Ghost Readings and Elimination of Radar Effect

Radar’s Interaction with Its Environment

The trained radar operator has to view the operation of his unit in the context of the total environment. Unfortunately, that environment includes both electrical and mechanical sources of Interference with the radar unit’s operation. Radar reads the speed at which objects move, and in a traffic environment we find that many things, including wind-blown trees and signs may be moving. The officer is also under constant electronic bombardment, with ambient electronic interference being emitted by his own radio, CB radios, cellular phones, intrusion alarms, Amateur Radio operators, air traffic radar, telephone microwave links.

Some or these can create “ghost” readings, but are readily identifiable as such in the presence of a valid target with proper operating techniques. These ghost readings will disappear and be replaced by the target reading when the actual target vehicle gets close enough, and will not affect the accuracy of the target reading. Most radar units have an automatic gain control, which automatically increases the sensitivity of the unit when there is no signal present. As a result, “ghost” readings are far more likely to occur when there is no other traffic evident. These readings generally disappear as soon as a legitimate target comes into the beam.

Fan and External Mechanical Interference

This is one of the most common types of radar effect, and has been demonstrated on national television. Typically, the radar is picking up a signal from some mechanical device, frequently from fans or ventilators. The radar may either be reading the actual speed of the fan blades, or may be picking up the electrical signals generated by the fan motor. This type of reading is distinguishable primarily through the use of the audio Doppler tone. Unlike the rising or falling tones of an automobile, the fan will generally be heard at a constant pitch.

One of the keys to locating and eliminating this type of interference lies with the recurrence of such readings under similar conditions. If you start receiving ‘ghost” readings in the 20-60 MPH range every time you run the air conditioner you can either momentarily turn off the air conditioner or reject all readings under those conditions. None of these sources of interference will sound like a car on the audio Doppler, and the audio remains the key to valid interpretation of the radar unit.

To eliminate fan noise, try the following steps in numerical order:

  1. Find a location (by moving the antenna) inside the vehicle that is free of fan noise; such as a corner of the dash away from the fan. The lower left side of the dash is a recommended location.
  2. Ensure that the antenna beam is not reflected back into the vehicle by anything in its path such as wipers, window trim, or anything mounted on the dash. Do not mount the counting/display unit or antenna/power cables in front of the antenna on the dash.
  3. Locate the antenna as close to the inside glass as possible (preferably less than 1/2 inch).
  4. Turn the fan off while operating the radar in stationary mode, SAME mode or moving mode with patrol speed under 30 mph.
  5. If the above suggestions fail, mount the antenna completely outside the vehicle.

RFI Noise Effects and Sources

As mentioned above, there is a constant flow of radio signals in our environment no matter where the patrol vehicle is positioned. Under certain circumstances, that ambient “noise” may be read by the radar unit. We’ll discuss specific sources of radio frequency interference below.


The power connection between the radar unit and the vehicle is a major potential source of interference. Electrical devices on the vehicle all produce a certain amount of feed-back into the system which may be read by the radar unit, much the same as a blender or vacuum cleaner will affect a nearby television. A poor connection through a dirty cigarette lighter socket may interrupt the unit’s power, causing “ghost” readings because of the power surge. Connecting the radar unit directly to the vehicle battery (using a female cigar receptacle – do not cut off the cigar plug on the radar) can easily eliminate this type of effect.


A two-way radio operated within the same vehicle as the radar unit will almost always result in swamping the radar signal, frequently resulting in an erroneous display. Therefore, officers should never transmit on radio equipment while the radar is in operation. Radar may also be affected by radio equipment broadcasting in other vehicles. The range of this effect is generally limited to 100 feet, more if an illegal linear amplifier is in use. Again, Interpretation of the audio Doppler will prevent the officer from misinterpretation of this reading. Both the quality and tone of the signal will change as radar unit shifts from reading the automobile to reading the RFI from the CB radio.


Police and business radios (often as powerful as 100 watts) will cause the same types of interference as CB radio. Because of this, officers should disregard any readings, which occur while they are broadcasting on the radio, or when another unit is broadcasting in close proximity. Cellular Phones will cause interference – even when not in use. This is because the cell phone transmits to the towers automatically when driving along to relay information on your location.


Neon and fluorescent lights are another potential source of radio interference. Radar units tend to be affected not only by devices operating at or near their assigned frequencies, but also by devices operating at multiples of that frequency. For instance, a fluorescent lamp operating at 60 cycles/sec will generate a speed reading of around 20-40 MPH on an radar unit. Officers can avoid this by parking their patrol units well away from illuminated areas, and again, by carefully listening to the tone.


In all moving radar systems, the radar unit would constantly read the patrol speed in the target window if it were not for the radar’s built in “harmonic filter”. This is because the radar always “sees” a reflection of the patrol car in the roadway, guardrails, and signs ahead of the radar. For instance, if you were running toward a mirror you would see a reflection of yourself running back toward you. Consequently the radar rejects any readings for target speed that match the patrol speed and displays either “HAR”(monic) or “–” or on most radar units, simply a blank screen. If a vehicle is coming toward the patrol car and no reading is displayed (even though a clear audio tone is present), then try slowing down or speeding up a little so that your patrol speed does not match his speed and the radar will begin reading speeds again.


When radar encounters an irregular surface, the beam reflected back to the unit may also become irregular. When that object has a regular pattern, such as a chain-link fence, the signal may be reflected back in pulses, which can be read by the radar as a Doppler shift. The audio tone is the key to the correct interpretation of any such reading, and officers should use particular care when using moving radar alongside evenly placed structures such as fences, walls or guard railings.

Motorcycle Application Considerations

Any experienced motor officer can tell you that all of the equipment on a bike takes a tremendous beating, even to the point where the rivets of metal ticket-book holders are vibrated out of place. As a result, radar units used in these applications will require extra care and maintenance. Special care should be taken that the unit is handled gently and carried in a padded enclosure. The radar should never be left on in a metal saddlebag, as this may cause the Gunn-effect diode to fail.

Effects of Windshields and Obstructions

Just as a windshield reflects, distorts and refracts light, it will change the nature of the radar beam. The radar signal may be reflected down the defroster openings, it may be bent and distorted by the curvature of the glass, or it may be blocked by the presence of the windshield wipers. Care should be taken in mounting the radar unit behind the straightest section of windshield, where it has a clear, undistorted view of the roadway.

Beam Reflection Considerations

Officers should be aware of the fact that the radar signal will reflect off many objects. A unit parked with the antenna facing a reflective surface, such as a highway sign or metal-sided truck, may well be reading traffic in the opposite direction. Officers can check this by carefully monitoring the activities of cars in both directions, and avoiding parking in locations, which are near such reflective objects.

Weather Effects of Traffic Radar

Poor weather conditions require officers to be extra alert to their radar units. Fog, snow and rain all tend to reduce the range of the radar unit. A driving rain may show on the unit as a “ghost” reading. Also, the slick pavement may make it difficult for the radar to read a patrol speed, so special care should be taken in avoiding moving cosine angle error.

Effects of Heat Build-up

Electronic components in radar units are generally not designed to withstand changes in temperature below freezing or above 140 F. However, even on an 80 F day the temperature of a radar unit on a patrol car’s dash may climb to well over 145. As the temperature changes, the values of the components change and the unit may drift. Frequent calibration checks are in order on exceptionally warm or cold days. Generally, heat failures in radar units are catastrophic. Special care should be taken to keep radar units protected from any extremes of temperature.

Panning Effect

As stated before, the radar unit reads the motion of objects relative to the antenna. If you swept a radar antenna across the front of a wall, it would show a speed, based on its relative speed. This type of error is particularly prevalent when the patrol car sweeps through a U-turn, or when hand-held radar units are used to follow traffic in a sweeping or panning movement. The radar antenna should never be moved while speeds are being clocked by the radar unit, and if it is moved any resultant readings should be rejected.

Batching Effect

Batching errors occur when the patrol car changes speed while using moving radar. Many of the older radar units take samples of both the patrol and violator speeds, but not necessarily at the same instant. Thus, if the patrol car speed changes prior to the next sample, and erroneous speed reading may occur. While using moving radar, it is very important that the patrol speed be kept at a constant, steady rate.

Shadowing Effect

Shadowing errors occur when the ground speed (patrol speed) side of a moving radar unit locks onto a moving object rather than the ground. The most common example is when patrolling at lower speeds, the radar’s patrol speed locks onto an oncoming vehicle rather than the ground and the radar reads the sum of the patrol car and the oncoming vehicle in the patrol window. This effect can be reduced by aiming the antenna more down toward the road, and if necessary slightly to the right away from the oncoming traffic.

Another example is when the patrol car overtakes a large truck traveling at 45 MPH. As the patrol car traveling at 55 MPH overtakes the truck, it suddenly reads the patrol speed as 10 MPH. (It “thinks†it is passing a stationary object at 10 MPH) It then subtracts 10 MPH instead of 55 MPH from the closing speed of a violator and indicating speeds of 100 MPH or more for the violator. The alert operator readily rejects such indications. This error may be easily prevented by always comparing the radar-indicated patrol speed against the speedometer for verification.

Multi-Path Signal Effect

This type of error is very rare, but is one, which the operator should be aware of. Under certain circumstances, radar may bounce off the target, strike another vehicle or object and bounce off the target a second time before returning to the radar unit. Under such conditions there is usually a very rapid shift in the Doppler tone and a very high speed is indicated which will not agree with the operator’s visual estimate.