185
rn
ise.
scan 665 with
ita of both the
Lai naviagion
ne tagged so
ìan time correc-
; and the an-
:ric correction
(8)
itenna coordi-
rmula is the
: cross section
±>er to which
;ed power
^stem's inter-
:he direction
, For the
i) a value was
external cali-
>een measured
this function
respect to the
.on) . The re-
i elevation
ixample given
iction 0 o is
most accurate
tin direction
le last figure
itenna coordi-
lin function
J'Jor :::
i
J
3».
integrated
.+ 30 deg.).
SCAN 665 0 = 63°
Figure 4. Calculated normalized radar cross section.
FIELD :
Figure 6. Along-track field-averaged gamma.
4 SIR-B EXPERIMENT
As already told in the introduction unfortunately the
SIR-B experiment has been partly insuccesful. Orig
inally, the DUTSCAT measurements should take place at
5 days within a 10 days flight scheme of the Shuttle
in early September 1984. However, launching had to be
delayed twice and the program reduced to 2 days, 11
and 12 October 1984. On these days the collection of
ground data of the test area occured under rather wet
field conditions due to extreme high precipitation in
the preceding month af September. The Shuttle radar
failed to produce data for our test area but scat-
terometer data were gathered along with flight par
ameters and video recordings. The last are meant to
facilitate the interpretation of the measurements.
The test area in the Flevopolder consists of large
agricultural fields (75 ha). We see it in fig. 5.
The lengths of the straight lines in this figure are
proportional to the number of scans where the half
power azimuthal antenna beamwidth is completely within
the field. So field-averaging is depending upon
flightparameters e.g. altitude, heading, pitch, roll,
as well as incidence angle and of course fieldsize and
beamwidth. With a videocamera attached to the scat-
terometer antenna recordings are made containing time
information from which the position of fieldboundaries
in the radardata can be extracted independent of the
measurement geometry. To obtain the earlier discussed
accuracy of 1 dB in relation with equation (5) to (7)
more than 10 scans must be available for averaging
within one field.
Figure 7 gives an impression of the multi-angular
gamma values for the 19 different fields. We see that
the data follow an S-like curve within a range of
10 dB.
FIELD 1 ...19
Figure 7. Gamma versus incidence angle.
The homogeneity of these fields in terms of flatness,
soiltype and agricultural practise is unique. For an
impression of the area the reader is referred to the
literature (de Loor 1982).
The flight track is located diagonally over the test
area and has a length of 10 km. At the time of the
measurements 19 different fields were covered. These
fields were surveyed and sampled in detail which
resulted in data about soil moisture, surface rough
ness and vegetation (Stroosnijder 1984).
The radar cross section 0 o of (vegetation covered)
soils depends on a limited number of predominant
surface parameters, such as those just mentioned. The
relative importance of these parameters depends on
radar parameters such as incidence angle, wavelength
and polarization. The objective for the experiment for
which data is available was to produce estimates of
soil moisture and roughness by using an inverse scat
tering model and field averaged O values for various
incidence angles.
An example of along-track field averaged a divided
by the cosine of the incidence angle, or the backscat-
tering coefficient y is given in fig. 6 (<0>=63 ).
5 SOIL MOISTURE AND ROUGHNESS
As discussed in 2.1 the across-track ground resolution
degrades for decreasing incidence angles. Because of
this resolution problem we will restrict ourselves for
the moment to large incidence angles. For this case
the backscattering coefficient y can be written as
(Attema e.a. 1982).
y = F.(l-exp(-p 2 )) (g)
p = 2kacos0, k=2TT/A
with a the rms value of the height distribution and
F an unknown intensity factor. Based on experimental
evidence, especially for bare soil, it can be assumed
that F is independent of incidence angle and related
to the soil moisture content (Attema/Krul 1979). From
eq. (9) we see that many moisture content - roughness
combinations are possible at one y value and incidence
angle, so that hardly any discrimination can be at
tributed to one single y value. Even if a significant
part of y versus incidence angle is known it might not
be possible to distinguish roughness from moisture.
