TCO Information Update  accessed on 1 March 2003

The Speed Camera

The speed cameras used by the T.C.O. are supplied by Gatsometer BV in Holland. The Gatsometer MRC System, referred to as the Gatso Type 24 MRC Slant Radar is operated by trained members of the Victoria Police Traffic Operations Group. Before operating the cameras, police members complete a 4 day training course in which they are examined in both theory and practical aspects of the speed camera. Cameras are operated in areas which have been identified as having speed related problems, areas identified as high risk roads based on validated speed complaints, or in traffic accident blackspots. They can be used anywhere throughout Victoria at any time of the day or night.

Radar Speed Camera Set-up

The radar unit may be set up on a tripod or mounted in an unmarked police vehicle positioned at the side of the road. The radar beam is transmitted at an angle of 20 degrees across the road. The Camera Control Unit is set up by programming certain information such as the time, date, film magazine number, speed zone, film type, the direction of the traffic to be covered, and the threshold speed, and connecting it to the camera. The 35mm camera operates as an ordinary camera, but it is controlled by the Camera Control Unit and Radar Control Unit, and can photograph 2 speeding vehicles per second.

How the Gatso Type 24 MRC Slant Radar Operates

The radar unit transmits a radar beam, at a frequency of 24.125 GHz, which is 5 degrees wide, 20 degrees high and slanted at an angle of 20 degrees across the road. As a vehicle travels through the beam, the reflected radar frequency is changed (Doppler effect) and the beam is reflected back to the radar antenna. The antenna receives any signals which arrive from the same 5 degrees by 20 degrees and converts this into the speed of the vehicle. If the vehicles speed is greater than the threshold speed set by the operator, a photograph is automatically taken of the vehicle. A camera is capable of taking two photographs every second, and detects the speed of vehicles travelling in either direction.

Calculating the Speed of a Vehicle

Trigonometry can be used to calculate the speed of a vehicle travelling along a road, detected by a radar beam transmitted across a road. Because the radar beam of the Gatso Type 24 Unit is slanted across the road at 20 degrees, the Doppler frequency shift recorded will indicate a speed that is slower than the targets true speed. In the radar unit there is an automatic calculation for the slant angle so that the true speed in the direction of travel is determined.

Determining the Speed and Travel Direction of the Vehicle

Determining vehicle travel direction using the Doppler Principle The radar unit emits a continuous wave at a frequency of 24.125 GHz at a 20 degree angle across the roadway. When a vehicle enters the radar beam, the reflected frequency changes due to relative motion between the radar and the vehicle. If the relative motion brings the target closer to the radar, the reflected frequency will be increased. If the relative motion takes the target further away from the radar, the reflected frequency will be decreased. Conceptually, if the vehicle is travelling towards the radar unit, the returning beam will be compressed. If the vehicle is travelling away, the returning beam will be expanded. How much the frequency is increased or decreased is directly proportional to the speed of that relative motion. What is most important about the Doppler effect is that the frequency change happens only when there is relative motion between the objects. If both objects are standing still (eg. parked vehicles) there is no relative motion, and the reflected signal has the same frequency as the transmitted signal (ie. no change in frequency = no speed reading). Police traffic radar merely measures this change in frequency and converts it to a speed reading.The Gatso Type 24 Slant Radar has the capability to determine in which direction a vehicle is travelling. Basically, this works by the radar unit determining increase or decrease in reflected frequency determines direction of travel, and the size of the increase or decrease determines the speed of the target. Further explanation of the Doppler effect can be obtained from textbooks.