First Results of the Signal Processing of INDERA
W. Sediono
1,2
,A. A. Lestari
1,3
1
Radar Communication Systems, Jl. Prof. Dr. Satrio KAV 6, Jakarta 12940 – INDONESIA
2
Agency for the Assessment and Application of Technology, Jl. M. H. Thamrin 8, Jakarta 10340 – INDONESIA
3
International Research Centre for Telecommunication and Radar - Indonesian Branch IRCTR-IB , STEI-ITB IRCTR-TU Delft,
Jl. Ganesha 10, Bandung 40132 – INDONESIA
ABSTRACT
INDERA is the first Indonesias maritime radar designed to be installed and operated on a ship. This radar is based on the FMCW principle for which a reliable method for processing the radar signal plays an important
role. In this paper the method used for processing radar signals received by the Rx-antenna is discussed. Thereafter first results from the field test of INDERA will be presented at the end of the method description.
Keywords
: INDERA, maritime radar, FMCW, signal processing.
1. INTRODUCTION
In the frequency-modulated continuous wave FM- CW radar, the transmitter frequency is changed as a
function of time in a known manner, e.g. sawtooth shape modulation fig. 1.
Fig. 1: Sawtooth shaped FMCW radar signal. The frequency of the transmitted signal is linearly increased about
ΔF within the sweep time t
s
.
After reflecting the radar signal a target at a distance R from the origin of radar will generate a beat signal
with the frequency [1,2]
R c
F f
m R
2 =
, 1
where F
m
is defined as
s m
t F
F Δ
=
. 2
By applying the Fourier transform we will obtain the spectrum of this beat signal. The range or distance of
the target can be determined by measuring the peak distance from the axis origin.
2. METHOD
The stepwise signal processing of an ideal beat signal received by the Rx-antenna is demonstrated in the
following computer simulation fig. 2.
1 2
3 4
x 10
-4
-1 -0.5
0.5 1
time [s] y
t 0.1
0.2 0.3
0.4 0.5
0.2 0.4
0.6 0.8
1
f f s
| Y f
|
0.2 0.4
0.6 0.8
1 0.2
0.4 0.6
0.8 1
R R max
| Y th
re s
h o
ld |
100 200
300 400
500 0.2
0.4 0.6
0.8 1
Range cell | Y
av g
|
Fig. 2: Computer simulation step by step: ideal beat signal top left, range-FFT top right, thresholding bottom left
and smoothing bottom right. The distance of the signal peak from the origin determines the range R of target [3,4].
In consideration of the fact that the incoming raw beat signal from Rx-antenna is not ideal this raw data need
to be preprocessed first. Starting from that point we can apply the Fourier transform FFT to the signal
resulting from the mentioned pre-conditioning stage. Then, in the last step we perform a post-FFT
processing before the signal can be finally sent to the radar monitor or PPI display. So, the basic principle of
the processing of an FMCW radar signal can be shown in the following block diagram fig. 3.
27
Fig. 3: Basic principle of the signal processing of an FMCW radar.
As shown in the above figure the analog beat signal coming from the Rx-antenna must be converted first
into digital signal using an ADC. A sample of such signal is shown in figure 4. We can see that the
received raw data is far from an ideal beat signal.
Fig. 4: Beat signal coming out from the ADC. The raw data is sampled at the rate of 2.5 MHz.
Consequently we must preprocess the data before conducting the Fourier transform. A first step we need
for that purpose is equalizing the data to suppress the wandering curve baseline. Furthermore we must
suppress the ‘burst’-like peaks within the raw data since this part does not contribute to the end result
fig. 5. In some cases, after equalizing the raw data, additional steps like smoothing and enveloping are
necessary to be completed.
Fig. 5: Result of the preprocessing of the beat signal.
After the equalization stage we can perform the FFT to achieve the range data fig. 6. The next problem to
solve is how we can differentiate the real object from the clutter enclosed within the signal.
28
Fig. 6: Result of FFT. The peak indicates that a target is present in that distance.
A simple way to detect peaks reflected from real targets is by applying a uniform threshold to the
signal. But this technique does not work effectively if the clutter has a similar magnitude with the target. So
we need a smart algorithm to detect peaks from the FFT result. A good reference to this problem can be
found in [5]. An overview of all conducted steps in the processing of the radar signal of INDERA is shown in
figure 7.
Fig. 7: Steps of the signal processing of INDERA.
The result of the applied peak detection algorithm is shown in fig. 8 where the signal amplitude is given in
logarithmic dB scale.
Fig. 8: Result of the post-FFT processing that shows a target detected in the cell range pixel number 85.
As shown in the figure a target was detected in the cell range pixel number 85. According to the current
setting of the radar range scale this number is equivalent to a distance of 0.66 NM from the origin of
the radar.
3. RESULTS
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