RAdio Detection And Ranging (RADAR) is the most common type of speed measuring device in Virginia. Unlike LIDAR, which is "point and shoot," RADAR requires training and experience to produce reliable results.

How Does Police RADAR Work?

Modern police radar in Virginia emit microwaves at an angle of approximately twelve degrees for a distance of over 4,000 feet (or until contact is made with an object). When the microwaves bounce off of a moving object, the waves' frequency shifts either up or down (this is called Doppler shift). The more altered the returning frequency, the higher the relative velocity of the object.

What is Stationary RADAR?

Stationary RADAR, is when an officer uses their radar while their police cruiser is stationary. The normal police speed trap uses stationary radar. Most speeding tickets in Virginia involve stationary radar.

What is Moving RADAR? / How Does Moving RADAR Work?

Most RADAR devices can be used while the officer's cruiser is moving. In moving mode, the RADAR device must measure the cruiser's ground speed as well at the relative speed of the target vehicle. Most RADAR devices accomplish this by using two different radar frequencies. One frequency measures the speed of all moving objects within the RADAR's beam and the second frequency is dedicated to measuring the relative speed of the ground directly in front of the police cruiser.

What is The RADAR "Hot Spot"?

The small portion of the moving Radar beam dedicated to measuring ground speed in front of a cruiser is called the "hot spot". Most of the moving mode RADAR errors are caused by interference to the hot spot.

RADAR Error: Moving Cosine Error

All RADAR devices are designed to measure the speed of an object that is traveling directly towards the RADAR device or directly away from the RADAR device. Any time the target vehicle is moving at an angle to the RADAR device the measured speed will be lower than the real speed. In stationary mode, this error benefits the drivers, but in moving mode this may cause an erroneously high ticket.

RADAR Error: Shadowing

When a RADAR device is operating in moving mode, any error that affects the RADAR's ground speed measurement also affects the accuracy of the overall speed measurement process. When the hot spot (the portion of the beam dedicated to measuring ground speed) locks onto a moving object (like another car), it creates an incorrect speed reading called a "shadowing error".

RADAR Error: Target Identification Error

The most common problem with RADAR units is that they detect and measure all objects in the radar beams but only display one or two speed results at a time. After a RADAR unit displays the speed of an object, it is the police officer's job to decide which vehicle is responsible for the speed displayed. Establishing a tracking history is essential to guaranteeing that the officer has pulled over the correct vehicle.

RADAR Error: Rapid Changes in Speed

RADAR units cannot track a target vehicle or the cruiser's own ground speed if that speed is changing too rapidly. Some units cannot accurately measure speed if the cruiser or the target vehicle is accelerating or decelerating more than one mph every .1 – 2.0 second (this is particularly a problem with older units).

RADAR Error: Harmonic Frequency Errors

Large targets such as trucks and reflective road signs close to the radar unit or the target vehicle can cause microwave echoes or "harmonics". Reflecting off of multiple moving objects causes multiple shifts in the radar frequency and creates inaccurate speed readings.

RADAR Error: Auto Locking

In some older RADAR units, when a speeding target is detected, an alarm sounds and the RADAR unit only displays the target's speed at the time it triggered the auto-lock. This feature is very unreliable because an erroneous signal may cause a high reading for a split second, thus triggering the auto-lock. Most law enforcement agencies around the U.S. have banned auto-lock features and have banned RADAR devices that auto-lock as their default setting.

These devices are not approved for use in Virginia. However, in Virginia many speed measuring devices will manually lock if the officer hits a button. If done improperly, manual locking can be just as bad as auto locking.

RADAR Error: Radio Frequency Interference

Radio Frequency Interference (RFI) can be caused by substations, high-power antennas, and the two-way radios found in police cruisers. Garage door openers, motion sensors, and the abundant electrical equipment found in a police cruiser and along highways can also generate RFI errors. RFI can cause erroneous speed readings. Some of these readings are continuous and steady while others are sporadic.

RADAR Error: Improper Radar Antenna Mounting

The fan blades of a police cruiser's AC unit or heater can produce a radar signal of approximately 15 to 45 mph. This most often occurs when the radar antenna is mounted with the antenna pointing across a defrosting fan vent.

RADAR Error: Weather

Weather conditions such as precipitation, temperature, and humidity can limit the range of radar and contribute to false readings. The Virginia State Police are instructed not to operate RADAR (or LIDAR) when there is any measureable precipitation.

RADAR Calibration

Law enforcement officers should test their RADAR units at the beginning and end of each shift. RADAR units are tested by using special tuning forks or a calibrated speedometer. The tuning forks must be calibrated every six months.

The officer must bring the tuning fork calibration certificate to court in the proper formatting. Additionally, the officer must be able to testify how they performed the tests.

RADAR Detector Laws in Viriginia

The possession of devices used to thwart speed measurement devices is illegal in Virginia. In Virginia, some law enforcement officers have radar-detector detectors and patrol with these detectors to find and ticket people who are using RADAR or LIDAR detectors or jammers.

Simply having a radar detector in the car violates Virginia law, and the prosecution does not have to prove that the device was functional. The only legal way a person can have a radar detector is if the device is not connected to a power source and not readily accessible by the driver (e.g. it is kept in the trunk). The fine for possessing a radar detector is approximately $140. But sometimes, possession of a radar detector is also used to justify an elevated punishment in speed cases.