HOW TRAFFIC RADAR WORKS
Stationary Mode - All traffic radar uses the Doppler frequency shift technique to measure the speed of moving vehicles. This
technique is based on the Doppler Principle, which states that a radar signal reflected from a moving target will experience a
frequency shift that is proportional to the speed of the target relative to the radar. Circuitry in the traffic radar then processes the
reflected signal to obtain the frequency shift and translate this frequency shift to speed.
In stationary mode, the transmitted signal strikes a moving target and is reflected back to the antenna. The traffic radar then measures
the frequency shift to obtain the target speed.
Prior to the introduction of the S
DSR line of products, traffic radar could not sense the direction of vehicles in the radar beam.
In conventional traffic radar, targets both closing and moving away generate the same Doppler frequency shift, and it is not possible to
distinguish their direction. Since the conventional radar cannot distinguish the direction of the targets in its beam (closing or away)
the operator had to rely on visual observation to determine target direction.
Now, the S
DSR 2X has the ability to filter out Doppler signals based on their direction. For example, while monitoring front
closing targets, the 2X can reject Doppler signals from all front away targets.
S
DSR line is the first practical radar to use a dual-channel antenna design. Each antenna actually has two sets of microwave
circuits and two sets of amplification/digitizing circuits. The two microwave circuits are designed to provide two simultaneous
Doppler signals with a 90° phase difference depending on direction.
Both channels of digitized Doppler information are sent to the DSP (Digital Signal Processor) circuit in the counting unit. The high-
speed DSP circuit then performs a Complex Fast Fourier Transform
computation simultaneously on each channel to obtain relative
direction for each target.
Opposite Lane Moving Mode - In opposite lane moving mode, two (2) signals must be processed to determine target speed.
The first signal, patrol speed, results from the radar signal reflecting from the roadway ahead of the radar. Since the Doppler shift is
proportional to the relative velocity between the radar and the roadway, the Doppler shift of this signal will be proportional to the
speed of the patrol vehicle. The second signal, closing speed, results from the radar signal reflecting from an approaching or retreating
opposite lane moving target back to the patrol vehicle. The Doppler shift of this signal will be proportional to the sum of the patrol
speed and target speed, or closing speed. To determine the target speed, S
DSR 2X subtracts the patrol speed from the closing
speed.
Same Lane Moving Mode - In same lane moving mode, two (2) signals must be processed to determine target speed. The first
signal, patrol speed, results from the radar signal reflecting from the roadway ahead of the radar. Since the Doppler shift is
proportional to the relative velocity between the radar and the roadway, the Doppler shift of this signal will be proportional to the
speed of the patrol vehicle.
The second signal, the difference speed, results from the radar signal reflecting from an approaching or retreating same lane moving
target back to the patrol vehicle. The Doppler shift of this signal will be proportional to the difference speed between the patrol and
target vehicles. If the target vehicle is moving faster than the patrol vehicle, the difference speed will be added to patrol speed to
obtain target speed. If the target vehicle is moving slower than the patrol vehicle, the difference speed will be subtracted from the
patrol speed to obtain target speed. To reduce user confusion, front same-lane targets and rear same-lane target will be rendered by
the S
DSR 2X using two different Doppler tones. Front same-lane targets will use the same Doppler tones as used for
opposite-lane targets and stationary targets. Rear same-lane targets will use a lower tone that is proportional to the difference Doppler
shift between the patrol vehicle and the target.
Prior to the introduction of the S
DSR line, a radar operator had to observe the relative speed of the target vehicle and “tell the
radar” whether to add or subtract
the difference speed from the patrol speed as described above. These older same lane radar models
require that the operator select the “correct speed” by the “correct position” of the “Slower” key on the remote control.
The unique Direction Sensing ability of the S
DSR 2X allows the radar to automatically (without the traditional “slower key”)
determine the correct speed of all
same lane targets in the radar beam.
Fast Mode - S DSR 2X offers a feature called Fast Speed Tracking. Fast Mode display can be easily turned ON/OFF in the
Operator Menu. See Page 6. In addition, FAST target locking
(for both same and opposite lane targets) can be turned ON/OFF in the
Options Menu. See Page 46.
The addition of the fast mode allows the ability to track small high speed targets that normally could not be tracked because a stronger
target shields the weaker target from normal speed measurement. The classic example is where a speeding sports car passes a slower
moving eighteen wheeler. The faster sports car, although clearly speeding, previously could not be measured because the strongest
truck target captures the target display window. S
DSR 2X, in this example, will display the speed of the strongest truck in the
target window, while the speed of the faster sports car will appear in the middle fast window. Tracking of both targets may be
performed simultaneously.
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