Radar (english)

Thomas Hochhuber, summer semester 2025

RADAR stands for Radio Detection and Ranging. It is a technique using electromagnetic waves for detecting and imaging of structures and determining the position and velocity of objects.^[1]

Introduction:

RADAR systems send out electromagnetic pulses in the radio frequency via an antenna. Objects in the observed area reflect portions of the waves, a computer records the intensity of the reflection and the time from sending to receiving reflected signals. With this data, distances to objects can be calculated based on the waves travel time to the object and back and differentiated from their surroundings by different reflection signals.
First RADAR systems were developed during the early 20^th century for civil use to detect metallic objects like ships to avoid collisions. Military research from the 1930s onwards led to significantly improved capabilities, size and variants of radar devices and allowed great impact of radar systems on the Second World War.^[2]
Today, RADAR is also used in many car driver assistance systems and body scanners. Dedicated variants called ground-penetrating Radar (GPR) are used in geological applications and in civil engineering as a non-destructive testing method for buildings and underground structures.

Physical basics:

RADAR Systems primarily use electromagnetic radiation in the radio spectrum, comprising wavelengths from 1 m up to 100 km (frequencies from 300 MHz to 3 kHz). The propagation of electromagnetic waves follows the maxwell equations, which describe the interaction of electric and magnetic fields.^[14]


Figure 1: Radio frequencies in the electromagnetic spectrum. Source: Thomas Hochhuber, adapted from leifiphysik.de.^[4],[14]

In Vacuum, radio waves propagate with the speed of light, c₀ = 299.792.458 m/s. In other mediums, the velocity is reduced due to interactions with the material, depending on the wavelength and the material specific dielectricity ε and the magnetic permeability µ.

If the difference in ε of two materials is significant, electromagnetic waves can be reflected and refracted on the interface.

Electrical conductivity σ characterizes free charge movement. Charge displacement results in energy storage in the material, which dampens the incoming electromagnetic wave. The higher the conductivity, the stronger the Damping δ, and the weaker the reflected signal.

In ground penetrating radar applications ε and σ are performance relevant, while µ only plays a minor role. In practice, radar is most useful in low-electric-loss materials. High conductivity is present in metals and water-containing materials, reducing the wave penetration. Additionally, spherical spreading as a common wave phenomenon distributes the pulse energy over a quadratically increasing area with increasing travelling distance, leading to a quadratically decreasing pulse amplitude. Parts of the pulse are also scattered or deflected on imperfections and small particles in the material, further decreasing the amplitude and blurring the response signal. This limits the operational depth of radar systems.^[3]

Principle of the measurement:

Most Radar methods depend on detection of reflected signals (echo radar). Short radio pulses are sent via a transmitting antenna (transmitter) and propagate spherically from the source, then time is measured until the reflected or scattered pulse can be detected by a receiving antenna (receiver). A computer then calculates the distance to the reflecting object. To locate an object, multiple measurements at different locations must be taken, so the location can be interpolated from individual distances. For correct interpretation of the travel times, the ε-Value of the material must be known. This is often done with standard values for materials like dry concrete or gravel. Moving over an object like a pipe as shown in figure 2 causes a characteristic reflection hyperbola in the radargram. Together with the top and bottom layer reflections, it is the basis for determining its exact placement in the specimen. In the radargram, the measurement path is plotted on the X-axis, the runtime of the reflections on the Y-axis. The location of the hidden Object can be determined at the top of the shown hyperbola.


Figure 2: Radargram of a cross-section from a concrete specimen. Source: NDT Wiki (NDT-CC licence).^[7]


Figure 3: Handheld GSSI Concrete Scanner. Radar System with integrated monitor showing interpreted measurement as cross section picture at the TUM Chair of Non-Destructive Testing. Source: Thomas Hochhuber.

Transmission (Transillumination) radar works by placing the transmitter on one side of the object to be examined, then measuring the scattered signal on the other side. Metallic objects or reflecting interfaces reduce the transmitted signal.^[5]

Resolution

The resolution of a radar measurement is divided into horizontal and vertical resolution in respect to the examined surface. Horizontal resolution describes how big objects must be to be detected and how close two objects can be located to still be differentiated. It strongly depends on the wavelength used, with smaller wavelengths allowing higher resolutions (smaller r). The maximal possible resolution, called Rayleigh resolution is λ/4. The horizontal resolution decreases with increasing measurement depth, due to scattering of waves, as can be seen in the following formula:

$\begin{array}{l}r=\sqrt{\frac{λ*z}{2}+\frac{λ^2}{16}}\end{array}$

r = horizontal resolution [m]
λ = wavelength [m]
z = measuring depth [m]

Vertical resolution describes how precise the depth of a reflecting object can be determined. It mostly depends on the materials’ permittivity. High permittivity allows for good vertical resolution. Furthermore, the pulse length T of the transmitted signal is of great importance. The following formula applies:^[6]

$\begin{array}{l}d= \frac{T*c}{2 \sqrt{ε_r}}\end{array}$

d = vertical resolution [m]
T = Transmitted pulse length [ns]
c = speed of light [m/ns]
ε_r = permittivity

Penetration

The penetration depth is primarily determined by damping in the material. Damping is frequency dependent and based on conductivity σ, and polarizability of the material ε. Magnetic permittivity is less relevant in geological applications, unless there is a significant amount of magnetic minerals like magnetite in the ground. In low conductivity ground materials like dry sand, a depth of investigation (DOI) up to 50 meters can be reached with long wavelengths. Measurements in rock salt achieved several hundred meters.^[5] In ice, depths of up to over 100 meters can be investigated. The presence of liquid water with ions and metals significantly reduces penetration depth by absorbing electromagnetic radiation. Most environments limit the penetration depth to less than 20 times the wavelength, so most systems have measuring depths between several centimeters up a few meters, with wavelengths chosen for a good compromise between resolution and penetration.^[5],[8],[12]

Types of Radar systems

Pulse Radar

This most common application works in the time domain as shown in the example for the measurement principle. Short pulses in the range of nanoseconds are emitted and reflected signals are detected and interpreted by a computer. The unit is typically mounted on a cart to be moved over the survey site. Only with multiple measurements from different locations, a 3D model of the underground can be calculated. One single scan without moving only gives distances to the dominant reflecting objects / interfaces, but no spatial distribution.


Figure 4: GSSI Pulse Echo Radar Antenna for Ground surveillance at the TUM Chair for Non-Destructive Testing. Source: Thomas Hochhuber.

Radar Interferometry

This method uses two different transmitter antennas at fixed positions relative to each other and the receiver. Receiving two signals from known emitters allows to precisely triangulate the location of reflective objects without the need to move the measurement device. This is especially helpful for detecting moving objects or small, complex structures in the centimeter range.

FMCW Radar

FMCW stands for Frequency Modulated Continuous Wave. It is a continuous wave radar, meaning the emitter is always active. During the measurement, the working frequency is swept over a fixed bandwidth. This enables higher penetration depths while keeping a high resolution near the surface by using optimal wavelengths for both tasks and combining the results. The received signal is recorded in the frequency domain and must be converted in the time domain to get usable results.^[3]^,[10]

SFCW Radar

SFCW stands for Stepped Frequency Continuous Wave. It is very similar to FMCW Radar systems, except the Stepped-frequency-method changes the used frequency stepwise while leaving the amplitude of the signal constant. This method combines results from scans with multiple well-defined wavelengths and thus provides better resolution and penetration than a radar with only one frequency or swept frequency range. It is the most expensive but often also the more capable GPR System.^[3]


Figure 5: Handheld proceq GP8000 SFCW Radar Antenna for Concrete-GPR at the TUM Chair for Non-Destructive Testing. Source: Thomas Hochhuber.

Ground Penetrating Radar Applications

Ground Penetrating Radar Systems are used in Geology and in Civil Engineering. They are optimized for working in solid ground like concrete or rocks. Depending on the task, different types of radar systems with frequencies ranging from 25 MHz for deep ground surveillance up to 15 GHz, which is already in the microwave spectrum. for detecting small, shallow buried objects like cables in civil engineering.^[9]
Most of them use frequencies from 100 MHz to 1500 MHz.^[7]

Typical Applications for GPR Systems are listed below:^[8]^,[10]^,[11]^,[13]

Geological Applications

(between 25 MHz and 100 MHz)

Mapping bedrock configurations
Locating groundwater levels
Determining ice sheet thickness
Locating lava tubes
Exploring resource deposits

Archaeological Applications