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The History of Radar | Radar History: Isle of Wight Radar During The Second World War | Radar: The Basic Principle
Radar Technology: Main Components | Radar Technology: Side Lobe Suppression | Radar Technology: Airborne Collision Avoidance
Radar Technology: Antennas | Radar Technology: Antenna Beam Shapes | Radar Technology: Monopulse Antennas | Radar Technology: Phased Array Antennas | Radar Technology: Continuous Wave Radar | Theoretical Basics: The Radar Equation
Theoretical Basics: Ambiguous Measurements | Theoretical Basics: Signals and Range Resolution
Theoretical Basics: Ambiguity And The Influence of PRFs | Theoretical Basics: Signal Processing | Civilian Radars: Police Radar | Civilian Radars: Automotive Radar | Civilian Radars: Primary and Secondary Radar
Civilian Radars: Synthetic Aperture Radar (SAR) | Military Applications: Overview | Military Radars: Over The Horizon (OTH) Radar
How a Bat's Sensor Works | Low Probability of Intercept (LPI) Radar | Electronic Combat: Overview | Electronic Combat in Wildlife
Radar Countermeasures: Range Gate Pull-Off | Radar Countermeasures: Inverse Gain Jamming | Advanced Electronic Countermeasures
The idea to mount a radar on a car isn't a new one. Since the 1960s, various ideas have been developed and tested, but the devices were either too bulky or too expensive for mass production. At the time of writing, several manufacturers around the world have developed small and lightweight devices, and efforts at making them affordable for the average driver are underway.
The main advantage of radar over competing devices, such as laser or infrared vision equipment, is the radar's ability to look through rain, fog and snow. Current devices are concentrating on Adaptive Cruise Control (ACC) and collision warning, with features such as 'collision avoidance' or 'autonomous driving' still on the list of things to come.
Functions of Automotive Radars
Navigation - the radar indicates road bends and intersections, even in bad weather. Coupled with an electronic road map and perhaps a GPS receiver, it is also able to give directions to your destination.
Collision warning - the radar continously scans the area ahead of the car and takes appropriate action when a collision is imminent. A collision may occur with stationary objects found on the road, with cars driving in the opposite direction, or with a car driving in the same direction but slowing down. 'Appropriate' action can be the activation of an alarm or even the hitting of the brakes. However, implementation of the latter function will depend on local legislation. Usually, all responsibility rests with the driver of the vehicle but, if such a drastic interaction is done without the driver's consent, they may plead 'not guilty' and claim lawsuits against the car or radar manufacturer.
Cruise control - the radar maintains either a preset and constant speed, or a constant distance to the car ahead.
Airbag pre-crash sensing - if it is determined that a collision is imminent, then the radar may set off the airbag with an optimised timing, even before the hostile object has made contact with the car body.
'Stop and go' functionality - this is an adaptation of cruise control for lower speeds in dense urban traffic. The driver may relax and read a newspaper while an observer would believe the car was being towed by the car in front.
Parking aid - a minimum clearance between the boundaries of the car and any other object can be maintained because the radar starts buzzing and blinking well before contact is made. Depending on the implementation, even the car's sides can be put under surveillance.
There are two sets of requirements for an automotive radar. Cruise control and other 'highway' functions demand the capability to measure unambiguous ranges of around 200m and velocities from zero up to some 500km/h if a scenario of two Porsches shortly before collision were set as the worst case. The radar should cover some 8° to 15° either side of the front direction, to be aware of adjacent lanes and to follow road bends. A phased array antenna would be desirable because mechanically-scanned antennae are subject to wear and tear. However, phased array antennae operating in the frequency band 76 - 77GHz are still quite expensive. For that reason, arrangements using several fixed antenna beams and performing monopulse evaluation between the outputs of any two of them are also under consideration. Angular resolution1 is a critical parameter and some 0.1° are deemed necessary.
The second set of requirements applies to traffic in urban environments. Velocities from zero to 140km/h can be expected. Angular resolution is less critical but the radar must cover significantly more than some 15° of azimuth when driving in a stop-and-go situation. Full 360° azimuth coverage is required when the radar is supposed to sense a side-crash too. The maximum range is of the order of 50m, but distances of a few centimetres must be measurable when the radar is used as a parking aid.
Hot candidates for the signal to be employed are frequency modulated continuous wave, high pulse repetition frequency and, most prominently, a combination of a 'simple' pulse and a 'chirp' because they all are suitable for detecting moving targets against a background of stationary ground clutter.
As promising as the prospects are, there are some problems to overcome before a device can be called ready for practical use.
Big and Small Targets
Lorries are big targets2, motorbikes and bicycles are small ones. The radar must be able to see them both, especially when an approaching lorry is being overtaken by a motorbike and the echo of the latter appears before the big return originating from the lorry. An even more complex task for the signal processor is when the same situation happens within a tunnel where the concrete walls are the most prominent source of reflection.
Detection of Obstacles
At first glance, anything that features different reflection properties than the concrete of the road must be either another vehicle or something dangerous. Examples are sheets of ice, rabbits, kangaroos or goods that have fallen off some lorry.
However, this approach is too simple. Road surfaces can exhibit lots of non-regular features that a radar can easily detect, but do not represent danger. Such non-obstacles are, among others: road markings (especially within construction areas), changes of the material the road surface is made of, duct covers, and metal bars that protect the air gap where a bridge begins or ends. A device that announces every such anomality as an obstacle would not be successful on the market because of its high false alarm rate.
Layers of hot air and water puddles reflect radar signals just like they reflect light. Hence what the radar 'sees' is a upside-down mirror image of the scene further ahead, rather than the puddle itself. Therefore a puddle can be easily mistaken for a very deep hole in the road.
On the other hand, the signal processing may take advantage of mirages because they provide a means to 'look around' the car in front by examining the road under its belly from a grazing angle.
Bridges and Tunnels
Imagine a scene where the road runs over the top of a hill and there's a bridge that spans the road at right angles. At first glance, the bridge would appear like a wall that has been placed across the road. The free space under the bridge (or the hole that makes up the tunnel) becomes visible only at closer distance. There isn't much time for the radar to take a decision if you consider speeds in excess of 250km/h, which aren't uncommon on Autobahns in Germany. Without second source information this situation will lead to a false alarm. The second source could be a digital road map stored in the vehicle's navigation system with extra records of these kind of features.
Obviously, one specific feature is lacking from all automotive radars currently in development: no manufacturer has included the capability to warn against police radar. Thus, the avoidance of speeding tickets is still left to the driver's responsibility.
No matter how far a radar can see through fog and haze, it doesn't influence the quality of a car's brakes or the friction between tyre and road. Like any other part of a car, the radar can break at any time without warning, and no manufacturer is able to guarantee that their equipment has 100% detection capability. Thus, you should take the information derived from a radar as a means to increase safety, not speed.
It is interesting to note that bats' acoustic sensors have long been employing continuous wave and chirped pulses. Moreover, bats have also developed suitable means to avoid mutual jamming when hundreds of them are simultaneously active in a given place. Radar manufacturers still have to think about that.
Other Entries in This Project
- Basic Principle
- Main Components
- Signal Processing
- Side Lobe Suppression
- Phased Array Antennae
- Antenna Beam Shapes
- Monopulse Antennae
- Continuous Wave Radar