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How Radar Works

This guide describes the fundamental principles and operation of radar.
The intended audience is radar users, system integrators and installers that wish to better understand how to install and operate commercial radar equipment to achieve maximum performance.

Definition of RADAR

The word RADAR is an acronym formed from the expression Radio Detection And Ranging. Common to all radars is the concept of extracting information from a reflected radio signal.

Basic principle of operation

RADAR is fundamentally an electromagnetic sensor used to detect and locate objects.
Radio waves are radiated out from the radar into free space. Some of the radio waves will be intercepted by reflecting objects (targets).
The intercepted radio waves that hit the target are reflected back in many different directions. Some of the reflected radio waves (echos) are directed back toward the radar where they are received and amplified.
With the aid of signal processing a decision is made as to whether or not a target echo signal has been detected. The target location and other information can then be acquired from the echo signal.

radar echo

Evolution of radar

The first step towards radar was a simple device to prevent collisions between ships, patented in 1904 by Christian Huelsmeyer. It used crude spark-gap transmitters similar to Marconi's early wireless equipment. Large metallic ships directly ahead of the equipment would increase the spark intensity and cause a bell to ring. Range to target could not be measured, but the principle of targets and echos was established.

The invention of radar is generally attributed to the British as they operated very early radar systems prior to, and during the Second World War. These military equipments were designed to detect and locate enemy ships and aircraft and played a decisive role in the Battle of Britain in 1940. Since then enormous investment has been made in military radar systems and electronic warfare equipment.

The steady evolution of military technology has resulted in smaller, more sophisticated and cheaper electronic components that have found subsequent uses for civilian applications. Many of the radar principles that have been proven and refined for military use can be directly applied to commercial radars.
Advances in microprocessor speeds coupled with the development of inexpensive radio components for mobile telephones means it is now possible to produce small sophisticated radar sensors with automatic target detection capabilities at a suitable price for cost-sensitive commercial applications.

chain home Early British radar systems used low frequencies so antennas had to be very large to detect distant aircraft - modern microwave radar is much more compact.

Why does radar use radio waves?

Parking sensors on cars use ultrasonic (sound) waves to assist when parking. However, ultrasonic and sound waves only travel at around 330 metres per second, so can only be used over very short ranges.
Radio waves are invisible electromagnetic waves that have no mass and travel at the speed of light, approximately 300,000,000 metres per second. The high velocity of electromagnetic waves is ideal for quickly travelling long distances to measure distant objects with minimal delay.

There are many different types of electromagnetic waves, such as infrared, X-rays and visible light. Radio waves are used for radar for a number of reasons:

  • It is simple and inexpensive to generate radio waves using electronic components.
  • Radio waves can pass through fog, rain, mist, snow and smoke.
  • Radio waves cannot be confused with infrared energy emitted by fire, heat haze, warm objects, hot gas or the sun.
  • Radio waves do not need light to work so radar can operate in total darkness as well as bright sunshine without performance being affected.
  • Radio waves are non-ionising so are safe unlike X-rays or gamma rays.

Radio waves have wavelengths between 10,000 km (30Hz frequency) to 1mm (300 GHz frequency). When smaller than 30cm (1 GHz and higher) they are referred to as microwaves. Many radar systems use microwaves because the antennas can be physically smaller as wavelength decreases. Depending on the application the radar designer will select the appropriate operating frequency for best performance.

electromagnetic spectrum

How radar measures range to target

Radars transmit invisible electromagnetic radio waves that travel at the speed of light, approximately 300 million metres per second. Although this is extremely quick, there will still be a brief delay between the transmission of the original signal and the reception of the echo. The time delay is directly proportional to the range to the target.
Long-range radars use very short pulses and measure the time difference between the original pulse and echo pulse to establish range to target. At shorter ranges a different technique (FMCW) is normally used where the radar constantly transmits but the frequency is modulated so there is a frequency difference between the echo signal and the instantaneous transmitted signal. The radar measures the difference in frequency, which is directly proportional to the range of the target.

In both cases, the radar makes a direct measurement of the echo signal to determine range to the object. Compared to optical systems where a large object at long range appears similar to a small object at close range, radar range measurements are not fooled by target size.

FMCW radar block diagram Basic block diagram shows a FMCW radar using two antennas to measure range to target by frequency comparison technique.

How radar measures size of target

Generally larger objects reflect more radio waves than smaller objects, however the target angle and shape also has an effect. Radar Cross Section (RCS) is the term used to describe the combination of shape and size and is usually expressed in square metres. Targets with higher RCS reflect more radio waves and cause a stronger echo signal to be detected by the radar, so this information can be used to aid in target classification. Although echo strength diminishes with increasing range to target, the radar knows the range from the echo so can compensate for this effect.

Typical RCS figures:

  • Human 0.5 square metres
  • Car 10 square metres
  • Building 10,000 square metres

In reality the RCS does vary to some extent based on the target angle, so a building or vehicle that is normal to the radar presents a large flat surface and has a slightly higher RCS than one that is angled so less reflected energy is directed back toward the radar.

Walking humans have an interesting characteristic where the swinging arms and legs causes the RCS to cycle higher and lower in sync with the walking motion.
Crawling humans have a lower RCS than walking humans because they have a physically smaller cross section. This can present some difficulties as RCS is similar to small wild animals.

F-117A Nighthawk Stealth military aircraft attempt to reduce their RCS by reflecting radio waves away from transmitting radar - note how surfaces are deliberately angled to reflect energy away from radar.

How radar measures speed of target

Target speed is measured directly by measurement of the Doppler frequency shift. The Doppler effect is a phenomenon that is regularly experienced even in everyday life. For example when a police siren is heard in the distance the tone changes and rises until the police car drives past and the tone starts to fall again.

For radar systems, the Doppler effect causes moving objects to shift the frequency of reflected radio waves based on the speed of the object. A Doppler shift is seen for objects moving radially, that is, directly toward or away from the radar. Doppler measurement is effective at detecting moving targets and ignoring targets that don't move, which is particularly important for ground surveillance radar where many reflections are seen from stationary targets.

doppler shift

How radar measures direction of target

Radar antennas typically have a narrow field of view that is scanned across a wider area. When a target is seen the direction in which the antenna is pointing corresponds to the direction of the target. In principle this is like using a telescope to determine the bearing of distant objects.

There are many possible antenna methods that can be used, with choice being determined by required size, weight, power and cost.
The simplest method is to physically rotate the antenna. When the radar sees the target echo, the direction of the antenna directly corresponds to the direction of the target.
Rotating antennas do have moving parts that can wear, however with clever engineering and use of very small and light materials, the expected lifetime can be extremely long.

Some radars have fixed antennas that are steered electronically, a so-called phased array, although this is often much more expensive than simple physical rotation. Another method uses two (or more) antennas to mathematically calculate the angle of arrival by comparing two (or more) echo signals. This method is cheaper than a phased array, but has limitations such as inability to distinguish multiple targets at the same distance and lower sensitivity.

telescope

How the radar maximum range is determined

There are three main factors that determine the maximum range:

  • The radar must receive sufficient echo energy to be able to make a detection.
  • The radar must have a direct line-of-sight to the target.
  • Limitations in the receiver circuitry may limit the range that can be measured (instrumented range).

Where the echo energy is the limiting factor, the radar will have different specified ranges for targets of different types, with higher RCS targets being able to be detected further away.
Where the limitation is due to the instrumented range the maximum range specification will apply equally to all sized targets.

Since microwaves do not bend round corners the target can only be seen if there are no large objects in the way. Mounting the radar higher up allows it to look over objects that are in the way.

Generally it is not possible to increase the transmitted power to attempt to increase the detection range, as the radio regulations are very strict to enforce a maximum power level. This rule is in place to prevent other radio users being negatively affected by interference caused by high power transmissions.

radar horizon At very long ranges the curvature of the Earth limits the so-called "radar horizon". For short range radars the line-of-sight is usually limited by buildings or obstructions.

How radars avoid interfering with each other

When multiple radars of the same type are used in close proximity there must be a method to avoid them causing mutual interference. A common technique is to synchronise equipments to a common clock then slightly offset each radar from its neighbours so they do not transmit on the same frequency at the same time.

Another technique is to simply use different operating frequencies that do not overlap at all. Although this doesn't require synchronisation, it severely limits the number of equipments that may be located nearby and is extremely wasteful of the allocated frequency spectrum.

How clutter affects radar performance

Clutter refers to sources of unwanted echoes generated by objects that reflect radio waves.

Clutter is caused by reflections from the ground. Any radar that detects targets on, or close to, the ground will see more clutter than radars that look upwards into the air, especially if the clutter moves.

In the ideal case, the ground would be a very flat concrete expanse with a target located in the middle. Unfortunately this is rare. Often there are fixed objects such as cars, posts, walls and fences that all contribute to the background clutter levels. Fixed clutter can mask the presence of a target by reflecting the radio waves before they can reflect off the target.

Radar signal processing tries to ignore fixed clutter either by filtering objects with no Doppler shift or by comparing the current scan to previous scans to identify fixed objects.
Even so, large fixed objects such as tall fences or buildings generate high clutter levels that make it difficult to detect much smaller targets that are next to the clutter due to an effect called scintillation where there are small changes in the echo amplitude of the large object.
Consider a large building with RCS of 10,000 square metres exhibiting 0.1% RCS change due to scintillation. This presents a background RCS variation of 10 square metres that is easily enough to swap the RCS of a walking human (1 sq. metre).
Orientating the radar so the building RCS is reduced will improve the situation.

Moving clutter, such as long grass, bushes, trees and water is very difficult to mitigate using signal processing. Moving clutter generates a Doppler shift and varies from scan to scan so cannot be distinguished easily from real targets. Radar systems will have a higher detection threshold in areas where there is lots of moving clutter to avoid excessive false alarms. The most effective way to improve the performance is to remove or reduce the moving objects that generate the clutter.

cluttered environment Radar will be swamped by the amount of clutter and largely unable to detect the person

The function of antennas in radar systems

The basic job of the antenna is to emit radio waves when fed with an electrical signal. Antennas are reciprocal, that is, they work just as well in reverse, so the antenna also captures radio waves and emits an electrical signal.
Radar may use a single antenna that is shared by transmitter and receiver or may have two antennas, one that transmits, another that receives. Usually pulse radars share a single antenna and FMCW radars use two.

Antennas focus radio waves much like a magnifying glass focuses light. Antenna gain is a measure of the energy increase due to the antenna focussing the radio waves. There is no amplification per se; the gain is simply from focussing more energy into a smaller area. Therefore increasing antenna gain naturally decreases beam width.

Antenna gain is influenced by physical dimensions and operating frequency. High frequency microwaves are often used for radar to achieve high antenna gain with physically small antennas.

The main beam width will be halved every time either antenna surface area or the operating frequency is doubled. Effectively this means low frequency radars need physically larger antennas for the same beam width as higher frequency radars that have much smaller antennas.
To allow comparison between different radar equipments a common antenna parameter is the -3dB beam width, that is the angle between the points where the antenna power is half that of the peak power measured on boresight.

antenna beam width The -3dB point is used as a figure-of-merit to compare antennas

How antenna sidelobes can affect radar performance

All antennas exhibit sidelobes, which are stray signals emitted or received in unwanted directions. It is not possible to produce theoretically perfect antenna that have a single narrow beam with no stray signals at other angles. In reality all antennas are expected to have some imperfections, but the amount of imperfection determines the degradation in system performance.

Sidelobes are characterised relative to the wanted energy of the main boresight beam. High sidelobe levels can cause false detections.

For example if a radar sensitivity is set to detect a walking human in the main beam, a sidelobe that has 10% (-10dB) of the energy of the main beam could falsely detect a large target with RCS that is 10x higher than a walking human, for example a large car.
Since the radar always assumes the target is seen in the main beam, the direction to the target will be wrong so would register as a false detection.
Clearly if the sidelobe is improved to be 0.1% (-30dB) of main beam energy then the RCS needed for a false detection must be 1000x greater than the human, which would rule out most vehicles, making the radar much less likely to generate false alarms.

Radar manufacturers tend not to specify sidelobes on product datasheets, but estimates can be made from typical antenna pattern graphs (if available) to judge if one system is significantly worse than another.

sidelobe plot Example plot shows excellent antenna sidelobes with amplitude rapidly reducing away from boresight (0 degrees).

How antenna beam width affects radar operation

Broad antenna beams see more targets (and clutter) at any moment than narrow beams. Narrower main beams aid the radar signal processor in distinguishing one target from another by increasing the angular resolution. Narrow beams also receive fewer reflections from the rest of the environment making it easier to ignore clutter at other bearings.
Radars using angle-of-arrival antennas are unable to use narrow beams, so may struggle compared to systems that rotate a narrow beam.
In simplistic terms imagine trying to listen to one person in a crowded noisy room. The angle-of-arrival system is similar to concentrating very hard to try to follow what is being said and to ignore all the other background noise. A narrow antenna is equivalent to blocking out much of the background noise to make it easier to hear the wanted conversation without requiring extraordinary levels of concentration.
Narrow beams are most useful in the azimuth (horizontal) plane.

How vertical beam spreading can improve performance

A spread beam is an antenna main beam that has been artificially broadened. This differs to a simplistic wide beam by being designed to distribute the radio waves in a specific pattern to achieve good performance at all ranges.
A spread beam cannot be usefully characterised by comparing -3dB points and is not usually symmetrical. This type of shaping is common for radar systems because the range to target has a large effect on the echo power (every time the range doubles the echo power reduces to 1/16 th of what it was), so spread beams distribute the antenna energy to compensate for this effect.
Beam spreading is usually in the elevation (vertical) plane only.

spread antenna beam The spread beam (red) has much better coverage than the simplistic broad beam (blue)

Radar operating frequency considerations

Radar users must be aware of the operating frequency and all applicable local and national radio regulations that may prohibit the use of equipment operating at certain frequencies. Broadly speaking, there are two types of frequency: licensed and unlicensed (license exempt).

  • License exempt frequencies are the simplest to understand as the equipment may be operated without first obtaining permission. The manufacturer certifies the equipment conforms to license-exempt conditions and the user operates it without further thought.
  • Licensed equipment requires the user to first obtain a license from the national radio regulator prior to operating the radar equipment. Licenses may provide benefits for some applications but in almost all cases there are additional fees that must be paid for the privilege.

Conclusions

All radars are based on a compromise between ideal performance and the requirements for size, weight, power consumption, cost and regulatory approval.
When comparing different radars from different manufacturers, it is important to consider what trade-offs have been made in the design and if they are compatible with the intended application.
For best radar performance there should be a clear line-of-sight to the target and a low level of clutter around the target.



Relevant application notes:

Advantages of Radar Controlled CCTV
Radar vs. Thermal Cameras
Difference between roadside and vehicle-fitted radar
Comparison of SVD technology

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