Radar Defenses


Radar technology is arguably the most common speed enforcement device in Virginia. Unlike Lidar, which is essentially “point and shoot,” using Radar requires significant training and experience to produce reliable results.

All Radar units consist of a counting unit, the display, and at least one antenna. In handheld Radar guns all three components are contained inside a single gun-shaped body. However, most Radar units in Virginia are not handheld portable units. They are mounted inside police cruisers. The Radar antennas are usually mounted on the front and back dashboards of police cruisers, the counting unit is usually mounted in the dashboard or center console, and the display (if separate from the counting unit) is also on the dashboard.

In Virginia, modern police Radar emits a continuous wave (instead of pulses). This continuous emission of microwaves spreads out at an angle of approximately 12 degrees for a distance of over 4,000 feet (or until it strikes an object).

The microwaves continuously bounce off of all objects within the beam’s path and return to the Radar unit to be detected. Moving objects cause shifts in the returning microwaves’ frequency (Doppler shifts). The more distorted the return frequency, the higher the speed of the target.

This graphic explains the principle of Doppler Shift in police radar

Objects moving toward the Radar unit cause the frequency of the Radar beam to rise while objects moving away cause the Radar frequency to fall (this phenomenon is the same phenomenon that makes an approaching car sound higher and a receding car to sound lower). Thus, by measuring the rise or fall of the Doppler frequency, the Radar unit can calculate the relative speed and relative direction of any object within the scope of its beam.

When a police cruiser is moving, the Radar unit is put into moving-mode and measures the cruiser’s speed by measuring the relative speed of the ground in front of the cruiser. The unit’s computer then takes the cruiser’s speed and adds it to or subtracts it from the relative speed of the target object depending on whether the object is moving away from or towards the cruiser.

Some moving-mode Radar units require the cruiser to be moving above a given minimum speed (usually around 20 mph) or require the target object to be moving above a certain minimum relative speed in order to give an accurate reading (usually more than 3 mph; i.e. the cruiser and the target car cannot be going within 3 mph of each other).

Because moving Radar requires the device to accurately measure the speed of the cruiser and the speed of the target vehicle there is more potential for error. In order to verify the accuracy of moving Radar, an officer must verify the Radar’s perceived ground speed against a calibrated speedometer at the time of measurement while simultaneously estimating the target vehicle’s speed. A proper tracking history is very important.

Moving Radar
relative speed radar
This graphic explains the concept of relative speed.

Most Radar devices can be used while the officer’s cruiser is stationary or 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. The small portion of the 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.

hot-spot police radar
This graphic explains the “Hot Spot” on police moving radar.
Moving Cosine Error
This graphic explains moving cosine error in police radar while the radar device is in moving mode.

All Radar and Lidar devices are designed to measure the speed of an object that is either traveling directly towards the speed measuring device or directly away from the speed measuring device. Any time the target vehicle is moving at an angle to the speed measuring device the measured speed will be lower than the actual speed. In stationary mode, this error benefits drivers.

However, in moving mode, if cosine error affects the ground speed measurements then the target vehicle’s measured speed may be higher than the target vehicle’s actual speed. If the police cruiser is near large reflective objects (such as road signs) or if the road is reflective (such as when it is wet or icy) the hot spot may lock onto a stationary object that is not directly in front of the cruiser. The greater the angle between the cruiser and the object the greater the cosine error will be. The greater the cosine error the lower the ground speed measurement will be. Cosine error is a major cause of erroneously high speed readings in moving mode.

This graphic explains “shadowing error” in moving radar.

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 it creates an error called shadowing.

When the hot spot locks onto a moving object the relative speed of the moving object and the cruiser is lower than the ground speed of the cruiser. The lower ground speed will always cause an erroneously high target speed reading when the target vehicle is moving in the opposite direction from the cruiser, and may sometimes cause a higher speed reading when they are moving in the same direction.

Target Identification Error
Fastest signal mode
This graphic explains “Fastest Signal” mode in police radar
Strongest Signal Mode
This graphic explains “Strongest Signal” mode in police radar.

The biggest problem with any Radar unit is that it detects and measures any and all objects in the beam’s path but only displays one or two speed results per antenna (depending on the make, model and operating mode of the device). The Radar unit will display only the speed of the strongest signal it receives and/or the fastest speed it receives.

The strength of the signal has to do with the target vehicle’s size, distance, material make-up and the target vehicle’s location within the Radar beam.

After a Radar unit displays the speed of an object, it is up to the police officer to decide which of the objects around the cruiser is responsible for the speed on the Radar’s display.

In order to avoid target identification error, an officer should implement a complete tracking history. This means that the officer should:

  • Visually estimate the target vehicle’s speed prior to the vehicle entering the Radar beam
  • Note the change in the Radar’s readings when the vehicle enters the beams
  • Verify that his visual estimation and the Radar reading are reasonably similar
  • Observe the vehicle’s readings throughout its time within the beam
  • Listen for a continuous high audio signal from the Radar (a sign that the signal is not due to radio frequency interference or harmonic signal interference)
  • Note a corresponding change in readings when the vehicle exits the beam.

A proper tracking history, as defined above, is essential to ensuring that the officer has associated the right car with the right speed readings.

Error From Rapid Changes in Speed

Accelerating or decelerating more than one mph every .1 – 2.0 second can cause some Radar units (depending on the model and age of the unit) to be unable to track an object. This same weakness can affect the Radar’s ability to track the cruiser’s ground speed. If either the police cruiser or the target car are rapidly accelerating or decelerating, the Radar may have trouble tracking speed accurately.

If the cruiser decelerates rapidly while measuring a vehicle that is traveling in the same direction or if the cruiser accelerates rapidly while measuring a vehicle that is going the opposite direction, this phenomenon may cause a higher than normal speed reading. A proper tracking history which includes comparing the Radar ground speed against a calibrated speedometer will help expose this error.

Harmonic Errors
Harmonic Signal Interference
This graphic explains Harmonic Signal Interference in police radar.

Harmonics can significantly affect speed readings. Large targets, such as trucks or reflective road signs close to the Radar or target vehicle can create echoes. Echoes occur when a Radar wave strikes the target vehicle before or after it bounces off a nearby object(s) and returns to the Radar unit. In such a case, the Radar signal bounces off multiple moving objects and returns to the Radar unit excessively distorted, generating erroneous speed readings. These erroneous readings can be higher or lower than reality depending on the relative direction of the moving objects.


Auto Locking

The Radar auto lock feature can also cause problems. Certain older units are designed to display speed measurements only when a tracked object is traveling above a certain speed. When such a unit detects an object going above that speed, an alarm sounds and the device will not display anything but that tracked speed until it is reset.

This feature is problematic because a fluke signal that causes a high reading for only a split second will trigger the auto lock and any driver appearing to speed nearby will be blamed. Auto lock makes a proper tracking history impossible.

Many law enforcement agencies around the country have banned the use of this function. Devices that auto-lock as their default setting are not approved for use in Virginia. In Virginia, most speed measuring devices will lock only when the officer presses a button. However, manually locking a speed prior to establishing a complete tracking history is not much better than auto locking.

Radio Frequency Interference

Radio frequency interference from substations, power antennas, and the two-way radios common in police cruisers can also cause random readings. Motion sensors, garage door openers, and obstruction detectors on heavy equipment and on the tail of some high-end SUVs can also cause erroneous readings. The electrical equipment in a cruiser and along the highways can also generate signals that may alter speed readings.

Radio frequency interference (RFI) usually comes into play only when either 1) the officer has set up a speed trap next to a strong RFI source (e.g. under power lines), 2) the officer’s Radar unit power source has been wired using wires without RFI shielding or 3) the Radar unit’s antenna wiring is bundled too closely to wires carrying RFI sources (e.g. a stereo power source, CB radio power source, antenna, etc.). A proper tracking history will usually allow an officer to detect RFI interference.

Improper Radar Antenna Mounting

The moving blades of a cruiser AC or heater fan can produce a Radar signal of about approximately 15-45 mph. This is because most Radar antennas are mounted on the dashboard near fan vents.

A Radar antenna should be securely fastened to the dashboard and never point across the Radar’s counting unit or across fan vents. Fan vents or the counting unit can cause “ghost readings”, especially when no other stronger signals are around. Proper mounting and proper tracking history will help identify false readings caused by these types of error. An officer should also test for these errors by pointing the antennas along a deserted road, turning on the fan and turning up the Radar’s sensitivity. If a false signal is produced the mountings should be examined.

Weather Error

Weather conditions such as temperature, precipitation, and humidity can affect the range of Radar and produce sporadic false readings. This is phenomenon is particularly likely when there is water, ice, or snow covering the road and the officer is operating the Radar in moving mode or in a place where harmonic signal interference is likely.

For this and other reasons, the Virginia State Police are trained not to operate Radar when it is raining or snowing. As always, a proper tracking history is essential to prevent this error.

Radar Calibration

Officers are trained to calibrate their Radar device at the beginning and end of each shift. The device is calibrated by using tuning forks. The tuning forks are calibrated every 6 months to produce a frequency that is equal to the Doppler frequency of a vehicle moving at a specific speed (usually 35 or 65 mph).

The officer places the Radar unit in test mode and strikes the tuning fork and places it inches away from the Radar antenna and verifies that the Radar device is producing a speed measurement that is within one mph of the tuning fork’s calibrated speed.

If using the Radar in moving mode then the officer must use both tuning forks simultaneously and then again separately to perform the calibrations. Each device should be assigned its own specific set of tuning forks. Because different devices may use different Radar frequencies, the tuning forks from different devices may not be interchangeable.

Every 6 months, officers are required to have their tuning forks calibrated for accuracy. This is done at private and government laboratories within the state. Each tuning fork must be accurate to within 1 mph and the Radar unit must match the tuning fork to within 1 mph. Consequently, the tolerance for a Radar unit is 2 mph when in stationary mode and 3 mph in moving mode. (In moving mode the target speed is the sum of the Radar’s ground speed and relative target speed which further expands the tolerance by 1 mph.)

Radar Detectors

The use or possession of any devices used to thwart electronic measurement of speed by law enforcement is illegal in the Commonwealth of Virginia. In Virginia, some law enforcement departments have Radar detector detectors and officers sometimes patrol with these devices in order to find and ticket people who are using Radar detectors.

Just having a Radar detector in the car is enough to violate Virginia law, and the prosecution does not have to prove that the device was functional. The only way a person in Virginia can have a Radar detector in the car and not be fined is if the device was not connected to a power source and was not readily accessible to anyone. The penalty for having a Radar detector is about $140. Sometimes, however, mere possession of a Radar detector is used to justify a more severe punishment for a driver charged with reckless driving by speed.

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