RECOGNITION DIFFERENTIAL. Recog-nition differential pertains to a sonarmans ability to differentiate target noise from background noise. It is a function of target design, maintenance, and a targets mode of operation. Recognition differential was originally defined as the signal-to-background-noise ratio required at the receiver to recognize a target 50% of the time. However, using the 50% probability resulted in too many signals being classified as targets that were not targets. The inordinate amount of false alarms led to a more specific qualification of RD. Today, RD can apply to a specific probability of detection (50% or some other percentage) and a specified probability of a false alarm.

TARGET STRENGTH. The target strength of a reflecting object is the amount by which the apparent intensity of sound scattered by the object exceeds the intensity of the incident sound. This value depends on the size, shape, construction, type of material, roughness, and aspect of a target, as well as the angle, frequency, and waveform of the incident sound energy.

NOISE LEVEL. Noise level pertains to ambient noise and self-made noise at the location of the sonar. Noise level is a function of the environment and ships speed.

PROPAGATION LOSS. Propagation loss is the loss of signal strength (intensity) between the sonar and the target. In the active sonar equation, PL is a two-way loss of energy, since sound energy travels out from the transmitter and back to the receiver. Propagation loss in water depends on the following factors: 

1. Spreading of the sound wave front. The farther the sound wave moves from the source, the greater the size of the wave front and the spreading of the sound energy.

2. Conversion of the mechanical energy in a sound wave to heat (attenuation).

3. Scattering due to surface, bottom, and suspended particulate reflections.

4. Leakage of sound energy from layers of trapped sound (ducts and sound channels) and leakage of energy into areas where it is absorbed and is not capable of target detection (shadow zones) is known as diffraction loss. Velocity gradients that result in ducts and shadow zones cause this loss of energy.

5. Multiple path interference. When one or more sound paths change with time, intensity fluctuations occur.

DIRECTIVITY INDEX. Directivity is a function of the dimensions of a sonars hydrophore (receiver) array, the number and spacing of the hydrophores, and the frequency of the received acoustic energy. These functions enable the direction of a received signal to be determined. Directivity also reduces noise arriving from directions other than that of the target. The directivity index pertains to a sonars ability to discriminate against noise. It is defined as the signal-to-noise ratio (in decibels) at the terminals of a hydrophore array or a directional hydrophore, relative to the signal-to-noise ratio of a nondirectional hydro-phone. Thus defined, DI is always a positive quantity in the equation.

DI is not included in the reverberation-limited equation, because hydrophore directivity cannot distinguish against reverberations.

REVERBERATION LEVEL. Reverbera-tion is observed at the sonar receiver. The level of reverberation is a function of source level; range; and surface, volume, and bottom reverber-ation. When an active sonar is reverberation limited, RL is used in the equation in place of NL and DI.

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