The DSC axis is located at the depth of minimum sound speed in the deep sound channel. This sound speed minimum causes the sea to act like a kind of lens, as expressed by Snells law, where sound rays above and below the minimum are continuously bent by refraction toward the DSC axis. That is, as the ray enters the deep sound channel from above, the sound speed follows a negative gradient and the ray bends downward toward the depth of the minimum sound speed, the axis. Conversely, after the ray reaches the axis, the sound speed gradient is positive and the ray bends upward toward the axis.

This refraction pattern forms the low-loss deep sound channel, as a portion of the power radiated by a source in the deep sound channel remains within the channel and encounters no acoustic losses by reflection from the sea surface and bottom. Because of the low transmission loss, very long ranges can be obtained from a source of moderate acoustic power output, especially when it is located near the depth of minimum velocity, the axis of the sound channel. Note that not all propagation paths in the DSC are entirely refracted paths. When the source or receiver or both lie beyond the limits of the channel, only reflected paths that encounter either the surface or bottom or both are possible.

The ocean by no means is laterally uniform. Because the temperature structure of the ocean varies with location, the axis depth ranges from 4,000 feet (1,225 meters) in mid-latitudes to near-surface in polar regions. As the channel axis becomes shallower, low values of attenuation can be reported. For example, the channel axis becomes shallower with increasing latitude northward from Hawaii, so a shallow source finds itself closer to the DSC axis as it moves northward. As a result, the transmission becomes better than it would be if the DSC axis were at a constant depth. Also, signals in the DSC can be found to reach a maximum and then begin to decrease with increasing range instead of the normal linear decrease. This effect is attributed to poor sound channel conditions along part of the path. The horizontal variations of the DSC axis can be readily observed on the DSC product.

The deep sound channel is sometimes referred to as the SOFAR (sound fixing and ranging) channel. Its remarkable transmission characteristics were used in the SOFAR system for rescue of aviators downed at sea. In SOFAR a small explosive charge is dropped at sea by a downed aviator and is received at shore stations thousands of miles away. The time of arrival at two or more stations gives a "fix," locating the point at which the detonation of the charge took place. More recently, the ability to measure accurately the arrival time of explosive signals traveling along the axis of the deep sound charnel has been used for geodetic distance determinations and missile-impact locations as a part of the Missile Impact Location System (MILS) network.

There is one graphic output available with the DSC product. It is a shaded deep sound channel axis depth display. The amount of shading indicates the range of depth in feet. See figure 9-9.

Figure 9-9.-A shaded deep sound channel axis depth display.

SHALLOW SOUND CHANNEL AXIS DEPTH (SSX)

The SSX product displays the axis depth values used in determining whether useful shallow sound channels (or ducts) exist within the area specified.

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