Tracking History

www.pbelectronics.com/police_radar.htm accessed 3 March 2003

The most important part of a radar case is a tracking history. The radar unit will display a number, and that’s all. It doesn’t tell the officer which vehicle it is, or if there’s even a vehicle there. The officer has to track the vehicle to make sure that his observations match what the radar is showing him. Otherwise, the officer might stop the wrong vehicle or a common radar error might give an incorrect speed. In some jurisdictions, the officer has to visually estimate the violator’s speed within 5 miles per hour. The radar beam is a cone. It doesn’t pick out individual vehicles. It can’t even pick out individual lanes. The radar shows a speed based on three factors: – Reflectivity, Position and Speed.

This is generally referred to as biggest, closest, fastest. The radar usually picks up target that is the largest in its view. Therefore, it might pick up a motorcycle that was very close to it before a tractor-trailer a mile down the road. Many times the radar will display different speeds of different vehicles that are close together. The officer has to determine if he’s getting a good reading, and if so which vehicles’ speed is being displayed. This isn’t as hard as it might sound. Radars are equipped with a speaker which give a tone reflecting the doppler signal its receiving. If it’s a clear high pitched tone, then it’s getting a good solid reading from a vehicle. It will give a low raspy tone if it’s not getting a clear signal.

This happens when there’s something in between the radar and the target or when the vehicle is entering or leaving the beam. Once you have a solid tone, you look at how the traffic is moving. If there is a clump of vehicles that is moving at 65mph then a vehicle overtakes them at a high rate of speed, and the radar shows 85mph, it’s easy to figure out who was going that fast. Alternatively, if a group of vehicles is traveling together in a clump, where no one is overtaking or falling behind, all the vehicles in that clump will be at about the same speed. Some radars have a fastest vehicle button that will display the fastest vehicle in its cone. This is very useful for when there are large targets such as tractor-trailers in between the radar and a fast moving small vehicle.

Stationary Radar

Stationary radar is radar at its simplest. The officer sits on the side of the road, and watches traffic. When he observes a vehicle moving at high speed, he activates the radar. The radar goes through its basic decision factors (Reflectivity, Position, and Speed) then it displays that speed. The radar will give a tone. If the tone is clear and the displayed speed matches the officer observations, the officer can make the stop.

Mobile Radar

Moving radar is very similar stationary radar, but it’s looking for two different speeds. The radar looks for the largest object in its field, and it assumes that this is the passing background. Then it looks for the second most significant object that it assumes is the target. The radar actually measures the closing speed or separation speed between the target and the patrol vehicle. The radar’s counting unit will then use the following formulas. Target Speed (TS) = Closing Speed (CS) – Patrol Speed (PS) or Target Speed (TS) = Separation Speed (CS) – Patrol Speed (PS) The radar unit will then display two speeds. It will show the target speed and the patrol speed. The officer must compare the patrol speed displayed on the radar with that displayed on the car’s speedometer. This is an essential element of the radar case. The radar speed will be more accurate, but there are certain errors that this will detect. The speeds must be consistent.