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Dwell time (T D) in surveillance radar is the time that an antenna beam spends on a target. [1] The dwell time of a 2D–search radar depends predominantly on the antenna's horizontal beam width θ AZ, and; the turn speed n of the antenna (in rotations per minute or rpm, i.e. 360 degrees in 60 seconds = multiplied by a factor of 6).
Five definitions of the beam width are in common use: D4σ, 10/90 or 20/80 knife-edge, 1/e 2, FWHM, and D86. The beam width can be measured in units of length at a particular plane perpendicular to the beam axis, but it can also refer to the angular width, which is the angle subtended by the beam at the source.
A simple calculation reveals that a radar echo will take approximately 10.8 μs to return from a target 1 statute mile away (counting from the leading edge of the transmitter pulse (T 0), (sometimes known as transmitter main bang)). For convenience, these figures may also be expressed as 1 nautical mile in 12.4 μs or 1 kilometre in 6.7 μs.
The resolution of any radar depends on the width of the beam and the range to the target. For example; a radar with 1 degree beam width and a target at 120 km (75 mi) range will show the target as 2 km (1.2 mi) wide. To produce a 1-degree beam at the most common frequencies, an antenna 1.5 kilometres (0.93 miles) wide is required.
For example, consider a radar with a beam width of one degree and an antenna that rotates once every ten seconds, or 36 degrees per second. An object will be painted by the beam for only 1/36 of a second as the one-degree beam sweeps over it. If the radar has a PRF of 500, the object will be painted with 14 pulses per scan at most.
With this calculation, the horizon for a radar at a 1-mile (1.6 km) altitude is 89-mile (143 km). The radar horizon with an antenna height of 75 feet (23 m) over the ocean is 10-mile (16 km). However, since the pressure and water vapor content of the atmosphere varies with height, the path used by the radar beam is refracted by the change in ...
Radar engineering is the design of technical aspects pertaining to the components of a radar and their ability to detect the return energy from moving scatterers — determining an object's position or obstruction in the environment.
Diagram of AN/SPY-3 vertical electronic pencil beam radar conex projections. X band functionality (8 to 12 GHz frequency range) is optimal for minimizing low-altitude propagation effects, narrow beam width for best tracking accuracy, wide frequency bandwidth for effective target discrimination, and the target illumination for SM-2 and Evolved Sea Sparrow Missiles (ESSM).