One of the principal tools for the enforcement of vehicle speed limit laws has been doppler radar. In a doppler radar system, a microwave frequency signal is reflected from a target vehicle, causing a change in the frequency of the signal in proportion to a component of the velocity of the vehicle. The frequency shift is measured and scaled to miles per hour or other dimensional units and is displayed. Since no computations are required of the unit operator, the vehicle speed measurement is quick and simple. In theory, the officer only needs to observe the passing traffic, monitor the unit display and, if an excessive speed is displayed, pursue and issue a citation to the violator.
In practice, a problem arises in the identification of the vehicle whose speed is being displayed when more than one vehicle is in a position to reflect the radar signal. The signal beam of conventional police radars is generally conical, spreading at about six degrees from an axis of the antenna. As the range from the antenna increases, the width of the beam increases such that more targets may be illuminated by the radar signal. For this reason, it is not possible to precisely aim the antenna at a target unless the target is within a relatively short range. It would be possible to narrow the beam by increasing the diameter of the antenna. However, this is not considered a viable solution because of increased expense and decreased convenience. In all present day police radar systems, the range control is actually a receiver sensitivity control. If an operator adjusts a range control to limit the range, he is actually decreasing the sensitivity of his unit. If, however, a truck or other vehicle with a large radar cross section approaches, the officer may read the speed of this target at a range much greater than he expects.
Another aspect of the identification problem is based on the difficulty for even an experienced operator to judge precisely which of a number of vehicles is likely to be the best radar signal reflector. Certain structures and configurations such as headlights, certain grills, license plates, large flat metal surfaces, and the like are relatively efficient radar signal reflectors. When these structures are variously combined in a vehicle and a plurality of such vehicles are positioned at random distances from a radar antenna, the number of geometric variables decreases the reliability of target identification on the basis of target reflectivity.
The majority of radar units in current use by police departments are designed for use in a moving mode, that is, with the radar unit in motion while monitoring the speed of vehicles traveling in the opposite lane moving in the opposite direction from the platform. There have been attempts at designing moving radar units capable of use in a same-lane moving mode, with varying degrees of nonsuccess. In any moving mode radar unit, the speed of the patrol car or platform vehicle and the speed of target relative to the platform are measured and then mathematically combined in such a manner as to result in the absolute or ground speed of the target. In standard (opposite lane) moving mode radars, the doppler signals representing the platform speed and the relative target speed are separated from the return signal by means of filters. A problem arises in correctly combining the platform and relative target speeds. A same-lane moving mode radar unit can be used to measure the speed of oncoming of "opposite lane" targets or "same lane" targets.
Oncoming target speeds measured by standard moving radar are simpler to measure since with such targets the platform speed is always subtracted from the relative target speed to derive the absolute target speed. However, oncoming target radars have limited utility, and their use presents safety problems. The officer has to make a hazardous U-turn to pursue a violator, after which he is no longer capable of measuring the target's speed with the radar unit.
The use of same-lane moving radar units generally requires the capability of measuring the speed of closing targets wherein the distance between the patrol car and the target is decreasing and the speeds of opening targets in which the target is pulling away from the patrol car. In a closing situation the relative target speed is subtracted from the platform speed, while in an opening situation the relative target speed is added to the platform speed. Since the mathematical handling of the speeds is different for the different situations, a switch on the radar unit is necessary to signal the desired mode. In operation, the officer is required to observe the target and set the switch in the proper position for target situation. However, if the officer selects the wrong mode or the situation changes because either the patrol car or target changes speed or a second target is present, an erroneous speed can be displayed.
In addition to the problems described above which are inherent in present day moving mode radar systems, such systems also suffer the same target identification problems as stationary mode radars. Because of these problems in addition to increased expense because of greater complexity, police departments have been reluctant to acquire same lane moving mode radar systems.
Thus, while conventional police doppler radar units are capable of a high degree of accuracy with regard to vehicle speed measurements, great care must be exercised in the use of such units as far as attributing the speed displayed by a radar unit to the driver of a particular vehicle. There is a common, although often mistaken, assumption that a radar unit displays the speed of a vehicle in the front of a group of vehicles, particularly one that is spaced ahead of a group. While it is true that situations occur in which an officer can easily discern which of a group of vehicles might be speeding, there are many vehicle grouping situations in which the identity of the vehicle whose speed is displayed is indeterminate. Most citizens seldom have contact with law enforcement agencies. When such contact occurs, it is important that the citizens feel that they have been dealt with justly. Therefore, it is important that any traffic citations written be based on factual information that is associated with the correct car. Traffic citations and arrests based on assumptions which are often false leave authorities vulnerable to undesirable allegations of arbitrary and selective enforcement of the traffic laws.
One important factor which must be considered in the design of police radar units is economics. Whereas a radar unit or system which costs tens or hundreds of thousands of dollars might be considered commensurate with the needs and costs of an aircraft or ship, such a cost would be prohibitive to a city, county, or state government. Thus, the capabilities of a police radar unit must be strictly limitied to what can be afforded by such governments without sacrificing accuracy and reliability.