Through-Wall Imaging With UWB Radar System - KEMT FEI TUKE

2.4 Calibration and Preprocessing 10
Z
SXZ ( , )
Motion
X
TX RX
TX RX
tr tr [ x,z re re]
[ x,z]
E 1
T= [ x T,zT] E 2
n
BXn ( , )
Hyperbola
a) b)
Fig. 2.3.1: a) 2D SAR spatial model. b) B-scan of one point.
The basic 2D SAR spatial model is shown in Fig. 2.3.1 a). A transmitted wave
is reflected from the target to all directions uniformly. Because the antenna beam
is wide, the signal reflected from the target is received not only when the antenna
system is exactly over the target, but in a lot of positions that allow to ”see” the
target. This will cause that one point in S(X, Z) space will be represented in an
acquired B-scan B(X, n) (set of A-scans assembled together in a 2D structure) as
a hyperbola, as it is show in Fig. 2.3.1 b). The X is scanning direction and Z is
looking direction.
2.4 Calibration and Preprocessing
There are several types of calibrations and preprocessing methods that help us
to remove some unwanted artifacts from measured data. The most important
are: oversampling in time domain, time zero estimation, crosstalk removing and
deconvolution. Such signal processing methods affect the imaging results on a
large scale. For example crosstalk does not contain any information about the
scanned object and should be removed. Time zero estimation is very important
for correct focusing measured hyperbolas back to the one point, which will be
described in Section 2.5. An image of the investigated object would be blurred, if
time zero estimation is incorrect even in a few chips. All of these calibration and
preprocessing methods are applied individually to the A-scans.
2.4.1 Time Domain Interpolation
The measured B-scan B(X, n) (Fig. 2.4.1 a)) is oversampled by the cubic spline
interpolation [83] with an oversampling factor KO in the discrete time domain, so
X