PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012
864
Shadowing Effect Analysis at Multiple Moving Persons Tracking by
UWB Radar
Daniel Urdzı́k1 , Rudolf Zetı́k2 , Dušan Kocur1 , and Jana Rovňáková1
1
Technical University of Košice, Košice, Slovak Republic
Ilmenau University of Technology, Ilmenau, Germany
2
Abstract— Ultra wideband (UWB) radars appear as the suitable technology for detection and
tracking of moving people in critical situations and hazardous environments. Our experiences
with such applications of UWB radar systems have shown that for the multiple moving person
detection and tracking, a single radar with a small antenna array is capable to detect very often
the person moving closest to the radar antennas only, whereas the other target (persons) are
usually not detected. This is a serious and unsolved problem of UWB radar applications for
detection and tracking of multiple moving persons. In this paper, we will outline the results of
the series of measurements with short range UWB radar which were dedicated for examination
and quantitative description of this negative effect.
1. INTRODUCTION
The short range UWB radar appears to be an attractive and perspective solution for the detection
and localization purposes. The UWB radar system is the special type of noise radar and can be
used to detect and track moving targets in critical environments with an advantage. One of the
possibilities of the UWB radar systems is their use for the various security, rescue and surveillance
applications (e.g., through wall and through fire detection and tracking of moving targets during
police or military operations, protection of facilities with high industrial or financial importance
etc.) Their primary advantage comes from the large bandwidth of the transmitted stimulation
signals of up to several GHz (typically between 100 MHz and 5 GHz). The large bandwidth results
in an excellent localization precision of such UWB devices and to the ability to penetrate many
common materials (e.g., walls, rubble, non-metallic obstacles). Then, such devices are able to
detect moving person by measuring changes in the impulse response of the environments [1]. For
detection and tracking applications outlined above, the short range UWB devices (range up to
20–25 m) are usually applied.
Our experiences with such applications of UWB radar systems have shown that at multiple
moving person scenarios, that a single radar with a small antenna array is able to detect very
often the person moving closest to the radar antennas only. The other target (persons) are also
detected but usually with a small reliability or none at all. This degradation of radar performance
can be explained in such a way that the person located in front of the radar antenna array acts as
an obstacle and creates an area with high attenuation of the energy of the electromagnetic waves
behind his/her (so-called shadowing person). The area with the additional attenuation is referred
to as the shadow zone. The shadowing person absorbs and reflects the energy of electromagnetic
waves transmitted by the radar transmitting antenna and/or reflected by the other targets and
hence, only a negligible part of the energy of the electromagnetic waves reflected by the other
targets can be received by the radar. The presence of such shadowing persons and hence the
shadow zones cause that the radar eventually cannot detect and track any of the persons which are
located within these zones. This effect is referred to as shadowing effect. The outlined problem of
due to shadowing effect has been addressed for the first time in [2], where a qualitative analysis
of shadowing effect has been described. The main contribution of this paper is the quantitative
analyses of attenuation within the shadow zone due to the person localized in front of the radar
transmitting antenna.
2. ANALYSIS OF SHADOWING EFFECT
In order to analyze the additional attenuation caused by the shadowing person the series of measurements had to be performed. The measurements took place in an empty large room (corridor) with
dimensions of 5.2 m by 2.6 m. The scenarios were chosen in such a way to examine the additional
attenuation in the shadow zone and its spatial shape. The measurements were performed with the
UWB pseudo-noise radar using maximum-length-binary-sequence (M-sequence) as the stimulus signal. The system clock frequency of the experimental M-sequence UWB radar was about 7 GHz,
Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 27–30, 2012 865
which resulted in the operational base band of about DC-3.5 GHz. The M-sequence order emitted
by radar is 12, i.e., the impulse response covers 4095 samples regularly spread over 1170 ns. This
corresponds to an observation window of 585 ns leading to an unambiguous range of about 47 m.
The radar system was equipped with one transmitting (Tx) and one receiving (Rx) omnidirectional
antenna. The radar data acquired by such radar device can be interpreted as a set of impulse responses of the environment [h(t, τ )], through which the emitted signals were propagated. They are
aligned to each other creating 2D picture called radargram, where the vertical axis is related to
the time propagationt of the impulse response and the horizontal axis is related to the observation
time τ .
The measurement of the spatial shape of the shadow zone and the values of attenuation within
was performed according to the scheme of the measurement shown in Fig. 1.
The transmitting antenna (Tx) was placed in the fixed position. The radar receiving antenna
(Rx) was situated subsequently in the different positions P (y, x) within the investigated area. At
each position, the impulse responses was measured by the antenna without the shadowing person. Then the shadowing person was situated in front of the radar transmitting antenna (P ) to
analyze the influence of his/her presence. In this position the person was performing a uniform
rotating movement along his/her body axis and hence the amount of electromagnetic waves reflected/absorbed by the person. The recorded radargram for each analyzed position of P (y, x)
contained a set of impulse responses with and without the information about the presence of the
shadowing person. In this manner there was total 55 positions of the investigated area analyzed.
By computing the corresponding attenuation for each of the positions of P (y, x), we were able to
derive the shape of the shadow zone.
In order to compute the attenuation for one position of P (y, x) the power level of electromagnetic
waves had to be calculated from all of the impulse responses of the radargram. Because the impulse
responses acquired by the radar device contain only the information about the magnitude of the
electromagnetic waves the power level was computed as follows:
Z t2
[PL (τ )] =
(h(t, τ ))2 dt.
(1)
t1
The symbol h(t, τ ) denotes the impulse responses of the radargram. The value of [PL (τ )] is one
dimensional vector of calculated power level of electromagnetic waves for the different time instants
of measurement. The power level calculated from all the impulse responses of one radargram is
shown in the Fig. 2.
The result obtained by the calculation of the power level contains two parts which corresponds to
the measured time intervals. The first part enclosed by the green rectangle [PLDW (τ )]ττ21 , represents
the values of calculated power level when there was no shadowing person present. The values in
the blue rectangle denoted as [PLAW (τ )]ττ43 , represent the calculated power level of electromagnetic
waves, while the person was present in front of the transmitting antenna and performing a uniform
1.4
1.2
Power [mW]
1
0.8
0.6
0.4
0.2
0
0
50
100
150
200
250
300
350
400
Time of observation [s]
Figure 1: Scheme of the measurement of the
shadowing effect.
Figure 2: Power level of the direct and attenuated wave.
PIERS Proceedings, Kuala Lumpur, MALAYSIA, March 27–30, 2012
866
rotating movement along his/her body axis. Because of the values of the non-shadowed electromagnetic waves appear as nearly constant in measured time interval hτ1 , τ2 i, one representative value
can be estimated from this interval:
PLDW R = mean([PLDW (τ )]ττ21 ).
(2)
The level of additional attenuation is then evaluated as the logarithmic ratio of the mean power
level of the direct wave and the values of power level of attenuated wave computed for the time
instant τ :
µ
¶
PLDW R
PdB (τ ) = 10 log
.
(3)
[PLAW (τ )]ττ43
From the values of PdB (τ ) which represent the additive attenuation at different time measured
instants τ , four quantities were estimated as the representative values of attenuation. These values
are estimated by these statistical functions:
PdBM AX = max(PdB (τ )),
PdBM IN = min(PdB (τ )),
PdBM EAN = mean(PdB (τ )),
(4)
(5)
(6)
and
PdBM EDIAN = median(PdB (τ )).
(7)
Hence we get four different representative values of additional attenuation for one position of the
antenna in the investigated area. By computing the additional attenuation for all of the 55 measured
positions in the shadow zone we were able to derive its exact shape and the levels of attenuation
within.
(a)
(b)
(c)
(d)
Figure 3: Experimental results of the measurement of the additional attenuation in the shadow zone. (a)
PdBM AX . (b) PdBM IN . (c) PdBM EAN . (d) PdBM EDIAN .
Progress In Electromagnetics Research Symposium Proceedings, KL, MALAYSIA, March 27–30, 2012 867
3. EXPERIMENTAL RESULTS
The experimental results show the values of additional attenuation measured at the height of
antenna 1.15 m in the operational base band of UWB radar device of about DC-3.5 GHz. The
results are displayed in the Cartesian coordinates to display the attenuation values at different
positions in the shadow zone behind the shadowing person. The results show the four cases when
the values of the additional attenuation in the investigated area was evaluated as the representation
of four different statistical estimations PdBM AX , PdBM IN , PdBM EAN and PdBM EDIAN .
From the presented results it can be seen that the person situated in front of the radar transmitting antenna causes significant levels of attenuations behind him/her. The additional attenuation
within the shadow zone due to shadowing person can take on values from 1 to +12 dB. By closer
inspection of the results it can be also seen that the significant levels of attenuation are focused
close behind the shadowing person and are proportionally downsizing with the distance from the
person.
4. CONSCLUSIONS
The experimental results obtained by the measurements by UWB radar have confirmed presence
of shadowing effect that is caused by a person located in front to the radar antenna apparatus. It
was shown that a person situated in front of any of the antennas creates a zone with an additional
attenuation within.
The obtained results show that the shape of shadow zones is approximately in the shape of the
trapezoid. In the most of analyzed cases it has been found that there is the correlation between
the width if the shadow zone and the distance between the antenna and the shadowing person.
However the shadow zone properties are needed to be investigated further from different aspects.
With regard to these facts, shadowing effect has to be taken into account in the case of the
design of radar systems for multiple moving person detection and tracking. Such solution could lie
in the utilization of two or more UWB radar sensor units for multiple target detection and tracking
or development of advanced signal processing methods for these purposes.
ACKNOWLEDGMENT
We support research activities in Slovakia/Project is co-financed from
EU funds. This paper was developed within the Project “Centrum
excelentnosti integrovaného výskumu a využitia progresı́vnych materiálov a technológiı́ v oblasti automobilovej elektroniky”, ITMS
26220120055.
REFERENCES
1. Withington, P., H. Fluhler, and S. Nag, “Enhancing homeland security with advanced UWB
sensors,” Microwave Magazine, IEEE, Vol. 4, No. 3, 51–58, Sep. 2003.
2. Kocur, D., J. Rovnáková, and D. Urdzı́k, “Mutual shadowing effect of people tracked by
the short-range UWB radar,” The 34th International Conference on Telecommunications and
Signal Processing (TSP 2011), 302–306, Budapest, Hungary, Aug. 2011.