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This chapter presents the design and fabrication of a low-cost continuous-wave ground-penetrating radar for detecting improvised explosive devices buried in the soil for use of security forces. It is low cost and simple because it uses a single frequency (920 MHz) and is designed only for the detection of the buried target. The work presented includes designing of transmitter system module, receiver system module, antennas, power module, graphical user interface module, and making a prototype that is compact and portable. The chapter explains the concept and illustrates a method to enhance isolation between antennas, which is a very important parameter for the effective functioning of ground-penetrating radar. The presented method of enhancing isolation by cavity-backing a rectangular microstrip antenna and keeping them separated at an optimum gap yielded high isolation of 52.6 dB. The prototype radar, using the enhanced isolation antennas, demonstrates the capability to detect up to the depth of 65 cm for a circular steel target of radius 12.5 cm buried in loose semi-dry pebbled soil. The prototype radar is sensitive enough to detect a plastic box, a small bunch of wire, a book (paper) buried in soil and a wooden slab and a steel scale buried in a sandpit.
Defence Institute of Advance Technology, Pune, India
Kamla Prasan Ray
Defence Institute of Advance Technology, Pune, India
*Address all correspondence to: krishraha@gmail.com
1. Introduction
Ground-penetrating radar (GPR) uses the principle of scattering electromagnetic waves for its operation. A target buried in soil will have different dielectric properties as compared to soil. This change in dielectric constant will cause a change in amplitude and phase of the signal reflected from the soil with the target as compared to the signal reflected only by the soil. Thus, identifying the change in phase and amplitude of the reflected signal will assist in detecting a target buried in soil [1]. This simple theory may be utilised to design and fabricate a GPR prototype for detecting improvised explosive devices (IEDs) buried in the soil.
Studies related to GPR are being conducted in all advanced countries for almost about three decades. These studies resulted in the development of methodologies that can be employed to develop rugged, portable, and multipurpose GPR. However, all these studies and methodologies aim at producing GPR, which can detect targets, ascertain the depth of the target and provide its pseudo-image [2, 3, 4, 5, 6, 7, 8]. The final GPR product become complex and cost-prohibitive incorporating all these functions concurrently.
For small detachments of security forces deployed in remote operational areas, the primary requirement of GPR is only to detect the target buried in soil, especially the IEDs. The other functionalities, such as determining the depth of the target and providing the pseudo-image of the target improve the detection value of the GPR, however, they make the system complex and cost-prohibitive and thus making it difficult for all the small detachments to procure them. The requirement is felt to design a low-cost simple GPR system that can only detect IEDs buried in the soil. This chapter intends to present a design and develop a prototype of a low-cost simple GPR for detecting IEDs buried in the soil. To make the product simple and low cost, only capable of detecting buried targets, the proposed system operates on a single frequency as opposed to wideband frequencies used in other commercial products.
The work presented in this chapter includes designing a continuous-wave transmitter and receiver module at a centre frequency of 920 MHz, designing a microcontroller-based module for detection of phase and amplitude variation, designing appropriate transmitter and receiving antennas with enhanced isolation between them, designing a user-friendly frontend and display and make the system online. The final objective of the work is to produce a portable prototype of GPR, which can detect IEDs buried in soil, for use of security personnel. The product design has been validated to detect both metals and non-metals buried objects in different kinds of soils and sand. The product is sensitive enough to detect a small bunch of wire and the maximum depth of detection achieved is 65 cm in loose semi-dry soil for a circular steel target of a radius of 12.5 cm. These experiments demonstrate that the prototype fabricated is capable of detecting IEDs buried in the soil.
The focus of the work is to design a low-cost GPR and use it only for the detection of the buried target. Thus, instead of operating using a wideband of frequencies [2, 3, 4, 5, 6, 7, 8] which provide other functionalities, such as providing exact depth and pseudo-image of the target, only a single-frequency operation is chosen to make the system simple and low cost capable of only detecting the target. A major problem in detecting a small target by the GPR is the masking of the low-power target reflected signal by relatively high mutual coupling between the antennas. To solve this issue, software pre-processing techniques, such as Background subtraction algorithm [9, 10], Displacement-based technique [11, 12], blind sources separation (BSS) techniques [13, 14], and hardware-based pre-processing techniques, such as filtering [15, 16], antenna polarisation technique [17, 18] and time gating techniques [19, 20], have been reported. These techniques make the system complex and cost-prohibitive too. This chapter explains in detail a simple low-cost but highly effective technique of enhancing isolation between transmitting and receiving antennas to resolve the issue of mutual coupling between antennas affecting the detection capability of the GPR. It is demonstrated that by introducing a cavity-backing on a rectangular microstrip antenna a destructive interface between direct and scattered radiation from the cavity rim yields maximum isolation for an optimised combination of cavity height and separation between two antennas. The cavity-backing yields maximum isolation of 71.4 dB and a minimum of 49.1 dB within a narrow BW of 5% for the centre frequency of operation at 920 MHz. The use of this proposed antenna with enhanced isolation makes the GPR highly sensitive and effective. The cavity-backing technique has demonstrated enhancing isolation in wideband operation also in which a double cavity-backing yielded uniform high isolation of more than 40 dB for a BW of 64% for the centre frequency of operation of 2.6 GHz.
Following sections of the chapter cover system implementation, the concept of enhancing isolation between antennas and implementation of the same, prototype fabrication, and results of various experiments and discussions.
The design and implementation process of a homodyne continuous-wave (CW) GPR to be used in detecting IEDs is discussed in this section. The frequency of 920 MHz has been used as the operating frequency for the product designed because, at this frequency, the depth of detection can be nearly 1 m with good resolution [3]. The work intends to satisfy the optimum hardware requirement of GPR at the aforementioned frequency and hence concentrates on the design and integration of oscillator, filter, power divider, in-phase, and quadrature-phase (IQ) demodulator, antenna, etc. A block diagram depicting the various components of the designed system is shown in Figure 1. The system consists of a transmitter subsection, a receiver subsection, antennas, a data acquisition system, and an online display system which has been elaborated on in the following sub-sections.
Figure 1.
Block diagram of the proposed low-cost GPR.
A single PCB has been designed for the transmitter, receiver, and microcontroller to make the system compact. The PCB is fabricated on a 1.6 mm FR-4 substrate with εr = 4.4 and tanδ = 0.02. The PCB designed along with all the components soldered is depicted in Figure 2(a). Figure 2(b) depicts Arduino Uno (microcontroller) and the 6 V DC, 48 AH battery connected to the PCB. Designing a transmitter, a receiver, and a data acquisition sub-system are discussed in the following subsections.
Figure 2.
PCB design (a) All components soldered (b) Ardiono and battery connected.
2.1 Transmitter sub-system
The transmitter comprises a voltage-controlled oscillator (VCO), which is designed to generate 920 MHz, as shown in Figure 2(a). The VCO unit has been designed using the BFP520 transistor in the Colpitts oscillator configuration [21]. BBY52 varactor diodes have been used for varying frequencies. A resistive power divider designed sends a part of this signal generated to the I-Q demodulator, which is used as a reference signal while demodulating.
2.2 Receiver sub-system
It receives the reflected signal from the ground and passes it through the low noise amplifier (LNA) for amplification. SGL 0622 LNA [22] has been used which provides a noise figure (NF) of less than 1.5 dB at 920 MHz and offers a gain of 30 dB. Inductive biasing is used to reduce noise. Following the LNA, a microstrip based three pole bandpass filter [23] is placed which is designed at the centre frequency of operation. Following this bandpass filter, a demodulator acting as phase and amplitude detector is placed. AD8347 [24] is used as a direct quadrature demodulator. It receives a reference signal from the transmitter end and provides amplitude ratio and phase difference between the transmitted and received signal. The amplitude and phase information are digitised, and the information obtained is displayed on a laptop.
2.3 Antenna design
Two identical transmitting and receiving rectangular microstrip antennas (RMSAs) resonating at 920 MHz have been designed using formulas given in Ref. [25]. The design of antennas augmented with the cavity backing to enhance the isolation between transmitting and receiving antennas is given in detail in Ref. [26]. It is of paramount importance to have enhanced isolation between the transmitting and receiving antenna so that a weak target reflected signal is not masked by the comparative high mutual coupling between the antennas.
The concept used to enhance isolation between the antennas is that when a cavity is introduced in co-located antennas, it makes two RF coupling paths between the two co-located antennas; one is direct and the other is via the cavity wall, as depicted in Figure 3(a). With an optimised height of the cavity rim at a given separation between antennas, comparable electric fields from these two paths can be made out of phase (180°), cancelling each other, which can lead to maximising isolation between them. However, this technique will work only for narrowband operation because coupling path length is dependent on operating wavelength. For wideband operation, a multi cavity-backed structure is required where each cavity provides optimum path length for a particular narrow band of frequencies.
Figure 3.
(a) Concept used to enhance isolation between antennas (b) Simulated structure of two cavity-backed antennas for measuring isolation (S21) between them.
Extensive simulations have been done using Microwave CST software to find this combination of optimum cavity height (h) and separation (x) between the antennas which yield maximum isolation. Figure 3(b) depicts a schematic diagram of two cavity-backed microstrip antennas with a separating distance of x for simulating isolation. Tables 1 and 2 give the results of these simulations. The work has been elaborated in Ref. [26].
Ser. no
x (mm)
|S21| (dB)
Ser no
x (mm)
|S21| (dB)
i.
30
35
vi.
120 (0.36 λ)
54.6
ii.
60
41.1
vii.
130
51.26
iii.
90
52
viii.
150
50
iv.
110
52.5
ix.
210
47.3
v
116
52.5
x.
240
46.8
Table 1.
Isolation (S21) obtained by keeping the cavity height (h) to 40 mm and varying the separation (x) between the co-located antennas at 920 MHz [26].
Ser. no
h (mm)
|S21| (dB)
Ser No
h (mm)
|S21| (dB)
i.
20
33.7
vi.
42
47.4
ii.
26
36.5
vii.
43
45.3
iii.
30
39.9
viii.
50
37
iv.
40 (0.12 λ)
54.6
ix.
60
32.2
v
41
−50
x.
80
28.04
Table 2.
Isolation (S21) obtained by keeping the separation (x) between the antennas fixed to 120 mm and varying the cavity wall height (h) at 920 MHz [26].
From the tables, it is noted that maximum isolation is obtained only at a particular combination of the height of the cavity wall and separation between the antennas. For the cavity height of h = 40 mm and separation of x = 120 mm, the isolation obtained is 54.6 dB. It is noted that the isolation is not enhanced either by increasing or decreasing the separation (of 120 mm) nor by changing the optimum cavity height (40 mm) for the 120 mm gap. It is, therefore, inferred that at the operating wavelength, destructive interference between direct and scattered radiation from the cavity rim yields maximum isolation for this combination of cavity height and separation between two antennas.
The top view and cross-sectional side view of the proposed antenna geometry are given in Figure 4(a). The CST microwave software has been used to optimise the parameters. Optimised ground plane (GL × GW) of the antenna is 19.5 × 19.5 cm2 and the radiating patch (L × W) is 13.5 × 13.5 cm2 with the coaxial feed located 4.1 cm from the centre. A radiating patch is suspended in the air at a height (h1) of 1.4 cm from the ground plane. A cavity wall surrounds the patch antennas. The height (Wh1) of the cavity wall is taken to be 4 cm as obtained from Tables 1 and 2. Using these optimised design parameters, prototype transmitter and receiver cavity-backed RMSAs, as shown in Figure 4(b), are fabricated. The radiating patch is made of copper plate suspended in the air with two Teflon supports at the centre line, and the ground plane and the cavity backing are made of aluminium. A comparison of measured and simulated isolation between the rectangular microstrip antenna without cavity-backing and with cavity-backing is given in Figure 5.
Figure 4.
(a) Top and cross-sectional side view of the proposed antenna configuration (b) photograph of fabricated cavity-backed antennas [26].
Figure 5.
Measured and simulated isolation at 0.36 λ0 separation of cavity-backed RMSAs for a cavity wall height of 0.12 λ0 [26].
It is depicted in Figure 5 that the introduction of cavity-backing improves the isolation by 25 dB at 920 MHz. Measured isolation with and without cavity wall for 0.36 λ0 (12 cm) separation is 52.6 dB and 27.5 dB, respectively. Within 5% designed BW, cavity backing of the RMSA yields maximum isolation of 71.4 dB and a minimum of 49.1 dB, whereas without cavity backing RMSA provided maximum isolation of 29.5 dB and a minimum of 27.5 dB only.
2.4 Data acquisition and online display system
The I and Q information of the reflected signal are passed through a low-pass filter with a cut-off at 20 Hz before feeding them to the A/D converter. In this work, an open-source Arduino Uno module [27] has been used as an A/D converter and microcontroller. For displaying amplitude and phase information of the target detected in real time a graphical programming environment (LabVIEW) has been used. LabVIEW design software has been integrated with Arduino [28] and a user-friendly graphical user interface (GUI) has been designed as depicted in the result section, to demonstrate the results. GUI design can calibrate the system according to soil conditions, fine-tune the frequency, and control the overall gain of the receiver. A basic flow chart for displaying amplitude and phase information in real time and designing the GUI with detection and calibration features is shown in Figure 6.
Figure 6.
Flow chart for online-display and calibration of the proposed low-cost GPR.
After fabrication and assembly of the single PCB and the integration of the power supply module, the final GPR system was integrated in such a way that there in minimum electromagnetic interference (EMI) amongst various subsystems. The unit was made compact and portable. In the final product, antennas are placed close to the ground at the base of the product. The antennas are grooved inside solid foam as shown in Figure 7(a). The separation between two antennas is fixed at 09 cm to maximise isolation between them. As obtained from Table 1, 09 cm separating gap yields 52 dB isolation (sim) which is comparable to the maximum isolation yield of 54 dB (sim) for a separating gap of 12 cm. Thus, to keep the overall system compact, the separating gap between the antennas is kept at 09 cm. The solid foam is sprayed with zinc oxide paint so that it acts as an EM radiation absorber and increases the isolation between the antennas. The PCB and the battery are placed at the back (the front surface is the surface facing the ground where antennas are placed) ensuring that the back lobe of the antennas does not interfere with them. The placement of the PCB and battery is depicted in Figure 7(b). The final product design is shown in Figure 7(c).
Figure 7.
Prototype of the fabricated GPR (a) front view of the base (b) back view of the base (c) final product with antennas, PCB, and power module at the base close to the ground rested on wheels and the display unit close to the user.
4. Experiment with targets buried in soil and sand
To test the detection capability of the prototype GPR, various targets were buried in the ground. The first experiment was carried out to establish the maximum depth of detection of the GPR outside the laboratory in semi-dry soil. IEDs can be made of metals and non-metals and may be buried in different kinds of soils. Thus, experiments were conducted to detect a plastic box, a small bunch of wire, and a book (paper) buried in soil and a wooden slab and a steel scale buried in a sandpit.
4.1 Experiment to determine maximum depth of detection of the GPR
To measure the maximum depth of detection of the product, an experiment as depicted in Figure 8(a), has been carried out. A circular steel target of a radius of 12.5 cm was buried in loose semi-dry soil with lots of small pebbles in it. The figure shows the target exposed but it has been buried in soil during the experiment such that the depth of soil over the target is about 65 cm. Next, the GPR prototype has been moved over the soil heap. Figure 8(b) depicts the detected signal. With no target present, the detected amplitude level varies from 0.3 V to 0.8 V. This variation is because of the presence of many small pebbles in the soil. When the unit moves and reaches the location below which the target is buried, the received amplitude becomes stable at the level of 1 V. After this, as the prototype is moved away from the target the amplitude level again starts varying. Also, it is noted that because the target is buried so deep and the target size is not so big, the amplitude level detected as compared to the reference level (i.e., when no target is present) is not much different (only it is more stable) and no phase information about the target is obtained. It is inferred that the GPR cannot detect a target smaller than the present one beyond the depth of 65 cm in this type of soil.
Figure 8.
Determining the maximum depth of detection of the GPR (a) experimental setup (b) GUI screen-shot for detection of a target at depth of 65 cm in the soil.
4.2 Detecting targets buried in soil and sand
After determining the maximum depth of detection of the GPR, various experiments have been conducted to ascertain its capability to detect IEDs buried in the ground. The blast effect of IEDs depends on their size i.e. explosive content, depth at which it is hidden, and type of medium in which it is kept. A particular target kept at the same depth will have more blast effect when kept inside sand than in soil. For the same kind of medium, a smaller target hidden at lesser depth may have the same blast effect as compared to a relatively bigger target kept at greater depth. Keeping these blast effects of IEDs into consideration, to make the experiments assess the detection capability of the GPR prototype in practical scenarios, the following experiments have been conducted for metal and non-metal targets.
A small plastic box of size 15 × 10 × 3 cm3, a bunch of wire, and a book (paper) of size 25 × 15 × 3 cm3 have been used as targets and placed at a depth of 20 cm inside the soil. Figure 9(a) depicts the result obtained using GPR for the plastic box target. The high-reflected power obtained here is on account of the difference between the dielectric constant of air (trapped in the plastic box) and the dielectric constant of the soil. Figure 9(b) depicts the result for detecting a bunch of wires. In this case, the reflected power is not stable because it is a bunch of wires, that is, it consists of many small plastic-coated copper wires with soil in between and is not a monolithic big target. Figure 9(c) depicts that the product can also detect paper buried in the soil. Next, the medium in which the targets are buried is changed. A dry wooden slab (10 × 10 cm) and a steel plate (15 × 15 cm) have been buried at a depth of 15 cm and 30 cm, respectively in a sandpit with a 50 cm horizontal separation between them. The result obtained is shown in Figure 9(d), which depicts that the product can detect both the targets buried in a sandpit.
Figure 9.
GUI screen-shots of the received amplitude and phase response of targets buried in soil and sand (a) plastic box buried 20 cm in soil (b) bunch of wire buried 20 cm in soil (c) book (paper) buried 20 cm in soil (d) A wooden target (buried at 15 cm) and a steel target (buried at 30 cm) in a sandpit.
4.3 Demonstrated detection capability
The detection capability demonstrated by the prototype GPR is given in Table 3. The table summarises the results of experiments conducted with the GPR operating at 920 MHz on the detection of various targets.
Ser. no.
Target type
Face area of target (cm2)
Land type
Detection depth (cm)
(i)
Circular steel plate
490
Pebbled semi-dry soil
65
(ii)
Plastic box
150
Semi-dry soil
20
(ii)
Bunch of wire
110
Semi-dry soil
20
(iv)
Book (paper)
375
Semi-dry soil
20
(v)
Wooden slab
100
Sandpit
15
(vi)
Steel scale
225
Sandpit
30
Table 3.
The detection capability of the prototype GPR at 920 MHz.
The prototype single-frequency CW GPR successfully detected metal targets as small as a bunch of wire buried at 20 cm in soil and non-metal, such as wood, paper, and plastic, buried in the soil. For a metallic circular plate of a radius of 12.5 cm buried in semi-dry pebbled soil, an experiment has been carried out for successful detection up to the depth of 65 cm for low-transmitted power (−10 dBm). In general, IEDs are placed at a depth, not more than 50 cm inside the soil, to have a substantial blasting effect. To have the same effect of the explosion with an IED kept at a greater depth, more explosives occupying a larger volume, will be required. Thus, the prototype designed will be effective in detecting IEDs in the field.
The proposed system exhibits highly sensitive performance because of the use of high-isolation and high-gain antennas. High isolation between antennas ensures that the low-reflected power from the small targets is not masked by relatively high-mutual coupling between the transmitting and receiving antennas. Table 4 compares the cavity-backed antennas used with other related work in yielding high isolation. As evident from Table 4, the proposed simple low-cost antenna yields higher isolation than all the refereed antennas. The isolation obtained may be further increased by adding RF absorbent between the antenna.
Comparison of Cavity-backed RMSA used in the low-cost GPR with other related works.
Most of the reported GPR uses software-based post-processing techniques, such as background subtraction algorithm [9, 10] to alleviate the issue of mutual coupling between antennas. However, the background subtraction algorithm assumes an environment without a target and subtracts it with the environment having the target. Thus, this algorithm is not suitable for real-life scenarios where it cannot be ascertained beforehand about the absence of the target. Considering the on-ground scenarios many works have reported hardware-based pre-processing techniques, such as filtering [15, 16] and time-gating techniques [19, 20]. However, all these techniques make the system complex and cost-prohibitive. In addition, the designed prototype work on a single frequency, unlike the other reported GPR systems [2, 3, 4, 5, 6, 7, 8] which operate on a wideband frequency. Hence, it is very simple and low-cost. However, due to a single-frequency operation, it cannot ascertain the depth of the target accurately, neither it can provide a pseudo-image of the target. Remote areas where this prototype is planned to be used are generally devoid of any unnatural foreign substances and thus detection of IEDs even without getting its pseudo-image fulfils the basic requirement. However, to find out the exact depth and pseudo-image of the buried target a band of frequencies would be required so that more information about the target is available. The broad bandwidth will also enable the required resolution in detecting targets. As the objective of the work is to have a simple low-cost device to be used by security forces working in remote areas, only a single operating frequency has been used to design the prototype.
For broadband continuous-wave operation, the band of frequencies has to be modulated so as to provide a marker for range estimation [1]. Frequency modulated continuous-wave (FMCW) utilising triangular wave [35] is one of the accurate methods for short-range detection. The transmitting and the receiving antennas also have to be designed broadband and high gain with uniform enhanced isolation throughout the band. The concept depicted in Figure 3(a) may be extended for providing broadband isolation by introducing multi cavity-backing. Multi-cavity backing has been demonstrated to be versatile in providing a wide BW of 64% for a centre operating frequency of 2.6 GHz, by introducing multi resonances and a high average gain of 12.6 dB by concentrating the RF energy in the desired direction [36]. Multi-cavity backing also demonstrates the potential to yield high uniform isolation of 40 dB [36] by each individual cavity-backing providing the optimum combination of cavity height and separation between the antennas to cause destructive interference as depicted in Figure 3(a) for a particular range of narrowband frequencies. The performance of a double cavity-backed antenna has been elaborated in Ref. [36]. Figure 10(a) depicts the fabricated double-cavity-backed antenna and Figure 10(b) depicts the isolation enhancement by introducing more than one cavity. As evident from Figure 10(b), single-cavity-backing enhances isolation at the lower frequency of operation, while the introduction of one more cavity to make it double-cavity-backed enhances isolation throughout the band of operation.
Figure 10.
(a) Double-cavity-backed microstrip antenna [36] (b) comparison of isolation (S21) yielded by without cavity-backed, single-cavity-backed, and double cavity-backed stacked multi-resonator microstrip antenna [36].
This chapter presents the development and fabrication of a portable compact low-cost CW GPR prototype operating at a single frequency of 920 MHz. A concept of enhancing isolation between co-located antennas has been explained and implemented. The prototype using the enhanced isolation antennas demonstrates the capability to detect both metal and non-metal targets buried in soil as well as in a sandpit. It is sensitive enough to detect a small bunch of wire buried 20 cm in the soil and the maximum depth of detection in a semi-dry soil is 65 cm for a metallic circular plate with a radius of 12.5 cm being used as a target. The prototype is planned to be utilised for detecting IEDs buried in soil for use by security personnel in remote areas.
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Written By
Krishnendu Raha and Kamla Prasan Ray
Submitted: 14 February 2022Reviewed: 30 March 2022Published: 24 May 2022
Open access peer-reviewed chapter
