Radar and lasers are the two most common methods of speed detection used in law enforcement. In both cases, the police target the suspected vehicle with a pulsed energy beam in order to read its speed. Radar uses microwaves and lasers use coherent light, but the basic principle is the same in both cases. Despite the similarities, there are a number of differences with regards to which methods of detection work best for each enforcement technology. Radar speed enforcement has been in use for a very long time, and both radar and detection technologies have gone through multiple generations, while laser technology is more recent in its application and so both the enforcement and the detection are still in their relative infancy.
Basics of Speed Enforcement Detection
The key to effective detection of speed enforcement, whether through radar or laser is the ability to pick up signals aimed at other vehicles. As a general rule, once a vehicle has been tagged by either laser or radar, it is too late for the driver to slow down. This is where detector sensitivity comes into play, the more sensitive the detector, the easier it is to pick up the part of the pulse that misses the intended target.
How Radar and Lidar Work
Even though police radar and lidar, also known as laser radar, do the same job, they work according to different principles. While some police radar can operate from a moving vehicle, the narrowness of a laser beam means that the guns need to be mounted on a tripod in order to get an accurate speed reading.
How Police Radar Works
Police speed radar is based on the Doppler effect. What happens is that when the police officer targets a vehicle with a radar gun, the speed of the vehicle causes a minute shift in the frequency of the reflection compared to the original beam. It is the same principle that causes the sound made by a motorcycle or train to become more or less loud to a person standing in one spot on the street. The first police radars emitted a constant stream of pulses, making them easy to detect, but later models switched to an "instant-on" mode which means the radar is not transmitting unless it is actively checking the speed of a vehicle.
How Police Lidar Works
Even though light is as affected by the Doppler effect as microwaves, police lidar guns do not rely on it for determining vehicle speed. Instead, they base their measurements on the speed of light. Because the speed of light is a constant, it is possible to use a laser to make a very precise measurements of distance by computing the time between when the gun emits a laser pulse and when it detects the reflection. By taking several measurements in quick succession, the laser gun can easily compute the vehicle's speed by the change in distance over time.
Overview of Radar Detection
A radar detector is essentially a superheterodyne microwave receiver tuned to the frequencies used by police radar. In other words, they are very sensitive radio receivers that listen for radar in the same way that a police scanner listens for radio communications traffic.
X-Band Radar Detection
The first police radar to be introduced was the X-band, which entered service in the 1950s. This relatively low-frequency high-power radar is very rarely used outside of New Jersey and Ohio. It is the easiest to detect due to its high power output and can sometimes be detected from up to four miles away, eight times the distance at which it can take a speed reading. Many supermarkets use X-band motion sensors on their automatic doors, which can lead to false positives.
K-Band Radar Detection
K-band radar saw the introduction of the now-universal radar gun. Operating at a higher frequency than X-band, it has a shorter range but can be used from a moving vehicle as well as a stationary point. It is also used in some motion sensors, but it is much more common in speed detection than its predecessor. It can be detected from a range of anywhere from a quarter of a mile to two miles distance.
Ka-Band Radar Detection
Working at still higher frequencies than K-band, Ka-band radar was originally used for photo-radar before being utilized in portable radar guns. The first Ka-band guns used exactly the same frequency as photo radar, but later models known as "Stalker" guns may operate at any frequency in the Ka-band between 33.4 and 36 GHz and so cannot be detected by devices tuned to the 34.3 GHz photo radar band alone. It is estimated that stalker guns are in use in over half the U.S. The reaction to stalker guns was the introduction of wider band detectors that can cover the entire Ka-band rather than just the narrow frequency used by photo-radar.
Overview of Laser Detection
Lidar is intrinsically easier to detect than radar because it operates over a narrower frequency band. The real issue with detecting radar has more to do with the tightness of the beam than any inherent difficulty in detection. The average lidar beam is only 18 inches in diameter at 500 feet, and since the sensor has to actually be in the path of the beam to work, it can be difficult to detect the beam before being targeted. Some detectors can pick up the scatter from a laser targeted on another vehicle, but this can be difficult and is not as reliable as even "instant-on" radar detection.
Comparison of Radar and Laser Detection
It is an odd fact that radar is technically harder to detect than lidar, but when it comes to practical situations it is easier to implement an effective radar detector than a lidar detector. Lidar detectors should be placed low enough to cover the headlights or front license plates, as those are the areas most often targeted by law enforcement. More sensitive detectors that can pick up the scatter of laser beams on other vehicles can be mounted higher up, but they provide minimal protection against direct targeting.
Radar detectors provide better distance coverage and can more easily detect beams aimed at other vehicles. They do suffer from more false positives, but they are more likely to detect enforcement before it is too late for the driver to do anything but pay the ticket.
The most effective option in most cases is to install a combination device that can detect both radar and lidar. While these detectors can be more expensive than single-purpose devices, they provide coverage against a greater range of threats, making them more practical for drivers who do not know which enforcement tool is being deployed where. Another benefit of the combination devices is that while they are more expensive than single-purpose detectors they do not generally suffer any performance degradation from having both technologies installed. Most radar detectors are multi-function devices anyway, so adding laser functionality is only a slight increase in complexity. The only real compromise may be in mounting position, as radar detectors are best mounted higher than laser detectors.
Buying a Combined Radar and Laser Detector on eBay
It is easy to buy a combination radar and laser detector on eBay. All you have to do is enter the relevant keywords in the Search box on the homepage and watch the results appear. Once you are on the search results page, you can use the filters on the sidebar to narrow the search to just the detectors you are interested in. You can limit by brand, by price, and even by what they are able to detect. After you have narrowed your options to just the units that meet your needs, you can sort the results by anything from price to distance, which can be very helpful to people who put a priority on either their budget or speed of delivery.
Once you have a list of possible purchases, the next step is to decide which of eBay's many reputable sellers is the best match for you. The first place to look is at the seller profile page where you can learn the seller's feedback rating, location, and policies, such as the preferred shipping carrier. After picking the seller you prefer to do business with, your detector is only a few clicks away.
Just as police radar and lidar reflect different approaches to solving the same problem, so do laser and radar detectors. In both cases, they are essentially receivers tuned to the frequency used for speed detection. While radar detectors are more likely to generate false positives, particularly in the X and K-bands, they are also more likely to detect enforcement in time to slow down. A strong laser signal, indicating that the vehicle has been directly targeted by the beam, usually means that a ticket is on the way if the driver was speeding. The only way to be sure of reacting to enforcement in time to slow down is to to detect transmissions targeted at other vehicles. This requires a sensitive detector, and this usually most effective when using a combination detector that is sensitive to both radar and lidar.