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Radar is an electromagnetic sensor used for detecting, locating, tracking, and recognizing objects of various kinds at considerable distances. It operates by transmitting electromagnetic energy toward objects, commonly referred to as targets, and observing the echoes returned from them. Energy is emitted in various frequencies and wavelengths from large wavelength radio waves to shorter wavelength gamma rays. Radar is an “active” sensing device in that it has its own source of illumination (a transmitter) for locating targets. It typically operates in the microwave region of the electromagnetic spectrum—measured in hertz (cycles per second), at frequencies extending from about 400 megahertz (MHz) to 40 gigahertz (GHz).
Ground-penetrating radar (GPR) is a geophysical method that uses radar pulses to image the subsurface. It is a non-intrusive method of surveying the sub-surface to investigate underground utilities such as concrete, asphalt, metals, pipes, cables or masonry. This nondestructive method uses electromagnetic radiation in the microwave band (UHF/VHF frequencies) of the radio spectrum and detects the reflected signals from subsurface structures. GPR can have applications in a variety of media, including rock, soil, ice, freshwater, pavements and structures. In the right conditions, practitioners can use GPR to detect subsurface objects, changes in material properties, and voids and cracks
GPR works by sending a tiny pulse of energy into a material and recording the strength and the time required for the return of any reflected signal. A series of pulses over a single area make up what is called a scan. GPR detects buried objects by emitting radio waves (ranging from about 10 MHz to a few GHz) into the ground and then analyzing the return signals generated by reflections of the waves at any subsurface discontinuity with different indexes of refraction such as at the boundary between soil and a landmine or between soil and a large rock.
Reflections are produced whenever the energy pulse enters into a material with different electrical conduction properties or dielectric permittivity from the material it left. Generally, reflections occur at discontinuities in the dielectric constant, such as at the boundary between soil and a landmine or between soil and a large rock. The strength, or amplitude, of the reflection, is determined by the contrast in the dielectric constants and conductivities of the two materials. This means that a pulse that moves from dry sand (dielectric of 5) to wet sand (dielectric of 30) will produce a very strong reflection while moving from dry sand to limestone will produce a relatively weak reflection.
While some of the GPR energy pulses is reflected back to the antenna, energy also keeps traveling through the material until it either dissipates (attenuates) or the GPR control unit has closed its time window. The rate of signal attenuation varies widely and is dependent on the properties of the material through which the pulse is passing.
Materials with a high dielectric will slow the radar wave and it will not be able to penetrate as far. Materials with high conductivity will attenuate the signal rapidly. Water saturation dramatically raises the dielectric of material, so a survey area should be carefully inspected for signs of water penetration. Metals are considered to be complete reflectors and do not allow any amount of signal to pass through. Materials beneath a metal sheet, fine metal mesh, or pan decking will not be visible.
GPR system
A GPR system consists of an antenna or series of antennas that emit the waves and then pick up the return signal. The GPR analyzes the return signals. A small computerized signal-processing system then interprets the return signal to determine the object’s shape and position.
A GPR system is made up of three main components: Control unit, Antenna, and Power Supply. GSSI GPR equipment can be run with a variety of power supplies ranging from small rechargeable batteries to vehicle batteries and normal 110/220-volt. Connectors and adapters are available for each power source type. The unit in the photo above can run from a small internal rechargeable battery or external power.
GPR Control Unit and Antenna
The control unit contains the electronics which trigger the pulse of radar energy that the antenna sends into the ground. It also has a built-in computer and hard disk/solid state memory to store data for examination after fieldwork. Some systems, such as the GSSI SIR 30, are controlled by an attached Windows laptop computer with pre-loaded control software. This system allows data processing and interpretation without having to download radar files into another computer.
The antenna receives the electrical pulse produced by the control unit, amplifies it and transmits it into the ground or other medium at a particular frequency. Antenna frequency is one major factor in depth penetration. The higher the frequency of the antenna, the shallower into the ground it will penetrate. A higher frequency antenna will also ‘see’ smaller targets. Antenna choice is one of the most important factors in survey design. The following table shows antenna frequency, approximate depth penetration and appropriate application.
Radar energy is not emitted from the antenna in a straight line. It is emitted in a cone shape (picture on left). The two-way travel time for energy at the leading edge of the cone is longer than for energy directly beneath the antenna. This is because the leading edge of the cone represents the hypotenuse of a right triangle.
Since it takes longer for that energy to be received, it is recorded farther down in the profile. As the antenna is moved over a target, the distance between the two decreases until the antenna is over the target and increases as the antenna is moved away. It is for this reason that a single target will appear in the data as a hyperbola, or inverted “U.” The target is actually at the peak amplitude of the positive wavelet.
Three-dimensional imaging
Individual lines of GPR data represent a sectional (profile) view of the subsurface. Multiple lines of data systematically collected over an area may be used to construct three-dimensional or tomographic images. Data may be presented as three-dimensional blocks, or as horizontal or vertical slices. Horizontal slices (known as “depth slices” or “time slices”) are essentially planview maps isolating specific depths. Time-slicing has become standard practice in archaeological applications, because horizontal patterning is often the most important indicator of cultural activities
Vehicle localization
A recent novel approach to vehicle localization using prior map based images from ground penetrating radar has been demonstrated. Termed “Localizing Ground Penetrating Radar” (LGPR), centimeter level accuracies at speeds up to 60 mph have been demonstrated. Closed-loop operation was first demonstrated in 2012 for autonomous vehicle steering and fielded for military operation in 2013. Highway speed centimeter-level localization during a night-time snow-storm was demonstrated in 2016.
Current ADAS features enabled by lidar and camera-based systems for autonomous vehicles are accurate to a certain degree but lack the reliability to deliver a consistently safe journey. Common driving conditions such as inclement weather, debris in the road, lack of clear lane markings or strong GPS signals can render the typical sensors useless and force drivers to take over — sometimes with little notice — or in the case of an AV, disengage, said WaveSense CEO Tarik Bolat.
Considering these difficulties, there is a lack of consumer confidence in advanced ADAS capabilities, with a 2021 AAA survey reporting that 80% of drivers wanted “current vehicle safety systems, like automatic emergency braking and lane keeping assistance, to work better.” To meet the needs of today’s drivers, automated and autonomous vehicles need WaveSense’s ground positioning technology to help mitigate common issues, deliver automotive grade reliability, and increase consumer confidence in ADAS programs.
The issue with today’s ADAS technologies such as lidars and cameras is that they rely solely on visible, static surface features like signs, buildings, or lane markings amid dynamic environments that are not always predictable—resulting in features that are hamstrung by their unreliability. Ground Positioning Radar (GPR) technology differentiates itself by peering directly into the Earth, which is very rich in features and stable over long periods of time, and provides centimeter-level precise positioning anywhere no matter what the conditions are on the surface. By adding WaveSense’s GPR technology, automakers are enhancing their vehicles with more reliable and accurate ADAS features—including autonomous parking and active lane keeping—safeguarding the automated driving experience.
Integration of WaveSense is straightforward in that it delivers a robust position that the vehicle uses – akin to a GPS that has cm level accuracy nearly all of the time – making vehicle localization a reality even in the most challenging road conditions. What this means for automakers is that it can be the primary positioning sensor going forward and will be complemented by the other more standard sensors in the stack. And unlike cameras and lidar, which fail under similar circumstances, WaveSense is uncorrelated from any other inputs, driving new levels of robustness since the likelihood of a common point of failure becomes vanishingly small.
Defense and Security Applications
Security, emergency measures and military uses of GPR abound. GPR’s unique sensitivity to non-metallic structures embedded in soils, rocks and building materials results in GPR seeing use in diverse applications such as search and rescue, tunnel location, intrusion detection, UXO, landmine and buried IED detection.
Military uses of GPR focus primarily on the location and detection of buried explosive devices. For area clearing, GPR is used on ranges and old sites to identify unexploded ordnance (UXO). More recent live campaign applications involve the real-time location and identification of buried improvised explosive devices (IEDs) and buried fusing mechanisms.
Security uses of GPR are wide-ranging. A common application is the location of embedded wires and cables in structures. Location of buried bunkers, tunnels and buried caches are areas of growing interest. The ability to sense human motion through walls and underground sees GPR being used for intrusion detection.
Wall-penetrating radar can read through non-metallic structures as demonstrated for the first time by ASIO and Australian Police in 1984 while surveying an ex Russian Embassy in Canberra. Police showed how to watch people up to two rooms away laterally and through floors vertically, could see metal lumps that might be weapons; GPR can even act as a motion sensor for military guards and police.
In May 2020, the U.S. military ordered ground-penetrating radar system from Chemring Sensors and Electronics Systems (CSES), to detect improvised explosive devices (IEDs) buried in roadways, in $200.2 million deal.
Husky Mounted Detection System (HMDS) is a counter-IED system
Husky Mounted Detection System (HMDS) is a counter-IED system able to detect underbelly IEDs and antitank landmines buried in primary and secondary roads. As a result, the HMDS is vital to route clearance packages (RCP). The system is a combination of the CSES VISOR 2500 ground-penetrating radar and the Husky vehicle from Critical Solutions International Inc. in Carrollton, Texas.
The HMDS helps the Army quickly clear roadways of anti-tank mines, roadside bombs, and other IEDs. The CSES VISOR 2500 ground-penetrating radar detects metallic and non-metallic explosive hazards, pressure plates, and antitank mines. It combines advanced real-time automatic-target-recognition algorithms, integrated metallic and non-metallic threat detection, automatic precision marking, and software in a ruggedized, supportable package.
CSES’s multi-panel high-performance VISOR GPR system functions on manned, blast-resistant vehicles to provide rapid ability to scope out anti-vehicle landmines and other explosive hazards on main supply routes and additional open areas as needed, CSES officials say. CSES’s ground-penetrating radar and an optional metal detector, when mounted on manned, blast-resistant vehicles, provides a rapid ability to scope out anti-vehicular landmines or any other type of buried explosive hazard.
Limitations
The most significant performance limitation of GPR is in high-conductivity materials such as clay soils and soils that are salt contaminated. Performance is also limited by signal scattering in heterogeneous conditions (e.g. rocky soils).
Other disadvantages of currently available GPR systems include:
- Interpretation of radar-grams is generally non-intuitive to the novice.
- Considerable expertise is necessary to effectively design, conduct, and interpret GPR surveys.
- Relatively high energy consumption can be problematic for extensive field surveys.
- Radar is sensitive to changes in material composition, detecting changes requires movement. When looking through stationary items using surface-penetrating or ground-penetrating radar, the equipment needs to be moved in order for the radar to examine the specified area by looking for differences in material composition. While it can identify items such as pipes, voids, and soil, it cannot identify the specific materials, such as gold and precious gems. It can however, be useful in providing subsurface mapping of potential gem-bearing pockets, or “vugs.” The readings can be confused by moisture in the ground, and they can’t separate gem-bearing pockets from the non-gem-bearing ones.
When determining depth capabilities, the frequency range of the antenna dictates the size of the antenna and the depth capability. The grid spacing which is scanned is based on the size of the targets that need to be identified and the results required. Typical grid spacings can be 1 meter, 3 ft, 5 ft, 10 ft, 20 ft for ground surveys, and for walls and floors 1 inch–1 ft.
The speed at which a radar signal travels is dependent upon the composition of the material being penetrated. The depth to a target is determined based on the amount of time it takes for the radar signal to reflect back to the unit’s antenna. Radar signals travel at different velocities through different types of materials. It is possible to use the depth to a known object to determine a specific velocity and then calibrate the depth calculations.
Mine detection technology
Ground-penetrating radar uses a variety of technologies to generate the radar signal: these are impulse, stepped frequency, frequency-modulated continuous-wave (FMCW), and noise. Systems use Digital signal processing (DSP) to process the data during survey work rather than off-line.
A special kind of GPR uses unmodulated continuous-wave signals. This holographic subsurface radar differs from other GPR types in that it records plan-view subsurface holograms. Depth penetration of this kind of radar is rather small (20–30 cm), but lateral resolution is enough to discriminate different types of landmines in the soil, or cavities, defects, bugging devices, or other hidden objects in walls, floors, and structural elements. GPR is used on vehicles for close-in high-speed road survey and landmine detection as well as in stand-off mode
In collaboration with partners from South America, engineers at the German Ruhr-Universität Bochum and Technical University Ilmenau are developing a new mine clearance technology, based on ground-penetrating radar. In the long run, they are aiming at creating a handheld device that will detect different mine types on rough terrain without fail and which can be used in the same way as metal detectors.
Lionheart and his team are developing ways to reduce the number of false positives when searching for mines. “So in a way, our challenge is not so much to find mines, but to detect that something’s not a mine,” he says. For example, it is common for landmine clearance teams to use metal detectors to locate the firing pin and metal percussion caps present in many landmines. Currently, all those bits of metal have to be dug out of the ground before an area can be declared safe and, according to Lionheart, this approach would mean of the order of hundreds of years before people could get their land back.
To improve the situation, Lionheart and his colleagues have developed the technology and the underlying maths of metal detectors to develop devices that can not only detect but also characterize metal objects in the ground. This makes it possible to disregard the signals that relate to harmless bits of scrap metal. Similarly, Lionheart’s team is developing a form of ground-penetrating radar with multiple sensors. This enables a far more detailed picture of the subsurface to be pieced together than is possible with conventional radar techniques.
Ground Penetrating Radar Market
The ground penetrating radar market (GPR) is expected to reach USD 726 million by 2024 from USD 493 million by 2019 at a CAGR of 8.1% from 2019 to 2024. The major key factors driving the growth of this market are concerns related to safety and protection of underground utilities, advantages of GPR systems over other traditional technologies, and government support for deployment of GPRs, among others.
Underground cables and pipelines are important to contemporary infrastructure and it is important that these buried assets such as electrical, telecoms fiber-optic cables, water and sewerage pipes, and gas pipelines are protected from future excavations. If these assets are not protected then they can be results in large disasters such as electrical arcs. Thus to protect these assets the demand of ground penetrating radar system is continuously increasing on global level. This increase demand can drives the growth of ground penetrating radar market.
The ground penetrating radar market for services is expected to grow at a higher CAGR during the forecast period. This growth is due to factors such as the rise in the practice of procuring GPR systems on a rental basis owing to their high cost of ownership, continuous innovation in GPR, and imposed regulations and standards by the government for using a suitable detecting device before commencing digging, demolishing, or constructing any infrastructure.
Cart-Based GPR systems are expected to account for the largest share of GPR market for the forecast period. These systems enable the addition of more antennas and display units. They can collect data in any environment for any application. The cart-based GPR equipment provide reliable and high resolution data about depths of subsurfaces.
IDS Georadar (Italy), Sensors & Software Inc. (Canada), Guideline Geo (Sweden), Chemring Group (UK), Geophysical Survey Systems, Inc. (US), Leica Geosystems AG (Switzerland), Radiodetection (UK), Penetradar Corp. (UK), Utsi Electronics Ltd. (UK), Hilti ( Liechtenstein), Pipehawk PLC (UK), and Geoscanners (UK) are among a few major players in the ground penetrating radar market.
