GHOST READINGS and RECOGNITION and
ELIMINATION of RADAR "EFFECT"
RADAR'S INTERACTION WITH IT'S 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
To eliminate fan noise, try the following steps in numerical
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
2. Insure 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
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 .
THROUGH POWER AND ANTENNA LEADS - 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
RFI INTERFERENCE - CB/HAM
RADIOS - 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.
RFI INTERFERENCE -
POLICE AND BUSINESS RADIO, CELLULAR PHONES - 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.
RFI INTERFERENCE -
LIGHTS - 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.
OWN SPEED CAPTURE
EFFECT "HARmonic detect" - 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.
AMPLITUDE EFFECT - 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.
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.
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 BUILDUP
- 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
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,
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.
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.