand probably saturated the
receiver therefore an upper limit is given.
Table 3. Equivalent two-way attenuation factors T e q
(in dB) in X-band, HH-polarization and at 45 degrees
incidence angle for poplar, oak and pine.
and crowns are mostly .small and light. Corners 1 till
4 inclusive and 6 were carefully placed behind groups
of crowns thereby avoiding illumination of the corners
through big gaps in the canopy. Corner 5 was placed
at a large open spot.
It can be concluded that the attenuation through the
forest canopy in general fluctuates strong and de
pends on relative orientations of gaps with respect
to the sensor. The attenuation of crowns of deciduous
trees is high. The attenuation of crowns of coniferous
trees may be lower but still is in the order of 20 dB
(two-way). Therefore at X -band contribution of scat-
terers of the understory and forest floor is prima
rily determined by canopy architecture (i.e. factors
like crown closure, crown shapes etc.) and angle of
measurement.
3.3. Modelling aspects illustrated by the cloud model
The attenuation properties of vegetation layers is an
important parameter in many backscatter models. In
one of these models; the cloud model formulated by
Attema and Ulaby (1978) and Hoekman et al. (1982),
gamma of a vegetation layer is modelled as single
scattering by a low density cloud of small identical
particles :
Y veg = °/ 2 q [l-exp(-2NQh/cos0i)] (2)
Flight over Roggebotzand test area
where N is the number of particles per unit volume,
tree species
run 1
run 2
h is the height of the vegetation
layer,
corner
i
poplar
>28.
>28.
O is the radar cross section of a
particle ■
corner
2
poplar
12.5
21 .5
Q is the attenuation cross section of a par-
corner
3
poplar
< 7.
10.0
tide.
corner
4
oak
22.
17.7
corner
5
oak
>22.
>21 .
Assuming the soil scattering to add incoherently to
corner
6
oak
>21 .
22.6
the vegetation scattering the total gamma
value be-
comes ;
Flight
over
Kootwijk test
area
Y ,=Y +exp(-2NQh/cos0 1 -) .Y .
'total 'veg v v 'soil
(3)
tree species
run 1
run 2
corner
i
pine
19.5
20.4
With T=exp(-2NQh/cos0^)
(4)
corner
2
pine
19.3
22.2
corner
3
pine
>24.
>24.
corner
4
pine
>17.
>17.
corner
5
pine
5.2
6.2
this equation is simplified to
corner
6
pine
11 .0
18. 1
Y total" AoP-d^-Vaoil
(5)
For an interpretation of these results a description
of the forest stand architecture is indispensable.
The stand of poplar (clone 'Robusta') is 25 years
old, with trees planted in rows every 8 meter. The
row spacing is also 8 meter, tree height is about
28 meter. The canopy features no crown closure;
distances between crowns range from 1 to 2.5 meter.
The measurement direction was in row direction at
45 degrees incidence angle. Corner 1 was placed in a
row. As a result 4 trees (2 crowns and 2 stems) were
between corner and sensor. This completely blocked
any perceivable signal from the corner. Corner 3 was
placed in the middle between 2 rows. Now only a few
branches were situated between sensor and corner resul
ting in a low attenuation. Corner 2 was placed be
tween rows, 2 meters from one row and 6 meters from
the other. Now the outer parts of 2 crowns were
between sensor and corner. The big difference in
measured attenuation factors (12.5 versus 21.5) could
be explained by between run variation of the relative
orientation of sensor, crown and corner.
The stand of oak (Quercus Robur) is 25 years old
and has a high degree of crown closure; there are
small gaps (<1 m 2 ) but almost no big gaps. The tree
height is about 12 meters. The pines (Pinus Sylves-
tris) at the Kootwijk site are 55 years old and about
12 meters tall. The site factors do not favour a
prosperous growth. The degree of crown closure is low
The parameters T as well as a and Q depend on frequen
cy, angle of incidence (for non-spherical particles)
and polarization (for non-spherical particles).
However the forest canopy is not expected to con
sist of identical scatterers. By assuming the vege
tation canopy to be composed of n discrete layers
with different types of scatterers the cloud model
equation can be extended. It follows from induction
that
Y veg =Yl+TlY2+TlT2Y3+ ‘‘- +TlT2 -• ,T n-l Y n ( 6a )
where index 1 stands for the top layer,
Y i =ai/ 2Q i *t 1_T i3
(6b)
aiK * Ti=exp (-2N£Q^d/cos0)
(6c)
With _ _ _ _
T t =T 1 T 2 T 3 ...T n
(7)
it follows Y t *. i“Y +T ^‘Y * i
'total veg t soil
(8)
When the cloud model assumptions are followed, the
equivalent attenuation factor T eq measured with the
corner reflector experiment, with corners placed at
the ground, is the same as the parameter T t of the n-
layer cloud model. Thus equation (8) can be written
as
