Typical values of relative permittivity and static conductivity for common subsurface materials at 100 MHz .
This chapter presents an approach for explosive-landmine detection on-board an autonomous aerial drone. The chapter describes the design, implementation and integration of a ground penetrating radar (GPR) using a software defined radio (SDR) platform into the aerial drone. The chapter?s goal is first to tackle in detail the development of a custom-designed lightweight GPR by approaching interplay between hardware and software radio on an SDR platform. The SDR-based GPR system results on a much lighter sensing device compared against the conventional GPR systems found in the literature and with the capability of re-configuration in real-time for different landmines and terrains, with the capability of detecting landmines under terrains with different dielectric characteristics. Secondly, the chapter introduce the integration of the SDR-based GPR into an autonomous drone by describing the mechanical integration, communication system, the graphical user interface (GUI) together with the landmine detection and geo-mapping. This chapter approach completely the hardware and software implementation topics of the on-board GPR system given first a comprehensive background of the software-defined radar technology and second presenting the main features of the Tx and Rx modules. Additional details are presented related with the mechanical and functional integration of the GPR into the UAV system.
- ground-penetrating radar (GPR)
- aerial landmine detection
- drone flight control
Ground-penetrating radar (GPR) is currently a well-accepted geophysical technique which has been successfully deployed with the aim of addressing important sensing problems that requires detection, imaging and identification of dielectric material discontinuities in the subsurface through the use of radio waves, providing a non-invasive method
Nowadays, there are several types of GPR commercially offered. However, within the wide range, two kinds of GPR systems can be identified upon the manner in which the data is acquired, either in
The topics presented in this chapter explore the potential of the software-defined radio (SDR) technology to provide flexible, cost-effective and low-weight radar prototypes for GPR application in the detection of metallic buried landmines. Landmine detection and clearance are one of the primary humanitarian necessities mostly in developing countries with internal war conflicts, for instance, Colombia, which is currently one of the most mine-affected countries in the world. Since 1990, the Colombian government has registered 10,751 victims of explosive landmines: 39% corresponding to civilians and 61% to the military. Although the internal conflict in Colombia is coming to an end, there still are many regions in the country with over 10,000 potentially hazardous areas that require urgent mine clearance, according to recent Colombian government statistics. Besides, most of the landmines in those countries are improvised explosive devices (IEDs) which impose the requirement of the prior knowledge of the target features in order to discriminate between the target and safe objects. IED detection is a challenge due to similarity between IEDs and common objects such as PVC pipes, cans and other objects that are used in the explosive device fabrication. Therefore, the use of SDR in GPR systems can significantly contribute to the identification of IEDs due to its intrinsic capacity of software adaptability.
This book chapter presents an approach for detecting buried landmines by using an autonomous drone equipped with a custom-designed SDR-based GPR system. Figure 1 details the proposed robotic setup, mainly composed by (i) the GPR system (USRP B210 board, RX and TX antennas and SMA connectors) and (ii) the drone’s on-board hardware (high-level and low-level processors, ZigBee communication module, IMU, GPS and LIDAR sensors). One key aspect to achieve a reliable GPR’s operation relies on the precise navigation of the drone, which must flight steady in spite of wind disturbances. In addition, the drone must flight at a very low altitude to allow the GPR’s emitted signals to properly radiate the subsurface (about 50
2. Comprehensive literature review
2.1. What are SDR and GPR technologies?
2.1.1. Software-defined radio (SDR)
The origin of the software-defined radio technology is related with the military field, specifically to the Defence Department of the United States with the Integrated Communications, Navigation, and Identification and Avionics (ICNIA) system in the 1970s. Later on in the 1990s, the SpeakEasy project started with the purpose of developing a software programmable radio systems operating in the band between 2 MHz and 2 GHz . This project can be considered as the base of the SDR technology.
In the mid-1990s, Joseph Mitola creates the SDR forum given a detailed description of the technology defining SDR from the engineering design, topological structure and computational structure perspectives . In summary, SDR can be understood as a reconfigurable radio system which substitutes the hardware components such as mixers, filters and modulators into software components by using computing embedded systems. The basic SDR systems are composed by an embedded system with a field programmable gate array (FPGA) interface with a digital-to-analog and analog-to-digital conversion (DAC and ADC, respectively) both adapted to a radio frequency trans-receiver system [8, 9].
2.1.2. Ground-penetrating radar (GPR)
GPR, being the acronym of
Currently, the landmine detection and improvised explosive devices (IEDs) using GPR are the subject of research. The GPR allows detecting both metallic and non-metallic targets in a non-invasive fashion . Unlike metal detectors, GPR technology increases the detection depth range and reduces the false alarm rate. Several GPR technologies and techniques have been addressed in literature oriented to perform a more efficient demining process .
There are several types of GPR; the main difference is the way in which data are acquired, either in
2.2. SDR for GPR systems
Since the introduction of the SDR concept, one of the most promising applications that have been taken from is radar. The advantages presented by the SDR technology suits perfectly with the oversize and overweight drawbacks of a traditional radar system . In this manner the term
In the follows, some contributions of SDRadar system are presented. Debatty in  presents a compressive state-of-the-art review of the SDRadar technology by approaching from the design concept and global assessment perspectives. In particular, the author mentioned the wide varieties of airborne SDRadar including the one for a UAV to sense and avoid collisions with other flying objects. The work in  presents the potentialities of the USRP-based software-defined radars presenting the design and implementation of an SDRadar system for target tracking and the experimental characterisation of the radar on a USRP board obtaining improved radar resolution results with respect to previous works. Other works like the one of Aloi et al.  approach in a more detailed fashion presenting the synchronisation issues and practical implementation of a radar system by using Simulink toolbox interface instead of GNU Radio in a USRP device. Recent contributions have shown novel software radar techniques; Costanzo et al. in  proposed SDRadar based on orthogonal frequency division multiplexing (OFDM) for soil discontinuity detection taking advantage of the well-known benefits of the multicarrier radar signalling technique employed in various application fields, such as remote sensing of wheatear forecasting, detection of buried objects and interpretation of urban scenes. In the other work presented by the same authors , a high-resolution L-band SDRadar is presented for target detection using the USRP NI2920 enhancing the radar bandwidth and range resolution by exploiting the Gigabit Ethernet interface of the SDR system.
2.3. UAV for demining applications
Unlike terrestrial landmine detection mechanisms, the use of unmanned aerial vehicles (UAVs) is clearly suited for covering a minefield without the risk of triggering landmines during the mission. However, the weight and size of the sensing systems used for demining are unlikely to be placed on UAVs due to their poor payload capacity. In , the authors proposed the fabrication of a small multi-frequency ground-penetrating radar (GPR) on-board a UAV quadrotor able to lift up to 1.1
So far, biological sensors used by animals (e.g. dogs and rats) provide the highest accuracy in terms of landmine detection. However, to avoid the use of trained animals for demining purposes, authors in  have proposed a blimp-based chemosensing UAV with a bio-inspired detection architecture composed by a six-grid array films responding to a wide range of volatile organic compounds. The system creates a map of the terrain with the information provided by the chemosensor. The advantage of this chemosensor technology relies on its small size; however, the sensitivity can go up to few particles per million (ppm), which might not be enough for detecting explosive particles such as Trinitrotoluene (TNT) or Dinitrotoluene (DNT).
In , an airborne LIDAR system integrated with laser scanner, GPS and inertial-measurement unit (IMU) is proposed. The system is able to detect TNT and DNT using sensitive biosensors based on the soil bacterium
3. SDR-based GPR design
As detailed in Figure 1, the open-hardware platform USRP for developing custom SDR configurations has been used to implement the GPR device. Here, GNU Radio is used, a free and open-source Python programming graphical user interface for software-defined radios to facilitate SDR development. By using the SDR functions contained within the GNU Radio framework, the most fundamental operations of the GPR, such as signal generation, filtering and up/down conversion, are easily implemented via software (unlike the traditional fixed-hardware implementations). Despite the development of SDR has gained a great impact, its full potential has not been fully exploited for radar-based applications applied to landmine detection, concretely, by integrating a GPR into a drone.
The GPR hardware system is composed by two main blocks, the transmitter (TX) and receiver (RX), as shown in Figure 2(a). The Tx module generates the pulse, which is then shaped by a root-raised cosine filter in order to reduce the frequency bandwidth of the signal due to the restrictions of the platform itself. The pulse modulates a carrier that is finally transmitted to the USRP platform to being radiated by the TX antenna. The radiated modulated pulse travels along the path air-soil, and in case of detection, there is a reflected wave, which is sensed by the RX antenna. The RX system down-sampled the signal. The amplitude and delay of the received signal are then post-processed using MATLAB aimed at generating the heat map shown in Figure 2(b). The forthcoming subsections detail on GPR modelling and design.
In order to design and develop the functionalities described, the design considerations described in Figure 3 have been established. In the first place, the technical specifications of the GPR are defined by taking into account the operational requirements, such as signal frequency operation and bandwidth, propagation speed in different media, noise, etc. Also, it is considered how the aforementioned variables might be affected by the target and material properties. This analysis enables to define and design criteria for developing the antennas, the required hardware and the signal processing algorithms. Finally, GPR storage and visualisation process refer the way how information is stored and presented to the final user.
3.1.1. Geometrical model
A simple geometrical scheme (shown in Figure 4) is proposed for setting the inclination angle for both antennas based on analysing reflection and refraction properties that depend on soil materials and target location. This model uses the following geometrical parameters:
where the relative air permittivity such as
|Material||Static conductivity (||Relative permittivity (|
|Soil (sandy, dry)||0.1–100||4–6|
|Soil (sandy, wet)||10–100||15–30|
|Soil (loamy, dry)||0.1–1||4–6|
|Soil (clayey, dry)||0.1–100||4–6|
|Soil (clayey, wet)||10–1000||10–15|
The dielectric material considered for the terrain in the model was a soil (sandy) material considering the limit between dry and low humidity with relative permittivity (
3.1.2. Signal power loss model
Power losses are a common phenomenon that must be involved in the development of a GPR system because as described in Figure 5 the signal faces different changes of medium not only from the air to the subsoil but also within the soil itself. Other signal phenomena should be considered as well like multi-trajectories by reflections of the same signal on the different media surrounding the measurement area. It is also necessary to take into account the distance and alignment between the transmitting and receiving antennas and the backscattered signals.
According to , the range of the
By replacing Eq. (3) into the power transmission loss coefficient and computing the power loss in dB, the expression for the transmission coupling loss can be defined as
Obtaining a terrain impedance
Dispersion is the phenomenon that occurs to the signal from the transmitter to the receptor due to non-homogeneities of the medium, especially within the soil that can be modelled as a stratified medium as shown in Figure 5 turning the wave propagation very dispersive. The losses due to propagation dispersion can be estimated as
Table 3 summarises signal attenuation values depending on the terrain materials (that are typically encounter in Colombia) and signal frequency.
3.1.3. Time-delay model
Besides modelling signal loss in Eq. (2), we have also considered the velocity of signal propagation (
In terms of depth resolution, some GPR applications measure depth by calculating the time involved between the signal reflection caused by the target and the receptor. However, this implies that the terrain has a clean subsurface (e.g. only ground besides the buried target). Clearly, landmine application demands to consider other types of buried elements. Those signals that are reflected by other elements that are not the target cause the clutter effect. The clutter can be defined as those chaotic signals that are measured at the same time and with similar spectral properties than the signal sample of interest. In order to identify the target among other elements (despite the clutter effect), the emitted signal must have a large bandwidth, and the antennas must have a high gain with significant aperture in the lower emitted frequencies. These features are called
In the model proposed both
Eq. (12) indicates that the
3.2.1. Software-defined radio (SDR) platform
Figure 2 detailed the main components of the proposed GPR system by following an SDR architecture. This section presents a brief description of the hardware components used in the GPR system. The SDR technology has two main hardware components: (i) PC and (ii) A software radio peripheral.
Nowadays, the most representative companies that provide development SDR platforms are Ettus Research (with the Universal Software Radio Peripheral (USRP)), National Instruments (NI-Universal Software Radio Peripheral), Pentek, DataSoft (Thunder SDR), FlexRadio (SDR-1000), Realtek (rtl2832) and lately low-cost SDR platforms as FUNcube Dongle Pro for amateur radio applications. Among the different alternatives, Ettus Research has the largest market segment with a wide variety of SDR platforms with different performances; therefore, the SDR platform used for the GPR application was the USRP B210 from Ettus Research, where the baseband signal processing was performed by the PC on-board the UAV.
The USRP B210 board is divided into two internal boards (Figure 2(a)), the
Thus, with the aforementioned information, the USRP B210 board has a frequency cover ranging from 70
3.2.2. GPR antenna design
The designed GPR is a time domain system where an impulse is applied to the antenna; there is a requirement for a linear-phase response, and this means that only a limited number of types of antenna can be used. The use of two separated antennas is due to the difficulty found with the use of a single antenna for transmission and reception, which would require an ultra-fast switch to operate in both channels, and since currently it is not possible to obtain commercially available switches to operate in the nanosecond region with sufficiently low levels of isolation between TX and RX ports, most surface-penetrating radar systems use separate antennas for transmission and reception in order to avoid interference from the transmitting antenna at the receiving antenna. Therefore, the cross-coupling level between the TX and RX antenna is a critical parameter in the antenna design for this kind of radars. Typically, a parallel dipole arrangement achieves a mean isolation of −50
On the other hand, the antenna’s performance is strictly linked with the terrain material, and in the case of the surface-penetrating radar sensing above the terrain, the antenna will radiate from the air into a half-space lossy material . Some works in literature have reported antenna’s behaviour over lossy dielectric materials  that summarise the cause modification of the antenna radiation pattern, both spatially and temporally, and should be taken into account in the system design. In addition, the propagation of electromagnetic pulses in a homogeneous conducting earth has been modelled in , and the dispersion of a rectangular pulse source suggests that the time domain characteristics of the received pulse could be used as an indication of distance.
In terms of frequency band, a typical antenna used in an impulse radar system would require to operate over a frequency range of a minimum octave and ideally at least a decade, 100 MHz–1 GHz. The input voltage driving function to the terminals of the antenna in an impulse radar is typically a Gaussian pulse, and this requires the impulse response of the antenna to be extremely short in order to not distort the input function generating time side lobes, which can illuminate clutter targets that are close to the target of interest degrading the radar resolution.
In order to have an antenna with a high bandwidth, the Vivaldi antenna was selected for the design. The Vivaldi antennas are part of the tapered slot antenna (TSA) family . This family belongs to the type of longitudinal-wave travelling antenna, i.e. plane antennas whose current and voltage distributions can be represented by one or more travelling waves, which usually travel in the same direction and propagate with a phase velocity less than or equal to the velocity of light [29, 30]. It provides an end-fire radiation and linear polarisation and can be designed to provide a constant gain-frequency performance. TSA are flat antennas that are built on a dielectric substrate. These vary according to the shape of the taper (i.e. the inner profile of the conductive material that goes over the dielectric). There are several kinds of profiles such as linearly tapered slot antenna (LTSA), constant-width tapered slot antenna (CWSA) and exponentially tapered slot antenna (ETSA). The Vivaldi antipodal antenna is characterised mainly by having a broader bandwidth with respect to the return losses of the antenna. Unlike the traditional Vivaldi antenna fed by a conventional microstrip line, the Vivaldi antipodal antenna separates the tapers by placing one on the front face of the dielectric and the other on the back face, as shown in Figure 7(a). In this structure, the feed is made by means of a microstrip line whose ground plane gradually narrows. The proper design of the transmission line ensures that this type of power is balanced and does not need the additional balun. The antipodal configuration guarantees having a wider bandwidth for the matching to the microstrip feed line . Additionally, recent works have shown that the introduction of slots in the antenna taper extends the bandwidth maintaining the good performance of the antenna in terms of radiation pattern and gain [30–33]. Similarly, the use of slots in the taper has been shown to be an effective technique to significantly reduce the size of an antenna without affecting its performance [34, 35], which is ideal for the on-board integration of the GPR with the UAV.
Two Vivaldi miniaturised antipodal antennas for the pulse transmission and reception are integrated to the on-board GPR system, specially designed and fabricated for radar application . As was aforementioned, this configuration is ideal for
The driver needed to work with the USRP B210 is the USRP Hardware Driver (UHD); it is a library written in C ++ designed to work on Linux, Windows and Mac OS. The main purpose of the driver is to provide control over Ettus products; the use of this software can be used stand-alone or by using other applications such as GNU Radio, LabVIEW, Simulink and OpenBTS. The software implementation of the GPR system can be done under GNU Radio software because it is open and free source and provides a friendly signal processing block interface. Additionally, it is a simulation tool that can be used together with RF hardware (USRP) to physically implement radio software systems.
The GNU Radio project  was started in 2001 and was founded by Eric Blossom with the aim of developing a framework for radio software. It consists of a set of files and libraries that provide signal processing blocks, allowing the design and simulation of systems based on radio software. This software tool can be used with additional external hardware such as the USRP, providing the possibility of physically implementing a system based on radio software. The operation of GNU radio can be conceived as a graph, where nodes symbolise signal processing blocks, and the interconnection between them will determine the path that the signal will follow starting from a source and terminating in a sink. Further details of GNU Radio software features, functionalities and applications can be found in its website .
GNU Radio applications can typically be programmed in two ways: (i) directly on Python or (ii) using the GNU Radio Companion graphical tool. The second option arises as a need to facilitate the task to the user as much as possible, thus minimising the application programming. The Tx and Rx GPR systems are programmed by using the GNU Radio Companion option, described as follows.
3.3.1. GPR Tx system
The GPR Tx system consists basically in the generation of the transmission impulse. Even though the theory dictates that the signal generated for impulse-based radar must be infinite band, in practical this is not possible because of the technology restrictions of the GPR system’s elements. In this case, the generated pulse is band limited since the USRP B210 card has a bandwidth approximately of 56
The classic rectangular impulse does not cause inter-symbol interference (ISI); however, an infinite bandwidth and significant transmission power are required. For a wireless communication channel, it is necessary to meet the Nyquist ISI criterion, the ISI is generated when consecutive signals are sent through the communication channel and the replicas of the previously sent signals generate interference to the signals that are currently going through the channel which makes the system less robust against noise. To minimise the ISI in a communication channel and concentrate the power within the desired bandwidth, the
The mathematical function of a RC filter is defined as follows:
In order to build a RC impulse in GNU Radio companion, first it is necessary to define a square signal and then filter it. However, GNU Radio does not have a square impulse signal generator block; therefore, a well-known method is used, which consists in the multiplication of four square signals with a useful cycle of the 50%, each of them with different frequencies following the rule (
With the generated square pulse, it is now necessary to give the form of a RC pulse passing through an elevated cosine root filter which in GNU Radio is known as
Within the RC Filter block, it is possible to set the decimation value, filter gain, sampling rate, symbol rate, roll-off factor (alpha) and number of taps for floating and real values according to the criteria of design and operation of the filter. The choice of decimating or interpolating is very important in the filter design, and the two are the equivalent of a down-sampling and up-sampling process, respectively. Both are integer values that allow increasing or decreasing the number of times a sample is replicated in the ADC process. The symbol rate (baud rate or modulation rate) is the number of pulses per unit of time (pulse/second).
The RC filter is a finite impulse response (FIR) filter used for
After the generation of the RC pulse, an additional modulation process is considered for further distortion reduction. The final transmission pulse is shown in Figure 11.
3.3.2. GPR Rx system
The received signal (Rx) is processed in order to detect a buried landmine. To this purpose, we need to introduce the following procedures: (i) signal filtering, (ii) setting a gain for quantifying the incoming power and (iii) designing the detection algorithm. By using the
On the other hand, as was mentioned before, the detection algorithm is based on a matched filter that enables to maximise the signal factor despite the noise (high signal-to-noise ratio (SNR)); in other words, it enables to detect the waveform of the signal (emitted pulse) despite the noise. The matched filter is a linear filter normally used in radar systems designed to detect a pulse shapes despite the presence of clutter noise. Once the drone has covered an entire terrain, the data captured by the GPR is post-processed in order to generate a heat map. Hence, Rx signal is of the form
By applying the inequality Cauchy-Schwartz in Eq. (19), the response of the filter is
By assuming white noise,
Based on the above considerations, the filter used as a matched filter is also a RC filter.
In the post-processing stage, the way of indicating a mine presence to the user has two approaches: by audio and by construction of a heat map. The audio recognition method is similar in operation to that of a conventional metal detector which emits an audible signal under the event of a positive mine detection. When a target is located, the received power is greater, and consequently the response’s amplitude of the matched filter is also greater; then the signal is processed by a function in GNU Radio to obtain the RMS value of the signal and is sent to a VCO where it fits the audible spectrum so that the sound is differentiable with respect of a non-mine event. The blocks in GNU Radio that describe this function are shown in Figure 12.
On the other hand, the method of recognition by construction of a heat map unlike the acoustic method requires further processing of the results. To this purpose, the GPR data are exported with a
4. SDR-based GPR integration on-board UAV
This section describes the on-board integration process between the GPR system and the UAV. The process includes the integration of hardware, software and the mechanical parts corresponding to the necessary supports for the correct coupling of both systems for a suitable flight plan for the GPR correct function. Being independent devices, the UAV and the GPR need a mechanical supports specially designed to fit with the physical area of the UAV system. In addition, it is also necessary to establish continuous communication between the equipment in such a way that the flight system is in charge of assigning processes, while the GPR system is a peripheral that executes those processes. Finally, the data obtained by the GPR together with the GPS data and positioner values of the board must be correctly archived, so that the base station can extract and post-process them for further analysis.
The mechanical integration of the radar with the UAV is realised by means of an adjustable, resistant and light support, which allows several antenna positions according to the height of flight and the depth distance of the buried landmines. For the support design, the geometric model (Figure 4) is taken into account for the signal transmission and reception. As was mentioned in Section 2.2, the designed GPR system is implemented using the Ettus USRP B210  card from Ettus Research and two antipodal Vivaldi antennas especially designed for radar applications explained in Section 2.2.2. Based on the above, the CAD models were designed for each of the necessary components.
For the SDR support CAD model, the physical dimensions of the SDR card were taken into account and designed in such a way that the card would slide through the support and be adjusted with the SMA connectors at one end. The model is shown in Figure 13.
For supporting the antennas and SMA cables, an adjustable rail system is designed in such a way that the separation between the antennas is variable between a minimum distance of 307.66 mm and a maximum distance of 669.69
The designed CAD models are fabricated using the 3D object professional printer using a simulated polypropylene material that gives strength and flexibility to the structure. The weight for the total and each piece of the mechanical support are given in Table 4. It is worth to notice that the total weight meets the restriction of the UAV payload.
|B210 support||20 g|
|Rail system with antenna support (×2 arms)||97 g|
|Landing gear||340.21 g|
|Antennas (×2)||68 g|
|USRP B210 SDR||99 g|
|Cables SMA-SMA (×2)||34 g|
The UAV has by default a 64-bit Linux operating system. However, due to compatibility problems, GNU Radio is installed over a bootable USB memory with a 64-bit Ubuntu 14.04 operating system. In this fashion, the UAV Mastermind instead of starting with the default operating system started with Ubuntu from the USB allowing the operation of the GPR system.
On the other hand, the communication between the computer on-board and the base station is made by means of the SSH protocol since it has strong security protocols and it ensures a stable process execution by keeping processes running on the server until the link is re-established when communication with the client is lost.
4.3. Graphical user interface (GUI)
The GUI enables the user to set up the desired trajectory (waypoint navigation) (Figure 19(a)), key parameters of the GPR and other features such as saving a log file of the flight or computing a terrain image mosaic with the sequences of images captured by the drone during flight. This work has been approached in a previous work cited in .
4.4. Landmine detection and geo-mapping
The Autopilot card of the UAV is programmed using the so-called variable
The data are stored in two self-contained folders in the mastermind’s desktop, one with the GPS and one with GPR data. The results of the GPR are stored automatically in GNU Radio in a binary file with different names for each of the trajectories. On the other hand, the GPS and IMU data are stored in a text file which is divided into six columns representing the data of each required value. The renaming and creation of radar and GPS files, respectively, are done autonomously without user intervention during the rest of the time of the UAV at the waypoints.
In order to geo-locate the identified landmine targets within an image (geodesical position), the odometry between consecutive images has been computed by using the on-board IMU data of the UAV . Once the landmine is geo-located, a map of the terrain is created by computing an image mosaic. Image mosaicking is a process for building a panoramic image that result from combining multiple photographic images taken with an on-board camera. The geodesic coordinates of the detected landmines are obtained by using the UAV on-board GPS through a robot operating system (ROS) package called
Recalling the workflow depicted in Figure 17, the steps followed in order to perform a mission are as follows: (i) the operator selects the GPS coordinates of the starting point of the mission (via Google Earth). By using the GUI of the ground station, the operator defines the path to cover a desired area. (ii) Before proceeding to real experiments, the operator must start the simulator (requires MATLAB) in order to verify that the drone is able to operate at the desired altitude and speed. (iii) Once the mission is validated, the operator must send the mission parameters to the drone (via clicking
5.1. Landmine detection results
Experiments with the complete system have been carried out nearby a small rural area. The drone covered a small terrain with an area of 35
In addition, other two types of metallic elements are buried working as false alarms. Figure 18(c) shows how these five elements (three landmines and two false landmines) are spatially distributed along the terrain. Elements #4 and #5 are fully metallic layers with 15 × 15 × 15
Experiments were conducted with an average ambient temperature of 14°C with an average solar radiation of 4.4
|True positives (TP)||4||Positive artefact indication-artefact in place|
|True negatives (TN)||2||Negative artefact indicator- artefact not in place|
|False positives (FP)||0||Positive artefact indicator- artefact not in place|
|False negatives (FN)||1||Negative artefact indication- artefact in place|
|True positive rate||80%||Correct positive results among all positive samples|
|False positive rate||0%||False alarm|
|Accuracy||85.7%||Reliability of the alarm|
|Positive predictive value||100%|
|Negative predictive value||66.6%|
Comparing the results from the heat map in Figure 18(d) against the location of the artefacts from Figure 18(c), note that the landmine (#1) (bottle-made artefact) was not properly detected. The corresponding GPR signal for landmine (#1) can be barely observed in the coordinates:
It was experimentally found that the limit with the measurements carried out for the artefact (#3): a tube-made artifact with an enclosure made of PBC with 30% of non-uniform metal component in the inside. Comparing Artefact (#3) with (#1), both are buried at the same depth with similar morphology, but the former has a larger diameter; thus, it has a larger area of 64
The corresponding GPR signal for landmine (#2) was fully detected in the coordinates: rows 1–5 with columns 2–4. This artefact was fully made of metal with a transversal area of 156
5.2. Geo-mapping results
In this subsection, experimental results of the geo-mapping process are presented. In overall, it has been analysed 28,029 images captured by the drone during flight. On average, the drone has covered terrain areas ranging from 15
This chapter has presented the development of a custom-designed lightweight GPR by approaching interplay between hardware and software radio. Additionally, the chapter introduces the integration of the aforementioned SDR-based GPR into an autonomous aerial drone (UAV). The performance of the GPR from the results obtained validates the possibility to integrate a lightweight radar system into a UAV.
In terms of GPR performance, the directional antennas radiated and received more power in a specific direction, which consequently increased the detection by means of reducing the interference caused by other sources. Also, thanks to the mathematical model derived for the
This work was supported by the Pontificia Universidad Javeriana (PUJ) in Bogota, Colombia, and was funded by the project: Modelling, Simulation and Optimisation of a Ground-Penetrating-Radar GPR On-Board a Drone. ID: 6802. We, the authors, would like to thank students E. Mendoza and L. Serrano for their contribution on the GPR design and testing.