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From eq. (2) and (3) the resolution cell follows as:
Aa = Ax.Ay = cxB a h/sin20
Which obviously has a minimum at 0 = 45 degre
example at a flight altitude of 300 m as used
SIR-B experiment we find that for DUTSCAT:
T = 100 ns
0a =0.22 rad
^^min = ^980 m^
In the search for relations between surface proper
ties and radar backscatter characteristics it must be
noted that the irregular nature of surfaces in general
causes the scattered electromagnetic field components
to be random functions. This means that the scattered
power as measured by the radar is different for each
resolution cell as it depends on phase relations with
in the resolution cell itself. Therefore the estimated
average of this scattered power, or the radar cross
section per unit area is the most commonly used para
meter in radar studies. After an incoherent averaging
known as "speckle reduction" the standard deviation
of the average received power P is:
SCAN 665 0 = 63°
a = p//n
(5)
Figure 2. Averaged received power for scan 665 with
an incidence angle of 63 degrees.
3 DATA PROCESSING
From the NLR we receive preprocessed data of both the
scatterometer and the airplane's inertial naviagion
system. These two types of data are time tagged so
that they can be combined and in the mean time correc
ted e.g. for the spatial spreading loss and the an
tenna weighing. This so called radiometric correction
is expressed by the radar equation:
with N the number of independent measurements caused
e.g. by the movement of the airplane. For N we find
the next expression (Ulaby e.a. 1982):
,3 4
P (4tt) R
r
(Ay.G./g 2 ) 1
(8)
N = Ax-(L a /2)
(6)
With L a the antennalength which is approximately:
L
a
A/6 a
(7)
with A the free space wavelength which is equal to
0.25 m for the frequency of 1.2 GHz used by DUTSCAT.
From (6) and (7) we see that an independent measure
ment is done after each movement of half the antenna
length. The received power appears to be exponentially
distributed and it can be calculated that for N=200
the averaged power is known with an uncertainty of 1
decibel within 90% of the time (Smit 1978). The along-
track distance required in our case to obtain this
accuracy is 120 m or 1.3 along-track resolution cells
at an incidence angle of 45 degrees (h=300m).
2.2 DUTSCAT data
The DUTSCAT scatterometer has a rather high internal
pulse repetition frequency of 78.125 kHz so that every
12.8 ys a pulse is transmitted. The received signal
is digitized with a sampling rate of 20 MHz which
means that the sample time is equal to 50 ns. Then a
coherent averaging is applied to the signals of sub
sequent pulses to improve the signal to noise ratio.
After this the previously described speckle reduction
is performed by averaging incoherently. Additional
averaging is done to reduce the data rate of the
digital output. Finally we have a pulse repetition
frequency of 4.77 Hz, so that effectively the across-
track direction is scanned once every 210 ms.
An example of an arbitrary resulting scan is given
in fig. 2. From the velocity of the airplane, that
carries the scatterometer, which is known to be about
50 m/s it directly follows that this particular scan
(665) is taken at about 7 km after the start of the
flight track. The information that at the time of
this scan the incidence angle was 63 degrees is ex
tracted from flight parameter data that simultaneously
has been recorded on tape along with the radar data.
The nominal incidence angle can be selected by the
operator inside the airplane and varied between 10
and 80 degrees. During the SIR-B experiment the flight
track was flown 8 times for one series of measurements
at intervals of 10 degrees. This was done on two
days, and in total three times so 24 recordings were
made.
P : received power (averaged)
P : transmitted power
G ^ : maximum antenna gain
Jg : Jg 2 (x,y)dx or R.Jg 2 (a,£)da
g : antenna gain function
a,£ : azimuth & elevation angle (antenna coordi
nates)
Not explicitely written out in this formula is the
fact that the resulting 0 O or the radar cross section
per unit area depends on the sample number to which
it belongs. Variations of the transmitted power P-t
can be monitored with the aid of the system's inter
nal calibration. The antennagain G in the direction
of maximum radiation has been measured. For the
DUTSCAT antenna (a 0.9 m parabolic dish) a value was
found of 17 dB. As another part of the external cali
bration the antenna gain function has been measured
over 60 degrees. According to eq. (8) this function
has to be squared and integrated with respect to the
azimuth angle (along-track or x-direction). The re
sult (see fig. 3) is a function of the elevation
angle in antenna coordinates. For the example given
in fig. 2 the calculated radar cross section 0 Q is
presented in fig. 4 where of course the most accurate
values are expected to be around the main direction
of the antenna. The vertical line in the last figure
corresponds to an elevation angle in antenna coordi
nates of -30 degrees below which the gain function
has not been measured.
F* 1.2 GHz
Figure 3. Squared antenna gain function integrated
with respect to the azimuth angle (-30..+ 30 deg.).
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-2«.
-3«.
5«.
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