The Basic
Radar Gun
The Basic
Radar Gun
Also see chapter 6.3 -- Laser Radar Operation
The basic microwave traffic radar is an unmodulated CW (continuous wave) transmitter that illuminates a target to measure the Doppler shift of target reflections. Most microwave traffic radars measure speed of one target at a time; range or angle information is not available. When multiple targets are present, the operator has no way to know for sure which target the radar is tracking. Most radars display the target with the strongest reflection (usually, but not always, the closest target); some models have the option to display the fastest (or a faster) target. Some models track and display two targets -- the fastest or faster target, and the strongest target.
Traffic radar detection range varies with model and unit, and target. Maximum range can be as high as a mile (1.6 km) or more for large targets under good conditions, or as low as a hundred feet (30 m) or less for small targets under bad conditions. Factors that effect detection range include;
A phenomena know as surface ducting (also referred to as a ground-based duct) can increase radar detection range significantly. Surface ducts have a height of about 30-70 feet (10-20 meters) and occur when upper air is exceptionally warm and dry COMPARED to surface air. Most of the time the greater the altitude the lower the temperature, when a layer of relatively constant cooler and more humid air forms near the surface a duct occurs. Conditions favorable to duct formation include heat radiating from the earth such as on cool summer night; cool air from the base of a thunderstorm can also form a duct. Long straight stretches of concrete (such as runways and highways) are known for forming microwave surface ducts. Large bodies of water tend to form ducts more often than land masses (water cools surface air).
OPERATION
(not all operations available on all models)
Single unit
Multi-unit (typical configurations and options)
Wideband Radar: (some Ka Band radars)
Intended to defeat radar detectors. Wideband radars (Ka band) operate on a single fixed frequency (operator selects one of several available), and/or in a frequency hop mode. A frequency hop mode radar dwells on a fixed frequency for a fraction of a second (on the order of 1/10 of a second or so) and hops to another frequency. The radar cycles between a number of different frequencies.
Continuous: Many radars continually transmit and process every target when the radar is turned on. Some radars only display targets above a preset speed (audio alert setting switch).
Instant On (some models)
Intended to defeat radar detectors. Instant On radars allow the operator to control radar transmission; the operator transmits only after selecting a target, and only long enough to get a speed reading.
Pulsed (some models)
Intended to defeat radar detectors. Some pulsed radars only transmit periodically on the order of every several seconds or so, and then only transmit long enough (100 ms or so) to get a speed measurement. Some pulsed radars transmit only one short burst on operator command. Pulsed radars, by design, do not establish a tracking history.
Stationary
Many traffic radars operate from a fixed position only -- stationary radar. Many stationary radars measure on-coming (approaching) traffic only; some measure on-coming and/or going (receding) traffic.
Moving Mode (Moving Mode Radar)
Most moving mode radars can operate in either stationary or moving mode; a few older models are moving mode only. Most moving radars also display patrol car speed, a few older models do not. Many moving radars measure on-coming targets only, some measure on-coming and/or receding traffic. Some moving radars can measure same-lane traffic in front of (forward looking antenna) and/or behind (rear looking antenna) the patrol car.
Target Direction (some models)
All stationary radars measure on-coming (approaching) traffic, some measure on-coming OR going (receding) traffic, and some radars measure on-coming AND going traffic (and some of these indicate target direction -- on-coming or going).
Most moving mode radars measure opposite direction on-coming targets (front antenna). Some radars measure opposite direction and/or same-lane targets. Moving radars with a rear antenna measure same-lane traffic and/or opposite direction (receding) traffic. Many radars that measure targets in more than one direction usually have a display that indicates target direction. Moving radar requires a minimum patrol car speed (typically a few to 20 mph); same-lane mode also requires a minimum speed difference between target and patrol car (typically a few to 5 mph).
The table below summarizes radar type and common configurations (varies with model) of radar mode (stationary / moving) with respect to target direction and front or rear antenna.
| Stationary mode | Moving-mode | |||
| Antenna | Front | Rear | Front | Rear |
| Stationary Radar |
on-coming and/or going |
on-coming and/or going |
- | - |
| Moving Radar |
" | " | on-coming and/or same-lane |
same-lane and/or going |
SIDE NOTE: A stationary radar (or moving mode radar set to function in a stationary mode) operated from a moving patrol car (and pointed in the direction of travel -- front) will measure the patrol car speed. The ground is a huge target that swamps out discrete target echoes such as from other vehicles (moving or not). The radar processes the ground echo, Doppler shifted by patrol car speed, as a target. If the radar is angled (off patrol car direction), the displayed speed reads low (see chapter 4 -- cosine effect error).
Antenna Front / Rear (some models)
Multi-piece single antenna radars usually mount the antenna pointed in the forward direction of the patrol car; some mount the antenna pointed in the rear. Some systems have two antennas, forward and rear, and the operator selects one or both antennas for use. Antennas can be mounted inside the patrol car (behind a windshield), or outside on the side of the vehicle between the roof and hood.
Manual Lock: operator manually locks target speed. (most models)
Auto-Lock: freezes target display (or LOCKED display) and/or photographs or video tapes automatically when a preset target speed has been met or exceeded. Only unattended photo/video radars or older models have this feature. Some courts will not accept the radar as evidence if this function is used.
Moving Radar
target speed: ± 2 mph
patrol car speed: ± 1 mph
CONTROLS
(not all functions on all models)
DISPLAYS / INDICATORS
LEDs (Light Emitting Diodes) and/or LCD (Liquid Crystal Display)
(not all indicators / functions on all models)
NOTE: radars that track two targets (strongest and fastest) generally use the middle display for the fastest target and the target display for the strongest target.
