RADAR PULSE CHARACTERISTICS

determining target size, strength, and location (fig. 2-7). RADAR PULSE CHARACTERISTICS Radar pulses travel at the speed of light (186,000 miles per second). Thus, the distance to a target can easily be calculated by monitoring a pulse’s elapsed time from transmission until its return. Half the distance traveled by the pulse determines the target’s range from the antenna. Pulse Length Pulse length (or pulse duration) is the measurement taken from the leading to trailing edge of a pulse and is a good indicator of the amount of power contained within the pulse (fig. 2-7). Generally, longer pulses emitted from a radar return more power, thus increased target information and data reliability. Longer pulses have the disadvantage in that fine details within the return echo may be lost. Pulse length is usually expressed in microseconds, but is also measured in kilometers. The WSR-88D incorporates a variable pulse length that may be as short as 1.57 microseconds (1,545 feet). Important aspects ofaradar pulse include minimum range, range resolution, and pulse repetition frequency. MINIMUM RANGE.—Pulse length determines a radar’s minimum range or how close a target can get to the antenna without adversely affecting operations. Minimum radar range is defined as any distance greater than one-half the pulse length. In other words, targets more than one-half pulse length from the antenna can be correctly processed, while approaching targets that get too close pose serious problems. If targets come within one-half pulse length or less of the antenna, the pulse’s leading edge will strike the target and return before the radar can switch into its receive mode. Some portion of the return energy is lost and the radar may become confused and discard the pulse. RANGE RESOLUTION.—A radar’s resolution is its ability to display multiple targets clearly and separately. Range resolution refers to targets oriented along the beam axis as viewed from the antenna’s position. Longer pulses have poorer range resolution. Targets too close together lose definition and become blurred. They must be more than one-half pulse length apart or they will occupy the pulse simultaneously and appear as a single target. The problem of range resolution will be discussed in more detail later. PULSE REPETITION FREQUENCY (PRF).—PRF is the rate at which pulses are transmitted (per second). It controls a radar’s maximum effective range by dictating the duration of its listening time. Increased PRF speeds the rate at which targets are repeatedly radiated. This increased sampling results in greater target detail, but the maximum range of the radar is reduced because of the shorter periods between pulses. The WSR-88D can emit anywhere from 318 to 1304 pulses per second. It has a maximum range of approximately 250 nautical miles (nmi). Listening Time Following the transmission of each pulse, the radar switches to receive mode awaiting its return. This break in transmission is appropriately called "listening time." When pulses do not return during their prescribed listening time, the radar assumes no targets were encountered and that the pulse has continued on its outward direction. Listening time determines a radar’s maximum effective range as it, in effect, limits the distance a pulse can travel. If listening time is reduced, pulses can cover less distance and effective range is decreased. Thus, a 50-percent reduction in listening time cuts maximum radar range in half. Only targets within the maximum effective range are detectable. Figure 2-7.—Radar Pulses 2-6