Thursday, November 11, 2010

Fire-control Radar

The fire-control radar is a radar that continuously provides information about the target position, such as elevation, azimuth, range, and velocity. These data are simultaneously fed into a fire-control system to calculate the information on how to direct weapons so that they hit the target. A fire-control radar features a very high pulse repetition frequency (prf), a very narrow pulse width, and a very narrow beam width. In other words, this type of radar emits a narrow, intense beam of radio waves to ensure accurate tracking information and to minimize the chance of losing track of the target. A fire-control radar must be directed to the general location of the desired target because of the narrow-beam pattern.
Some modern fire-control radars have a track-while-scan capability which enables it to function simultaneously as a fire-control radar and a search radar. This works either by having the radar switch between sweeping the search sector and sending directed pulses at the target to be tracked, or by using a phased-array antenna to generate two or more discrete radar beams and dividing them between both tasks.

A fire-control radar functions in three phases: 1) designation phase, in which the fire-control radar must be directed to the general location of the target due to the radar’s narrow beam width, ending when lock-on is acquired; 2) acquisition phase, in which the fire-control radar searches in the designated area in a predetermined search pattern until the target is located or redesignated; 3) tracking phase, in which the radar system locks onto the target, ending when the target is destroyed.

The efficiency of a fire-control radar is determined by primarily by two factors, radar resolution and atmospheric conditions. Radar resolution is the ability of the radar to differentiate between two targets closely located. The first, and most problematic, is gaining high range resolution. To do this in a basic fire-control radar system, it must operate at a high pulse repetition frequency and have a high receiver sensitivity. Bearing resolution is typically ensured by using a narrow beam width. Atmospheric conditions, such as moisture lapse, temperature inversion, and dust particles also affect radar performance. Moisture lapse and temperature inversion often cause ducting, in which RF energy is bent as it passes through hot and cold layers. This can either extend or reduce the radar horizon, depending on which way the RF is bent. Dust particles and water droplets cause attenuation of the RF energy, translating into a loss of effective range.