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Detection of targets behind walls using ultra wideband short pulse: Numerical simulation

Walid A. Chamma and Satish Kashyap

Defence R&D Canada √ Ottawa TECHNICAL MEMORANDUM

DRDC Ottawa TM 2003-226 November 2003

Detection of targets behind walls using ultra wideband short pulse: Numerical simulation

Walid A. Chamma and Satish Kashyap

Defence R&D Canada − Ottawa TECHNICAL MEMORANDUM DRDC Ottawa TM 2003-226 November 2003

© Her Majesty the Queen as represented by the Minister of National Defence, 2003

© Sa majesté la reine, représentée par le ministre de la Défense nationale, 2003

DRDC Ottawa TM 2003-226 i

Abstract This work describes the use of a full-wave time-domain numerical procedure, based on the finite-difference time-domain (FDTD) method, to investigate the capabilities and limitations in the use of an ultra wideband (UWB) radar system to detect targets behind walls. A room with a concrete floor and wooden walls is modelled inside the FDTD lattice. The modelling assumes a transmitting antenna located outside the room and radiating a UWB pulse with a centre frequency of 2 GHz. A conducting cubic target is placed inside the room and the scattered field due to the cube is calculated using FDTD and is recorded at several receiver points outside the room. The back projection procedure is used to construct an image of the target. The above investigation is then repeated with the single target placed at different locations inside the room. Using the field information at various points outside the room, the movement of the target inside the room is traced and displayed.

The effect of various parameters on the performance of the detection system is studied. These include the number of receiver and transmitter points and their separation distances both in a bistatic and monostatic radar setups. In the bistatic radar case it was observed that larger array aperture lengths produced a better-focused image while larger receiver separation distances produced higher sidelobes. However in the monostatic radar case, a better image is generated for smaller numbers of transmitter and receiver points.

Additional simulation for two conducting cubes with different sizes inside the room is done to investigate the capability to locate and distinguish their corresponding images using the received scattered field information. By gating the received responses an enhanced image of the farther target is obtained.

Résumé Le présent document décrit l’utilisation de la procédure numérique applicable au domaine temporel et aux signaux à double alternance, fondée sur la méthode des différences finies dans le domaine temporel (FDTD), dans l’étude des capacités et des limitations d’un système de radar à bande ultra-large (UWB) pour la détection de cibles derrière les murs. Une chambre avec un plancher en béton et des murs en bois a été fabriquée à l’intérieur d’un réseau en treillis FDTD. La modélisation comprend une antenne d’émission à l’extérieur de la chambre, qui émet une impulsion UWB dont la fréquence centrale est de 2 GHz. Une cible cubique conductrice est placée à l’intérieur de la chambre et le champ dispersé par le cube est calculé à l’aide de la méthode FDTD et est enregistré par plusieurs récepteurs placés à différents endroits à l’extérieur de la chambre. La rétroprojection est utilisée pour la reconstitution de l’image de la cible. L’étude ci-dessus est ensuite répétée avec une seule cible placée à différents endroits dans la chambre. À l’aide des données de champ provenant de différents points à l’extérieur de la chambre, le mouvement de la cible à l’intérieur de la chambre est retracé et affiché.

Les effets de différents paramètres sur le rendement du système de détection sont à l’étude. Cette étude comprend le nombre de points de réception et d’émission et leurs espacements

ii DRDC Ottawa TM 2003-226

dans les configurations radar bistatique et monostatique. Dans le cas des radars bistatiques, on a observé que les plus grandes ouvertures d’antenne génèrent une image mieux focalisée alors qu’un plus grand espacement entre les récepteurs génère des lobes secondaires supérieurs. Toutefois, dans le cas des radars monostatiques, l’image générée est meilleure lorsqu’il y a moins d’émetteurs et de récepteurs.

Une simulation supplémentaire a été menée avec deux cubes conducteurs de différentes grosseurs dans la chambre afin d’étudier la capacité à localiser et à distinguer les images correspondantes à l’aide des données de champ dispersé reçues. On obtient une image améliorée de la cible la plus éloignée en sélectionnant les réponses reçues.

DRDC Ottawa TM 2003-226 iii

Executive summary Various Canadian agencies involved in law enforcement, fire fighting, and counter terrorism are interested in being able to detect the presence of a person behind a wall. The ability to obtain information about the activities of a person/persons (movement, gun, hostages, …) would make the job of these agencies much easier, especially in dangerous situations. Most building materials such as wallboard and concrete are fairly transparent to radar frequencies making through-the-wall surveillance possible. Many researchers have proposed this application and DRDC Ottawa is currently involved in the investigation of this technology for DND purposes through computer modelling and field experiments.

This work describes the use of a full-wave time-domain numerical procedure, based on the finite-difference time-domain (FDTD) method, to investigate the capabilities and limitations in the use of a UWB radar system to detect targets behind walls. A room with a concrete floor and wooden walls is modelled with a transmitting antenna located outside the room and radiating towards the room. A representative conducting cubic target is placed inside the room and its scattered field is calculated and recorded at several receiver points outside the room. The back projection procedure is used to construct an image of the target. Using the field information at various points outside the room, the movement of an object inside the room is traced and displayed.

The effect of various parameters on the performance of the detection system is studied. In the bistatic radar case it was observed that larger array aperture length produced a better-focused image while, larger receiver separation distances produced higher side lobes and a less-focused image. However in the monostatic radar case, a better image is generated for small numbers of transmitter and receiver points.

Further work, which will include a full-scale human model carrying a gun, is in progress and is expected to be supported by measurements.

Chamma, W. A. and Kashyap, S., 2003. Detection of targets behind walls using ultrawideband short pulse: Numerical simulation. DRDC Ottawa TM 2003-226, Defence R&D Canada – Ottawa

iv DRDC Ottawa TM 2003-226

Sommaire Différentes agences canadiennes du milieu des forces de l’ordre, de la lutte contre les incendies et de la lutte anti-terroriste trouvent avantages à pouvoir détecter la présence de personnes derrière les murs. La capacité d’obtenir des renseignements sur les activités d’une ou de plusieurs personnes (mouvements, armes, otages, etc.) faciliterait grandement la tâche de ces agences, particulièrement dans les situations dangereuses. Les matériaux de la plupart des édifices tels que les panneaux de revêtement et le béton sont pratiquement transparents aux fréquences radar, ce qui rend possible la surveillance à travers les murs. De nombreux chercheurs ont proposé cette application et RDDC Ottawa participe actuellement à l’étude de la nouvelle technologie pour le MDN en effectuant des modélisations informatisées et des expériences sur le terrain.

Le présent document décrit l’utilisation de la procédure numérique applicable au domaine temporel et aux signaux à double alternance, fondée sur la méthode des différences finies dans le domaine temporel (FDTD), dans l’étude des capacités et des limitations d’un système radar UWB pour la détection de cibles derrière les murs. Une chambre avec un plancher en béton et des murs en bois a été fabriquée. Une une antenne d’émission à l’extérieur émet dans sa direction. Une cible représentative cubique et conductrice est placée à l’intérieur de la chambre et son champ dispersé est calculé et enregistré par plusieurs récepteurs à différents endroits à l’extérieur de la chambre. La rétroprojection est utilisée pour la reconstitution de l’image de la cible. À l’aide des données de champ provenant de différents points à l’extérieur de la chambre, le mouvement de la cible à l’intérieur de la chambre est retracé et affiché.

Les effets de différents paramètres sur le rendement du système de détection sont à l’étude. Dans le cas des radars bistatiques, on a observé que les plus grandes ouvertures d’antenne génèrent une image mieux focalisée alors qu’un plus grand espacement entre les récepteurs génère des lobes secondaires supérieurs et une image moins focalisée. Toutefois, dans le cas des radars monostatiques, l’image générée est meilleure lorsqu’il y a moins d’émetteurs et de récepteurs.

D’autres travaux, simulant à pleine grandeur un humain portant une arme, sont en cours et devraient confirmer les calculs.

Chamma, W. A. and Kashyap, S., 2003. Detection of targets behind walls using ultrawideband short pulse: Numerical simulation. DRDC Ottawa TM 2003-226, R & D pour ladéfense Canada – Ottawa

DRDC Ottawa TM 2003-226 v

Table of contents

Abstract ...................................................................................................................................... i

Résumé ...................................................................................................................................... i

Executive summary ................................................................................................................... iii

Sommaire................................................................................................................................... iv

Table of contents ........................................................................................................................ v

List of figures ............................................................................................................................ vi

1. Introduction ........................................................................................................................... 1

2. FDTD modelling.................................................................................................................... 2 2.1 Room geometry .......................................................................................................... 2 2.2 UWB excitation pulse ................................................................................................ 4

3. Simulation results and discussion.......................................................................................... 7 3.1 Bistatic UWB radar system ...................................................................................... 10 3.2 Monostatic UWB radar system ................................................................................ 17 3.3 Multiple target detection .......................................................................................... 20

4. Conclusions and future work ............................................................................................... 23

5. References ........................................................................................................................... 24

vi DRDC Ottawa TM 2003-226

List of figures

Figure 1. Room geometry and the bistatic UWB radar setup.................................................... 3

Figure 2. Antenna excitation and radiated UWB pulse. ............................................................ 4

Figure 3. Radiated UWB pulse at location a from the antenna ................................................. 4

Figure 4. Antenna excitation and radiated UWB pulse frequency response. ............................ 5

Figure 5. Frequency-domain transfer function of the excitation antenna (9.0 cm dipole). ....... 6

Figure 6. Room geometry with UWB radar showing a 10 cm conducting boxes at locations 1 and 2. ....................................................................................................................... 7

Figure 7. Recorded responses at receiver Rx41 for empty room and conducting box inside the room cases. ................................................................................................................. 8

Figure 8. Isolated response of the 10cm conducting box at locations 1 and 2 recorded at receiver Rx41 obtained without de-convolution of the antenna impulse response. ..... 9

Figure 9. Processed Isolated response of the 10cm conducting box at locations 1 and 2 recorded at receiver Rx41 obtained with de-convolution of the antenna impulse response...................................................................................................................... 9

Figure 10. Room geometry with UWB radar showing a 10 cm conducting box travel path inside the room from location 1 to 5. ....................................................................... 11

Figure 11. Generated image of the 10cm conducting box at locations 1-5 inside the room detected by ............................................................................................................... 12

Figure 12. Isolated impulse snap shots at receiver Rx41 of the conducting box inside the room moving from location 1 to 5. .................................................................................... 13

Figure 13. Generated image of a 10 cm box at location 1 inside the room detected by the bistatic radar setup for different values of n and d. (D = 240 cm). .......................... 15

Figure 14. Generated image of a 10 cm box at location 1 inside the room detected by the bistatic radar setup for different values of n and D. (d = 4 cm). .............................. 16

Figure 15. Room geometry with a monostatic UWB radar setup............................................ 17

Figure 16. Generated image of a 10 cm box at location 1 inside the room detected by a monostatic UWB radar setup for different values of n and D. (d = 10 cm) ............. 18

DRDC Ottawa TM 2003-226 vii

Figure 17. Generated image with filtered out sidelobes of a 10 cm box at location 1 inside the room detected by a monostatic UWB radar setup for different values of n = 9 and D = 80 cm. (d = 10 cm). ...................................................................................... 19

Figure 18. Room geometry with a monostatic UWB radar setup, with two conducting boxes of sizes 10 cm and 30 cm........................................................................................ 20

Figure 19. Generated images of a 10 cm box and a 30 cm box inside the room detected by a monostatic UWB radar setup (n = 9, d = 10 cm, D = 80 cm)................................. 21

Figure 20. Isolated impulse response recorded at Rx (0.0, 1.0 cm, 100 cm) of a monostatic UWB radar setup for the case of one and two boxes inside the room. ................... 21

Figure 21. Generated images of a 10 cm box and a 30 cm box inside the room obtained by the original and the gated impulse responses. .............................................................. 22

viii DRDC Ottawa TM 2003-226

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DRDC Ottawa TM 2003-226 1

1. Introduction Impulse radar can be used to detect the presence and movement of targets behind walls. To be an effective detection system, the radar should have the transmitted signal at a frequency low enough to be able to penetrate walls and have a very wide bandwidth so that targets behind walls are clearly identified. Bandwidths need to be several gigahertz to achieve high resolution of the order of a fraction of a meter. Ultra wideband (UWB) radar systems satisfy these low frequency and large bandwidth requirements; they are defined as those for which the relative bandwidth is equal to or greater than 25%. The UWB transmitted pulse usually consists of a very short pulse train of just a few cycles. The electromagnetic (EM) scattered fields from a target, illuminated by the UWB radar, are received at several locations and then processed to construct its corresponding image.

UWB radar systems have been used for a wide variety of civilian and military applications. Skolnik et al. [1] outlined considerations that characterize the design of UWB radar for the detection of low-altitude missiles over the sea. They discussed the factors that enter into the choice of frequency, the selection of the type of the transmitter, antenna, and the receiver as well as signal processing issues. UWB for minefield detection (ground penetrating radars) was also investigated by Carin et al. [2], where a full-wave model for EM scattering from buried targets is developed. Such systems were also studied thoroughly by Walton and his group at the Ohio State University Electro Science Laboratory [3]. In another unique application, a UWB radar system is used to detect and predict images of moving targets behind walls or non-metallic visually opaque boundaries. Nag et al. [4,5] introduced a hand-held wall penetrating radar unit that transmitted time-modulated UWB signals through barriers. This system is very useful in forced entry and hostage situations, as well as emergencies where there are victims under debris. Such applications are being investigated through simulation and experimentation at DRDC Ottawa [6-9].

This work describes the use of a full-wave time-domain numerical procedure, based on the finite-difference time-domain [10] (FDTD) method, to investigate the capabilities and limitations in the use of a UWB radar system to detect moving targets behind walls. A room with a concrete floor and wooden walls is modelled inside the FDTD lattice. The modelling assumes a transmitting antenna located outside the room at a 1.0 m height and transmits a UWB pulse with a centre frequency of 2 GHz. A conducting cubic target is placed at a 1.0 m height inside the room. The scattered field due to the box is calculated using FDTD and is recorded at several receiver points outside the room. The range location of the target can be determined by measuring the propagation delay time from the transmitting antenna to the target and back to the receiver point after accounting for the presence of the wall. Using the received field data at several observation points, an image of the target can be constructed. The above procedure is then repeated with the single target placed at different locations inside the room. Using the field information at various points outside the room, the movement of an object inside the room is traced and displayed.

This work also describes the effect of various parameters on the performance of the detection system. These include the number of receiver and transmitter points and their separation distances. Additional simulation is then conducted for two conducting cubes of different sizes inside the room to investigate the capability to locate and distinguish their corresponding images using the received scattered field information.

2 DRDC Ottawa TM 2003-226

2. FDTD modelling The finite-difference time-domain (FDTD) numerical method is a direct solution of the time-dependent Maxwell’s curl equations using the finite-difference technique. It is analogous to the finite-difference solution of fluid flow problems encountered in computational aerodynamics, where a numerical model is based on a direct solution of the corresponding partial differential equations. In FDTD the propagation of an EM wave into a volume of space containing a dielectric or a conducting structure (or a combination of both) is modelled. By time stepping, the incident wave is tracked as it first propagates to the structure and then interacts with the latter through current excitation, scattering, multiple scattering, penetration and diffraction.

2.1 Room geometry

A 2.36 m x 3.59 m x 2.62 m room with a concrete floor and wooden walls is modelled inside the FDTD lattice and is shown in Fig. 1. The wooden walls and the concrete floor are represented inside the FDTD lattice by their physical parameters, namely, a relative permittivity εr = 2.75 and conductivity σ = 3.0x10-3 S/m for the wooden wall and εr = 7.0 and conductivity σ = 5.0x10-2 S/m for the concrete floor. These values are chosen for an operating frequency f = 2 GHz. In this simulation the antenna source is an x-directed 9 cm centre-fed dipole located outside the room at an elevation z = 1.0 m as illustrated in Fig. 1a. A z-directed antenna can also be used in this investigation. The effect of the antenna polarisation in target detection will be investigated in future work. The target inside the room consists of a 10 cm conducting cubic box located inside the room at xb = -30 cm, yb= 94 cm, and zb = 100 cm, where (xb, yb, zb) are the coordinates of the conducting box centre as shown in Fig. 1b. Fig. 1 also shows the FDTD spatial and temporal resolution parameters used in the modelling and simulation of the target EM illumination.


Figure 1. Room geometry and the bistatic UWB radar setup.

Free space gap ( 8.0 cm)

Wood boards, thickness =1.0 cm εr = 2.75, σ = 3.0x10-3 S/m

100 cm 9.0cm Dipole Antenna @ 5.0 cm from wall

Concrete @ f = 2 GHz εr = 7.0, σ = 5.0x10-2 S/m

FDTD Simulation Parameters Lattice size Nx = 296, Ny = 445, Nz = 320 Spatial resolution dx = dy = dz = 1.0cm Time resolution dt = 19.25 x 10-12 seconds

z

y x

262 cm

n = 1 2 3 4

121 120 119

118

Rx1 (-120cm, 1cm, 100cm)

Antenna: 9 cm x-polarized dipole. Exc: 0.6 ns Gaussian Pulse (2 GHz mod) Location: Tx (0.0, 0.0, 100 cm)

Rx121 (120 cm, 1 cm, 100 cm)

x

y z

z = 100.0 cm

359 cm

236 cm

30 cm

10cm conducting cubic box at Location (1) : (xb= -30 cm, yb= 94 cm, zb= 100 cm)

d

D

(b) top view

(a) side view


2.2 UWB excitation pulse

To model the EM illumination of the conducting box target by a UWB short pulse, the dipole antenna is fed by a 0.6 ns Gaussian pulse modulated by a 2 GHz sine wave, and is shown in Fig. 2. This UWB short pulse has a bandwidth that is around 1 GHz. Since the dipole antenna used is not designed to have a very wide bandwidth, the resulting radiation pulse will be distorted by (convolved with) the antenna’s own impulse response. Fig. 3, showing the radiated fields recorded at different distances along the axis of the dipole, demonstrates how the radiated field is distorted and attenuated as it travels away from the dipole.

Delay time (ns)

Ex(V

/m)

0 0.5 1 1.5 2 2.5 3 3.5 4-1.25

-1

-0.75

-0.5

-0.25

0

0.25

0.5

0.75

1

UWB Excitation PulseUWB Radiated Pulse

Figure 2. Antenna excitation and radiated UWB pulse.

Delay time (ns)

Ex(V

/m)

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5-3

-2

-1

0

1

2

a = 65 cma = 55 cma = 45 cma = 35 cma = 25 cma = 15 cma = 5 cm

Radiated Field at distance "a" from dipole

Figure 3. Radiated UWB pulse at location a from the antenna


Fig. 4 shows the frequency response of the UWB excitation pulse and the radiated pulse. For the case of a plane wave with the UWB excitation pulse of Fig. 2 as temporal distribution, the pulse will propagate in free space without distortion or attenuation. Hence, the frequency-domain transfer function, Ht(ω), of the dipole antenna used in our simulation can be obtained through:

)()()(

ωωω

UWB

radt H

HH = (1)

Here Hrad(ω) is the frequency response of the radiated field from the antenna at a specified observation location in the near field and HUWB(ω) is the frequency response of the radiated field due to the plane wave at the same observation location. Fig. 5 shows the frequency-domain transfer function of the antenna Ht(ω). The knowledge of the impulse response of the antenna is necessary information that is used to obtain the impulse response the target detected inside the modelled room. This is done by deconvolving the received response of the target at each receiver location with the antenna impulse response.

Frequency (GHz)

Ampl

itude

0 1 2 3 4 510-13

10-12

10-11

10-10

10-9

UWB Excitation Pulse Frequency Response [ HUWB (ω) ]UWB Radiated Pulse Frequency Response [ Hrad (ω) ]

Figure 4. Antenna excitation and radiated UWB pulse frequency response.


Frequency (GHz)

Ampl

itude

0 1 2 3 4 510-2

10-1

100

101

102

Antenna Transfer Function ( Ht (ω ) )

Figure 5. Frequency-domain transfer function of the excitation antenna (9.0 cm dipole).


3. Simulation results and discussion Our initial investigation considers a 10 cm conducting cubic box inside the room shown in Fig. 6. A setup of one x-directed transmitter antenna, represented by the 9 cm dipole centred at (0.0,0.0,100 cm), and 121 receivers are considered. The receiver points in the FDTD simulation are observation points where the EM field magnitudes are recorded and saved for post-processing. Using receiver points as opposed to receiving antenna elements will improve on the processing efficiency without sacrificing the accuracy of our simulation. If antenna elements were used as receivers, their corresponding impulse response must be included in obtaining the impulse response of the detected targets. The receiver points Rx1 to Rx121 are distributed along the side of the room in the x-direction with coordinates x1 = -120 cm, x2 = -118 cm, …, x121 = +120 cm (a resolution d = 2.0 cm between each of the receiver points, with y = 1.0 cm and z = 100 cm), and an aperture length D = 240 cm. The room wall begins at y = 5.0 cm, so the transmitter antenna at Tx = (0.0, 0.0, 100 cm) is 5.0 cm from the wall, and the receivers points are 4.0 cm from the wall.

Figure 6. Room geometry with UWB radar showing a 10 cm conducting boxes at locations 1 and 2.

n = 1234

121120119

118

Rx1 (-120cm, 1cm, 100cm)

Rx121 (120 cm, 1 cm, 100 cm)

z = 100.0 cmx

yz

D

359 cm

30cm

10 cm conducting cubic box at Location 1 : (xb= -30 cm, yb= 94 cm, zb= 100 cm)

d 236 cm


Tx (UWB dipole) (0.0, 0.0, 100.0 cm)

D = 240.0 cm d = 2.0 cm

Receivers Locations y = 1.0 cm z = 100.0 cm Rx1 : x = -120 cm, Rx2 : x = -118 cm, Rx3 : x = -116 cm, …., Rx121 : x = +120 cm


Fig. 7 shows the recorded response at receiver Rx41 (x41 = -40 cm, y41 =1.0 cm, z41 = 100 cm) for the cases of an empty room and the room with a conducting box at location 1 with coordinates (xb = -30 cm, yb = 94 cm, zb = 100 cm). Reflections from the far wall can be clearly identified by the delay distance value of 8 m. This distance corresponds to the round trip travel distance from the transmitter dipole to the far wall and then back to receiver Rx41. However, the EM field disturbance due to the box (delay distance = 2 m) is relatively small in magnitude. Hence, signal processing procedures must be used to isolate target responses.

Delay distance (m)

Ex(V

/m)x

10-0

3

0 1 2 3 4 5 6 7 8 9 10-50

-40

-30

-20

-10

0

10

20

30

40

50

Empty Room ResponseRoom with Box Response

Figure 7. Recorded responses at receiver Rx41 for: (a) empty room case, and (b) conducting box inside the room case.

Fig. 8 shows the isolated recorded response at Rx41 of the conducting box at locations 1 and 2 inside the room at (xb = -30 cm, yb = 94 cm, zb = 100 cm) and (xb = -30 cm, yb = 294 cm, zb = 100 cm) respectively. The isolated recorded response of the conducting box refers to the difference between the responses recorded at Rx41 with and without the presence of the conducting box. The responses shown in Fig. 8 are those without the deconvolution of the antenna impulse response. Fig. 9 shows the responses with the deconvolution of the antenna impulse response implemented, resulting in the impulse response of the conducting box at the locations considered. A spatial shift is observed in the impulse response of Fig. 9 as compared with that of Fig. 8. This will result in producing the image of the conducting box at the correct location on the room image plane. The scale difference in the Ex field magnitude of Fig. 8 and 9 is due to the deconvolution process and will not effect the image generation procedure.


Delay distance (m)

Ex

(V/m

)x10

-03

1 2 3 4 5 6 7 8 9 10-10

-8

-6

-4

-2

0

2

4

6

8

Isolated Response due to boxat location (2)

Reflections from the far wall of the room

Isolated Response due to box at location (1)

Figure 8. Isolated response of the 10 cm conducting box at locations 1 and 2 recorded at receiver Rx41 obtained without deconvolution of the antenna impulse response.

Delay distance (m)

Ex(V

/m)x

10-0

5

1 2 3 4 5 6 7 8 9 10-10

-8

-6

-4

-2

0

2

4

6

8

Isolated Impulse Responsedue to box at location (2)

Isolated Impulse Responsedue to box at location (1)

Figure 9. Processed Isolated response of the 10 cm conducting box at locations 1 and 2 recorded at receiver Rx41 obtained with deconvolution of the antenna impulse response.


3.1 Bistatic UWB radar system

The transmitter and the receiver points of the above setup constitute a bistatic radar system. The time-domain response data recorded at each receiver point are generated directly by the FDTD method with no need for any Fourier transformation. These received signals are processed using the time-domain back-projection technique [11] to produce the target image. The back-projected signal at pixel (xi, yj) in the room image plane due to the recorded field E(t, n) at receivers Rxn {n = 1 to 121} at location (xRxn, yRxn) and a single transmitter Tx at (xTx, yTx) is given by

∑=n

ijji nntEyxI )),((),( (2)

where

( )

vnRxTx

nt ijijij

)()(

+= (3)

The speed of light in the propagation media is v and tij(n) is the total delay recorded for the transmitted signal to travel from the transmitter location to pixel (xi, yj), Txij, plus the delay from the pixel (xi, yj) to receiver n, Rxij(n). Txij and Rxij(n) are obtained from the following expressions:

( ) ( ) 22TxjTxiij yyxxTx −+−= , (4)

( ) ( ) 22)( RxnjRxniij yyxxnRx −+−= (5)

Hence, the target function at a given pixel (xi, yj) in the image plane is formed from the data in the received responses at specific delay (time bins) corresponding to the distance of this pixel point to each of the receiver points. Interpolation is used at instances where the delay time calculated does not correspond to a data point in the recorded responses. Using Hilbert’s transformation [11], G, the power envelope image can be obtained by substituting E(t, n) in Eq. (2) with ),(ˆ ntE , where

)),((),(),(ˆ ntEjGntEntE += . (6)

A power envelope image can be found by taking the amplitude of the complex image ),(ˆ ntE

in Eq. (6), this will enhance the intensity level of the produced image.


Using the above procedure the images of the conducting box at locations 1-5 inside the room are generated. The coordinates of these locations are shown in Fig. 10. Fig. 11 shows the generated images of the conducting box inside the room at locations 1-5. The box image for each location is displayed on a single image plane for comparison purposes. In addition, for clarity, the sidelobes of the generated images are suppressed by setting the threshold level in the plotting program to display only the peak magnitudes. These images were generated using only 25 receiver points. These receivers are separated by distance d = 10 cm and have an array aperture length of D = 240 cm. For location 1, the box image is displayed more clearly due to stronger return signal to the receivers on the side of the room, as compared to that of the other locations 2-5. As the conducting box moves farther inside the room and farther away from the UWB radar setup, the generated image is more out of focus than that of location 1. This is attributed to the decreasing magnitude of the reflected signal, the dispersion of the propagating pulse, and the magnitude of the reflected signal from the box at levels comparable to reflections from the room structure. Fig. 12 demonstrates this reasoning. It shows the receiving isolated impulse response at Rx41 as the conducting box travels inside the room in steps of 20 cm.

Figure 10. Room geometry with UWB radar showing a 10 cm conducting box travel path inside the room from location 1 to 5.

n = 1

234

121

120119

118

Rx121 (120 cm, 1 cm, 100 cm)

xyz

D

D = 240.0 cm d = 2.0 cm

Rx1 (-120 cm, 1 cm, 100 cm)

359 cm

30 cm

Loc. 1(-30 cm, 94 cm, 100 cm)

d 236 cm

Tx (UWB dipole) (0.0, 0.0, 100.0 cm)

Loc. 3(-30 cm, 194 cm, 100 cm)

Loc. 2(-30 cm, 294 cm, 100 cm)

Loc. 4(+30 cm, 294 cm, 100 cm)

Loc. 5(+80 cm, 294 cm, 100 cm)


In the present radar setup, the parameters that influence the resolution of the detected targets are similar to those that influence a synthetic aperture radar (SAR) system. In SAR, the range resolution R∆ (y-axis in the room plane) is given by:

BcR

2=∆ (7)

where c is the speed of light in free space and B is the bandwidth of the transmitted signal. The azimuthal resolution (x-axis in the room plane), θ∆ for an array of transceivers is given by

D2oλθ =∆ (8)

where oλ is the wavelength of the centre frequency and D is the array aperture length. Although our current radar setup is bistatic, the expressions in Eq. (7) still apply, however, the azimuthal resolution value will be reduced by a factor of half [12] since the effective array length will be D′ = D /2 :

D′=′∆

2oλθ (9)

Figure 11. Generated image of the 10cm conducting box at locations 1-5 inside the room detected by a bistatic UWB setup (n = 25, d = 10 cm, D = 240 cm).

Down Range (cm)

Cro

ssR

ange

(cm

)

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Loc. (1)( -30cm, 94cm, 100cm)

Loc. (5)( +80cm, 294cm, 100cm)

Loc. (2)( -30cm, 294cm, 100cm)

Loc. (3)( -30cm, 194cm, 100cm)

Loc. (4)( +30cm, 294cm, 100cm)

NormalizedAmplitude

x

y z


Figure 12. Isolated impulse snap shots at receiver Rx41 of the conducting box inside the room moving from location 1 to 5.

Ex

(mV/

m)

0 1 2 3 4 5 6 7 8 9 10-0.1

-0.05

0

0.05

0.1

0 1 2 3 4 5 6 7 8 9 10

box moving along the +x-axis

box moving along the +y-axis

ct(meters) ct(meters)

box moving along the +y-axis

location (1)

location (3)

location (5)

location (4)

location (2)


Next we investigates the effect of changing the resolution distance d between the receiver points and the array aperture length D on the quality of the detected image of the conducting box at location 1 inside the room. In Fig. 13 the image of the conducting box is generated for different values of d but for a fixed value of D = 240 cm. The number of receivers n used for each value of d range from 121 to 16 receivers and are noted in Fig. 13. With aperture length D = 240 cm, the resolution of the generated images, representing the conducting box, are the same for all the cases studied. However, for larger resolution distance d corresponding to lower number of receivers, higher sidelobes are generated. Fig. 14, on the other hand, shows the effect of changing the array aperture length D for a fixed value of resolution distance d at 4.0 cm. Here, as expected and illustrated in Eq. (9), the image representing the conducting box becomes more unfocused for smaller values of D. In addition, ghost images of the box at lower intensity start to show alongside the main image with decreasing value of D. In both Figs. 13 and 14, the transmitter is located at the centre of the room at (0.0, 0.0, 100 cm).


Figure 13. Generated image of a 10 cm box at location 1 inside the room detected by the bistatic radar setup for different values of n and d (D = 240 cm).

1.000.980.960.940.920.900.870.850.830.810.790.770.750.730.710.690.67

NormalizedAmplitude

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n = 18 , d = 14cm

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n = 16 , d = 16cm

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n = 25 , d = 10cm

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n = 41 , d = 6cm

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n = 21 , d = 12cm

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(cm

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room frontwall

n = 121 , d = 2cm

box

x

yz


Figure 14. Generated image of a 10 cm box at location 1 inside the room detected by the bistatic radar setup for different values of n and D (d = 4 cm).

1.000.980.960.940.920.900.870.850.830.810.790.770.750.730.710.690.67

NormalizedAmplitude

Down range (cm)

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ange

(cm

)

0 10 20 30 40 50 60 70 80 90 100110120130140150160-70

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room frontwall

n = 15 , D = 56cm

box

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n = 11 , D = 40cm

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n = 13 , D = 48cm

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n = 9 , D = 32cm

x y


3.2 Monostatic UWB radar system

Next, we consider a monostatic UWB radar setup consisting of a maximum of nine transmitters/receivers (Tx/Rx). In this case Eqs. (3) and (4) become

v

nRxnTxnt ijij

ij

)()()(

+= (10)

and

( ) ( ) 22)( TxnjTxniij yyxxnTx −+−= (11)

As shown in Fig. 15, these are used to calculate the image intensity at pixel (xi, yj) in the expression of Eq. (2).

This setup has a resolution distance d = 10 cm and an array aperture length D = 80 cm centred at the room wall as before and at a height z = 100 cm. The transmitters consist of nine x-directed 9 cm dipole antennas excited by the UWB pulse, while the receivers consist of response recording observation points. The azimuthal resolution for this setup is calculated using Eq. (8).

Figure 15. Room geometry with a monostatic UWB radar setup.

n = 1Rx1 (-40 cm, 1 cm, 100 cm)

Rx9 (40 cm, 1 cm, 100 cm) z = 100.0 cm

xyz

D

359 cm

30 cm


236 cm

d Tx5 (UWB dipole) (0.0, 0.0, 100.0 cm)

2

3

4

5

6

7

8

9

D = 80.0 cm d = 10.0 cm

Receivers Locations y = 1.0 cm z = 100.0 cm Rx1 : x = -40 cm Rx2 : x = -30 cm Rx3 : x = -20 cm . Rx9 : x = +40 cm


Fig. 16 shows the generated image of the same 10cm conducting box at location 1 for different values of D and the number of Tx and Rx using the monostatic radar setup. Here the generated image is displayed at the correct location even with a small number of transmitters and receivers since the azimuthal resolution has doubled. For the case of D = 20 cm, and Tx = Rx = 3 elements, ghost images alongside the main image start to show as predicted by Eq. (8) (reduced azimuthal resolution). As expected, the sidelobe levels are relatively high when compared to those of Fig. 13 with d < 10 cm. By setting the intensity display threshold level in the plotting software, these sidelobes can be filtered out and a clear image representing the box inside the room is obtained, as illustrated in Fig. 17 for the case of n = 9.

1.000.980.960.940.920.900.870.850.830.810.790.770.750.730.710.690.67

NormalizedAmplitude

Down range (cm)

Cro

ssR

ange

(cm

)

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room frontwall

(Tx/Rx) n = 9 , D = 80cm

box

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90(Tx/Rx) n = 7 , D = 60cm

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90(Tx/Rx) n = 5 , D = 40cm

Figure 16. Generated image of a 10 cm box at location 1 inside the room detected by a monostatic UWB radar setup for different values of n and D (d = 10 cm).


Down range (cm)

Cro

ssR

ange

(cm

)

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1.000.980.960.940.920.900.870.850.830.810.790.770.750.730.710.690.67

room frontwall

(Tx/Rx) n = 9 , D = 80cm

box

NormalizedAmplitude

Figure 17. Generated image with filtered out sidelobes of a 10 cm box at location 1 inside the room

detected by a monostatic UWB radar setup for different values of n = 9 and D = 80 cm (d = 10 cm).


3.3 Multiple target detection

The above discussion involves only a single 10 cm conducting cubic box. In this section a 30 cm conducting cubic box is added at coordinates (65 cm, 297 cm, 100 cm) at the back of the room (see Fig. 18). Using the monostatic UWB radar setup of nine elements discussed earlier (Tx = Rx = 9, d = 10cm, D = 80 cm), the generated image of the detected targets is shown in Fig. 19. In this Figure, the 10 cm box can be identified very clearly, however, the 30 cm box image is displayed with much lower intensity. This is attributed to the magnitude of the radar signal reaching the 30 cm box being less than that reaching the 10 cm box at location 1. The scattered signal from the 30 cm box is further attenuated as it travels back to the receivers.

Figure 18. Room geometry with a monostatic UWB radar setup, with two conducting boxes of sizes

10 cm and 30 cm.

Fig. 20 shows the isolated signatures received at the centre element of the UWB radar with and without the 30 cm box. This figure shows clearly a lower magnitude response resulting from the 30 cm box than from the 10 cm box. By gating the received responses in the image processing part to neglect responses from targets at distances that are relatively closer to the radar system, and normalizing to a new image peak value, an enhanced image of the target at the far side of the room can be obtained. By superimposing the image obtained from the gated response with that of Fig. 19, a clearer display of the target inside the room is obtained. Fig. 21 shows the resulting display.

Rx1 (-40 cm, 1 cm, 100 cm)

Rx9 (40 cm, 1 cm, 100 cm) z = 100.0 cm

x y z

30 cm

10 cm conducting cubic box at Location (1) : (xb= -30 cm, yb= 94 cm, zb= 100 cm)

236 cm

d Tx5 (UWB dipole) (0.0, 0.0, 100.0 cm)

2

3

4

5

6

7

8

9 30 cm conducting cubic box at Location : (xb= 65 cm, yb= 297 cm, zb= 100 cm)


Figure 19. Generated images of a 10 cm box and a 30 cm box inside the room detected by a monostatic UWB radar setup (n = 9, d = 10 cm, D = 80 cm).

Figure 20. Isolated impulse response recorded at Rx (0.0, 1.0 cm, 100 cm) of a monostatic UWB radar setup for the case of one and two boxes inside the room.

Down Range (cm)

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ange

(cm

)

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30cm box( 65cm, 297cm, 100cm)

NormalizedAmplitude

10cm box( -30cm, 94cm, 100cm)

Delay distance (m)

Ex

(V/m

)x10

-5

1 2 3 4 5 6 7 8-6.0

-4.0

-2.0

0.0

2.0

4.0

6.0

Isolated Response for one box (10cm)Isolated Response for two boxes ( 10cm & 30cm)

Reflections due to the 30cm box


Figure 21. Generated images of a 10 cm box and a 30 cm box inside the room obtained by the original and the gated impulse responses.

Down Range (cm)

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ange

(cm

)

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0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00

NormalizedAmplitude

10cm box( -30cm, 94cm, 100cm)

30cm box( 65cm, 297cm, 100cm)


4. Conclusions and future work In this work, bistatic and monostatic UWB radar setups used for the detection of targets behind walls were investigated using the FDTD numerical method. For the case of a 2.36m x 3.59m x 2.62m room with wooden walls and a concrete floor, the radar setup consisted of one transmitter and 121 receivers with an array aperture length D = 240cm, and resolution d = 2cm. This bistatic radar was used to detect the location of a 10cm conducting cubic box inside the room. The effect of changing the number of receivers, array aperture length, and receivers resolution on the image quality of the detected structure was investigated. It is observed that larger array aperture length, D, produces a better-focused image. However, larger values of resolution distance, d, produces higher sidelobes.

For the monostatic radar setup case, a better image is generated for a small number of transmiter/receiver elements and small array aperture length D. This is attributed to the doubling of the azimuthal resolution. Also, a relatively better image is generated for the case of the monostatic radar with three elements, D = 20cm, and d = 10cm. For a room with two targets, it is observed that lower image intensity is generated for the target that is farther away from the radar setup. However, gating the received responses to ignore early time data and overlapping of the generated images (generated using original and gated responses) produces better images for the detected targets.

Further investigation to utilize UWB-SP radar system for through-wall detection purposes through numerical simulation is continuing. This work is being done in conjunction with lab experiments conducted by the Radar System section at DRDC Ottawa. Different scenarios being investigated include:

1. Modelling of a room with concrete/brick walls with/without wooden or metallic studs.

2. Include wiring and water pipes in the walls.

3. Modelling of human phantoms of different sizes to represent adults and children inside the room.

4. Modelling of human phantom carrying a rifle.

5. Modelling of multiple rooms.

6. Using different frequency values, e.g. 10 GHz, and different pulse widths.

7. Standoff scenarios.


5. References

[1] M. Skolnik, G. Andrews, and J. P. Hansen, Ultra-wide Band Microwave – Radar Conceptual design, IEEE AES Systems Magazine, 25 – 29 (October 1995).

[2] L. Carin, N. Geng, M. McClure, J. Sichina, and L. Nguyen, Ultra-wide Band Synthetic Aperture Radar for mine-field detection, IEEE Antennas & Propg. Magazine, vol. 41, No. 1, 18 – 33 (Feb. 1999).

[3] E. K. Walton and S. Gunawan, Comparative Analysis of UWB underground data collected using step-frequency Short Pulse and Noise wave form, in: Ultra-wide Band, Short Pulse Electromagnetics 3, edited by Baum et al., (Plenum Press, New York, 1997), pp. 511 – 516.

[4] T. Payment, A low power Ultra-wide Band Radar Test Bed, proceedings of EuroEM 2000 (May 2000),

[5] S. Nag, H. Flluhker, and M. Barnes, Preliminary interferometric images of moving targets obtained using a time-modulated Ultra-wide Band through – wall penetration radar, Proc. of IEEE Radar Conf., 64 – 69 (2001).

[6] W. Chamma and S. Kashyap, Detection of Targets Behind Walls using Ultra Wide Band Short Pulse, in Ultra-wide Band Short Pulse Electromagnetics 6, edited by Mokole, E. et al., (Plenum Press, New York, 2003). pp. 493-506.

[7] S. Gauthier, W. Chamma, Through-The-Wall Surveillance, DRDC Ottawa Technical Memorandum DRDC Ottawa TM 2002-108, October 2002.

[8] E. K. Hung, W. Chamma, and S. Gauthier, UWB Receive Array Beamformer Output Images of Objects in a Concrete Room, DRDC Ottawa Technical Note DRDC Ottawa TN 2003-062, May 2003.

[9] S. Foo, EM Modeling of UWB Through-Wall Radar Imaging Using the High-Frequency Geometrical Optics (GO), DRDC Ottawa Technical Memorandum DRDC Ottawa TM 2002-166, December 2002.

[10] A. Taflove and S. C. Hagness, Computational Electrodynamics: The Finite-Difference Time-Domain, (Artech House, Boston, 2000).

[11] D. Mensa, High Resolution Radar cross-section Imaging, (Artech House, Boston, 1991).

[12] J. K. E. Tunaley, Through the wall UWB radar processing, DRDC Ottawa Contract W7714-01-0588, March 2002.

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1. ORIGINATOR (the name and address of the organization preparing the document. Organizations for whom the document was prepared, e.g. Establishment sponsoring a contractor’s report, or tasking agency, are entered in section 8.)

Defence R&D Canada – Ottawa 3701 Carling Avenue, Ottawa, ON, CANADA K1A 0Z4

2. SECURITY CLASSIFICATION (overall security classification of the document,

including special warning terms if applicable) UNCLASSIFIED

3. TITLE (the complete document title as indicated on the title page. Its classification should be indicated by the appropriate abbreviation (S,C or U) in parentheses after the title.)

Detection of targets behind walls using ultra wideband short pulse: Numerical simulation (U)

4. AUTHORS (Last name, first name, middle initial)

Chamma, Walid A. and Kashyap, Satish

5. DATE OF PUBLICATION (month and year of publication of document)

November 2003

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Technical Memorandum

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DRDC Ottawa TM 2003-226

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This work describes the use of a full-wave time-domain numerical procedure, based on the finite-difference time-domain (FDTD) method, to investigate the capabilities and limitations in the use of an ultra wideband (UWB) radar system to detect targets behind walls. A room with a concrete floor and wooden walls is modelled inside the FDTD lattice. The modelling assumes a transmitting antenna located outside the room and radiating a UWB pulse with a centre frequency of 2 GHz. A conducting cubic target is placed inside the room and the scattered field due to the cube is calculated using FDTD and is recorded at several receiver points outside the room. The back projection procedure is used to construct an image of the target. The above investigation is then repeated with the single target placed at different locations inside the room. Using the field information at various points outside the room, the movement of the target inside the room is traced and displayed. The effect of various parameters on the performance of the detection system is studied. This include the number of receiver and transmitter points and their separation distances both in a bistatic and monostatic radar setups. In the bistatic radar case it was observed that larger array aperture lengths produced a better-focused image while larger receiver separation distances produced higher sidelobes. However in the monostatic radar case, a better image is generated for smaller numbers of transmitter and receiver points. Additional simulation for two conducting cubes with different sizes inside the room is done to investigate the capability to locate and distinguish their corresponding images using the received scattered field information. By gating the received responses an enhanced image of the farther target is obtained.

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FDTD UWB RADAR Through the wall surveillance

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