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Police Radar

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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

You're driving down the motorway and you're passed by a little red sports car. Two miles down the road, you see blue lights on a car parked just behind the red car. Have you wondered how the officer knew how fast they were going? How does the radar work? Is there anything that I can do to avoid police radar?

Doppler Radar

Police radar is a doppler radar. It measures speed by looking for a redshift or a blue shift in light, similar to the way astronomers measure the velocity and distance of stars.

The radar antenna emits a beam of light in the radio frequency range1. The light bounces off the target and then returns to the police radar antenna. The velocity of the target will change the frequency of the radar signal. That change in frequency is interpreted by the radar unit and shown to the officer as the target's speed.

Radar Case

For the officer to make a speeding case the following need to be established:

  • Jurisdiction
  • Date and time of the offence
  • Roadway on which the offence occurred
  • Posted speed
  • Identity of the vehicle and the operator
  • Tracking history
  • Radar reading

It's often helpful for the officer to include other information, such as weather and traffic conditions, and any statements made by the violator.

The officer testimony will typically be something like this:

On 30 December 2001 at about 8:27pm, I was operating stationary radar on Highway 1 near Main Street, in the city of Centre, Georgia. The area is posted as a 45 mile per hour zone. I noticed a red Saturn SL1 traveling east on Highway 1 at a high rate of speed. I activated my radar. It gave a high-pitched clear tone and indicated a speed of 62 miles per hour. I stopped the Saturn and made contact with the driver, Ms. Blank.

Some jurisdictions may require additional information, such as the calibration information on the radar, the officer's certification to operate the radar, information establishing why the violator's speed was unsafe, etc.


The most important part of a radar case is a tracking history. The radar unit will display a number, and that's all. It doesn't tell the officer which vehicle it is, or if there's even a vehicle there. The officer has to track the vehicle to make sure that their observations match what the radar is showing. Otherwise, the officer might stop the wrong vehicle, or a common radar error might give an incorrect speed. In some jurisdictions, the officer has to visually estimate the violator's speed within five miles per hour.

The radar beam is a cone. It doesn't pick out individual vehicles. It can't even pick out individual lanes. The radar shows a speed based on three factors:

  • Reflectivity
  • Position
  • Speed

This is generally referred to as biggest, closest, fastest. The radar usually picks up target that is the largest in its view. Therefore, it might pick up a motorcycle that was very close to it before a tractor-trailer a mile down the road. Many times the radar will display different speeds of different vehicles that are close together. The officer has to determine if a good reading is being obtained and, if so, which vehicle's speed is being displayed.

This isn't as hard as it might sound. Radars are equipped with a speaker that gives a tone reflecting the doppler signal it's receiving. If it's a clear, high-pitched tone, then it's getting a good solid reading from a vehicle. It will give a low raspy tone if it's not getting a clear signal. This happens when there's something between the radar and the target, or when the vehicle is entering or leaving the beam.

When you have a solid tone, you look at how the traffic is moving. If there is a clump of vehicles moving at 65mph, then a vehicle overtakes them at a high rate of speed and the radar shows 85mph, it's easy to figure out who was going that fast. Alternatively, a moving clump of vehicles (where no-one is overtaking or falling behind) will all be travelling at about the same speed.

Some radars have a fastest-vehicle button that will display the fastest vehicle in its cone. This is very useful for when there are large targets such as tractor-trailers in between the radar and a fast-moving, small vehicle.

Modes of Radar


Stationary radar is radar at its simplest. The officer sits on the side of the road and watches traffic. When a vehicle is observed moving at high speed, the officer activates the radar. The radar goes through its basic decision factors (reflectivity, position and speed), displays that speed and gives a tone. If the tone is clear and the displayed speed matches the officer's observations, a stop can be made.


Moving radar is very similar to stationary radar, but it looks for two different speeds. The radar looks for the largest object in its field, and it assumes that this is the passing background. Then it looks for the second most significant object, which it assumes is the target. The radar actually measures the closing speed or separation speed between the target and the patrol vehicle. The radar's counting unit will then use one of the following formulae:

  • Target Speed (TS) = Closing Speed (CS) - Patrol Speed (PS)
  • Target Speed (TS) = Separation Speed (CS) - Patrol Speed (PS)

The radar unit will then display two speeds. It will show the target speed and the patrol speed. The officer must compare the patrol speed displayed on the radar with that displayed on the car's speedometer. This is an essential element of the radar case. The radar speed will be more accurate, but there are certain errors that comparing the speeds will detect. The speeds must be consistent.

Same-direction Radar

Same-direction radar was developed when engineers were examining the shadowing error. Same-direction radar is based on very different principles to moving or stationary radar. It also requires more a complicated tracking history.

Basically, it figures out the patrol speed. Then it looks for the bounced reflection from the other vehicle and measures the relative speed between them. This makes things more complicated because the officer must decide to activate the radar and let the radar know if the officer or the target is moving faster.

Radar Errors

There are several things that will affect a police radar unit. There's a famous example of a lawyer aiming a radar at the courtroom wall and clocking it at 19 miles per hour. Radars will pick up interference from things other than vehicles - power lines and the patrol car's air-conditioner are the most common things that a radar will register. This is why training and experience are important. Officers will learn where the power lines are and how the radar will react to them.


Police radar uses part of the electromagnetic spectrum. They can be influenced by any number of electromagnetic and physical phenomena. For instance, targeting radars on fighters use the same frequencies. Air-conditioning units in patrol cars can create a reading (generally 32 mph). Some high power-lines can also set off radars (generally around 92 mph or 101 mph).

Officers must have a good tracking history to confirm that their observations are matched by the speed displayed by the radar. If an officer is travelling along a road with a 35 mph limit and sees a vehicle traveling at around 50 mph, and the radar displays 100 mph, they know that the result is bogus. An officer should know their beat well enough that they are aware of the common sources of interference.

Some forms of interference, such as air-conditioning units, will disappear when the radar detects an actual moving object. Its decision factors will ignore any signal as weak from the air-conditioner.

I bet you didn't think police work would involve trigonometry.

Cosine Error

Cosine error is when the radar antenna is at an angle to the target. Instead of coming straight towards the antenna, the target is moving across the beam. Some of the speed is lost.

Imagine that an officer is sitting at the right angle in the figure to the right. The target is moving at five blocks per minute, but because light travels in a straight line, it's only measuring the speed along line b. It loses one block per minute of speed.

Basically, this means that if an officer is sitting at an angle to the flow of traffic, the speed indicated will always be lower than the actual speed of the target. In stationary mode, it's always to the advantage of the violator.

In moving mode, a cosine error can reduce the computed speed of the patrol vehicle. So, when the counting unit computes the target speed with CS-PS=TS, the target speed will be higher than it should be. To counteract this, the officer needs to check the speedometer against the patrol speed displayed by the radar.


Masking is a rarely observed error where the radar antenna is pointed at the counting unit (the part of the radar that shows the speed).


Shadowing occurs when an officer is behind another moving object. Usually it will be something large such as a tractor-trailer. The radar will interpret the tractor-trailer as the background instead of the actual background. Therefore, when an officer is running moving radar, the patrol speed showed by the radar unit has to be checked against the speedometer. If they don't match then there may be a shadowing error.


Batching is when an officer is accelerating and activates the radar. Most modern radars have internal error-checking that prevent this from being an issue.


Scanning is when you swing a radar antenna across a background. It's possible to get the radar to show a speed this way, but it is difficult.

Other Potential Issues with Radar

Officer Training

An officer must be trained to operate the radar. It doesn't take much to figure out how the radar works, but it does take some training and experience. In many states, the officer will have to be licensed to operate the radar, and it will be an element of the case that the officer will make in court. Asking the officer for this permit on the side of the road is probably a waste of time.

Two-officer Teams

On some occasions, officers will act in teams. One officer will operate the speed-detection equipment, and the other will issue citations. This is particularly common when the police use aeroplanes to find speeders.

In order to obtain a conviction, the officer who operates the radar must appear in court to identify the violation. The officer who issues the citation must come to court to identify the driver. The officers must also be able to say how they passed the information about the violation between them.

Radar Detectors

A radar detector is just a radio receiver that flashes a light and makes a noise whenever it receives a signal in a certain frequency range. Isn't that very useful? The answer is 'maybe'...

Just as there are numerous things that a radar picks up as interference, there are a number of things that will activate a radar detector. Furthermore, most police radars are equipped with an instant-on feature. The officer will activate a radar whenever they identify a potential speeder. Therefore, there may be no signal for the detector to pick up until it's too late.

That's not to say that radar detectors don't have value. If you're travelling across level ground, then you may pick up the radar signal when the officer checks a driver in front of you.


Radars should be checked for accuracy occasionally. Under Georgia law, the officer has to check it at the beginning and end of each shift. The check for accuracy consists of the following:

  1. A light check. The officer presses a button on the radar, and all the LED lights illuminate.

  2. An internal circuit check, which is accomplished by pressing a button on the radar unit.

  3. Tuning fork check. Tuning forks that are tuned to vibrate at a certain frequency are put in front of the radar antenna. The radar unit will display a certain speed.

If the radar doesn't perform within the manufacturer’s specifications, it has to be removed from service until it can be repaired.

Radars also have to be calibrated by specially trained technicians, usually once a year.

Other Methods of Speed Detection

There are other methods to detect speed. The most common are LIDAR (laser) and pacing.

LIDAR is one of the most accurate and easy-to-use technologies. The equipment is similar to a radar, but it is aimed like a rifle. The officer can choose to determine the speed of a specified vehicle. He simply aims the device, pulls the trigger, and the unit displays the speed and distance travelled by the target. Some newer models also take a digital picture of the target.

Officers can also pace speeders using their speedometers. The officer maintains a constant distance from the violator and watches the speed over a certain distance. The violator is then cited with the lowest speed that the officer observed. This method depends on the accuracy of the officer's speedometer. Officers must be able to testify that the accuracy of their speedometer has been checked or use a radar to confirm the officer's speed when following the violator.

Other Entries in This Project



Theoretical Basics

Civilian Applications

Military Applications

Electronic Combat

1Frequency bands (USA, Canada, United Kingdom):
  • X band: 10.500 - 10.550 GHz
  • K band: 24.050 - 24.250 GHz
  • Ka band: 33.4 - 36.0 GHz
  • Laser: 904 nanometres

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