Police radar operates by transmitting radio waves at a fixed frequency. A portion of the beam reflects from a target vehicle and returns to the radar. If the target is moving, a slight change in frequency occurs (Doppler shift). The radar does the heavy math and calculates target speed.
The picture gets more complicated if both radar vehicle and target are moving, the task for which moving radar was created in the early seventies by some very clever boffins at industry heavyweight Kustom Signals.
To calculate the speed of an oncoming vehicle in moving mode, radar sends out two signals, using one to determine the cruiser's velocity (patrol speed) and the other to read closing speed, the combined speed of cruiser and target. From this it subtracts patrol speed and target speed is displayed.
Moving and Stationary Radar
Two types of radar are used—stationary and moving. Stationary radar must be used from a static site, typically a patrol car parked alongside the road. The most common sort of stationary-only radar is the hand-held model popularized by decades of photographs of officers holding them, often pointed toward the camera, like giant pistols.
With one exception, moving radar is the tool of choice for state highway patrols. In Pennsylvania, the law permits only the State Police to use radar and then only in stationary mode. Hand-held radar, usually K-band Decatur models, have been the mainstay of the PSP for years. Local police and sheriff's departments must make do with time/distance computers like VASCAR and similar non-radar technology. Learn more.
Although a few hand-held models can be used in moving mode—at least if they're secured in a dashboard bracket—moving radar is different. It has discreet components, principally one or two antennae, a control/display module and a remote control. Designed for permanent mounting in a vehicle, occasionally it's seen on motorcycles.
With moving radar an officer can clock approaching vehicles while driving on patrol. If the radar has dual antennas—one facing forward, the other aimed rearward—targets in the opposite lanes can also be clocked after they've passed by the cruiser.
Newer radar models have two features that make them particularly lethal. Same Lane mode allows the officer to clock same-direction vehicles ahead of the rolling cruiser. If the radar has a second, rear-facing antenna it can clock faster vehicles as they approach from behind.
Fastest Speed mode allows the radar to sample multiple targets and display the speed of the fastest. Using this feature it's easy to clock a motorcycle passing an eighteen-wheeler, an impossible feat for conventional radar. Learn more about this type of radar.
Newer radar models have two features that make them particularly lethal. Same Lane mode allows the officer to clock same-direction vehicles ahead of the rolling cruiser.
Fastest Speed mode allows the radar to sample multiple targets and display the speed of the fastest. Forget about hiding behind slower cars. Learn more about this type of radar.
Radar can transmit continuously or be placed on hold, ready to fire but not transmitting. With no signal present there's nothing to detect, neutralizing radar detectors. When a target draws near, a button-press on the remote triggers the radar and a speed appears almost instantly.
Officer preference dictates the choice. Some let it run constantly, content to get fewer customers except for the brain-dead. Aggressive officers are more likely to use instant-on, hoping to outwit drivers with detectors.
Photo radar, also called the mobile speed van, is an automated system that combines K-band or rarely, Ka-band, radar with a camera and powerful strobe flash. Mounted in a van or SUV, it shoots a beam of radar across the road at an angle and runs continuously. It's unusually difficult radar to detect and most detectors won't alert until too late.
Except for the earliest units in the fifties, X-band (10.5 Gigahertz or GHz) was the only frequency used for police radar until the mid-1970s. K-band arrived in 1976 and Ka band appeared in 1989.
At present about two percent of the 100,000-odd radar in service nationwide use X band, 18 percent use K band and 80 percent operate on Ka band.
Hoary old X band has nearly disappeared except in one holdout state, Ohio, whose highway patrol prefers X band and its wider beam, figuring it contributes to more foolproof target identification. But others aren't quite so particular and everywhere else there's been a wholesale shift to Ka band.
Ka band is a headache for the only proven radar defense, the radar detector. Skeptics dismiss detectors as mere ticket-notification devices, claiming that instant-on radar's 200 milliseconds of warning makes it unbeatable. And in a confrontation with instant-on radar, they're right—the cops usually prevail.
But a highly sensitive detector can spot these short bursts of radar from miles away. As proof, we cite the 14-plus miles of range given under ideal conditions by the phenomenal Escort RedlineXR.
Even in a worst-case situation like our Hill/Curve test site, its 0.8-mile warning allowed plenty of time to react before encountering the speed trap. So skeptics aside, be assured that the right radar detector can more than even-up the odds.
Ku band is an ancient European frequency centered at 13.45 GHz, sandwiched between X band and K band. Although approved for use in the U.S., no domestic manufacturer has ever produced a Ku-band radar and none has ever been sold here. The appearance of Ku-band radar detectors can be traced to a 2006 ploy by a radar detector manufacturer to drive sales. Learn more.
Radar target-capture range depends upon radar type, operating mode, target-vehicle frontal area, terrain, weather and a slew of lesser factors. For this reason there's a big disparity between theoretical target range and practical range.
On a busy highway, in moving mode the typical clock occurs at less than 1,000 feet, usually much closer. Radar is dumb, merely displaying a speed, and it's the officer's responsibility to determine which vehicle is producing that speed. He's required to establish a tracking history, a multi-step process to verify that he and the radar are looking at the same target.
Done properly, this can take several seconds, more than enough time for an alert, radar detector-equipped driver to spike the brakes. But officers not infrequently flout the rules and simply fire at any target of interest. Learn more about radar range.
Where To Look For Radar
On highways without a divider the most common encounter is to meet a cruiser approaching from the opposite direction. If traffic volume is moderate-to-heavy, it's common for the officer to drive in the fast lane to prevent other vehicles from blocking the radar beam. In light traffic he'll more likely be cruising at moderate speed in the slow lane.
If you're driving on a highway with an impassable center divider, look for radar in a cruiser parked on your side of the highway. That's because a center divider, usually a cable barrier, can prevent the instant U-turn necessary for an officer to work opposite-lane traffic in moving mode. Gaps in the median can be far apart; sometimes they don't exist. He'll have to park at roadside or sit in the median and monitor traffic heading in the same direction.
If he's using a dual-antenna radar, common in patrol vehicles but less so with motorcycles, he can alternate between the front and rear antenna. Engaging the rear antenna, he'll use his mirrors to watch targets as they approach. After they've passed, he'll use the front antenna, hitting them from behind as they depart.