Piranha: Firing Point

Initially the weapon sonar was programmed to be passive listen-only, in receive mode. Pinging was rarely to be allowed, only if the unit lost its target in passive mode, but the target was so loud ahead that losing it would be impossible. The four massive screws of the target aircraft carrier thrashed loudly in the sea, coming from exactly the same bearing the weapon expected to hear it.

According to the computer model of launching platform, ocean, and target position, the weapon calculated that it was halfway to the target. It had been set by the launching tube to “immediate enable” mode, meaning it was allowed to detonate at any time after leaving the tube rather than being required to count out a distance from the firing ship. Fully armed, the warhead was warmed up, awaiting only for the initial low-explosive charge to detonate.

Aft of the nose-cone-mounted sonar transducer electronic package but forward of the onboard computer hardware was a ring around the torpedo skin linked to several redundant electronic modules making up the hull-proximity sensors. One sensor was magnetic, feeling the lines of the earth’s magnetic force, which were evenly spaced through the sea, but tremendously focused by the huge iron mass of a ship, a target. The magnetic sensors saw the distant spacing of the magnetic lines of force as white light and the gathering together of many lines of force, focused by the ship mass, as darkness on a white field. When the electronic module saw a dark spot of increased magnetism, they fed a positive signal to the computer’s warhead-detonation software. The second sensor was a wideband optical sensor, looking outward to the sea and able to sense the darkness below and the light above from the surface; a dark surface ship’s hull caused a positive signal to the detonator. The third sensor was a blue laser, shining outward in all directions to the sea, able to sense the presence of something that was not water or surface reflection. To it a hull stood out in stark contrast to the rest of the environment.

For the surface-ship-target mode, the torpedo had enabled the magnetic proximity sensor and the blue laser, with confirmation coming from the less reliable visual sensor. The software wanted to see a “hard detect” on magnetic or a definite laser sighting, confirmed by optics if possible. The optics could be fooled by a sudden cloud obstructing the sun, and were fooled at night by the phosphorescence of a ship’s wake. A laser detect absent a magnetic detection would be a valid detonation signal, since the weapon would assume that either the magnetic sensor had failed, or that an antimagnetic anomaly device was in use. This was a new torpedo countermeasure employed by warships to alter the magnetic field surrounding the ship, a device that was only modestly successful.

The weapon sped on at low-approach speed, 60 clicks, putting out its 186 hertz tonal into the water and emitting broadband white noise at 83 decibels relative to the ocean’s background noise, in the 50-decibel range. A broadband white-noise receiver would have picked out the weapon from a distance of ten kilometers. The 186 tonal sound-pressure level was emitted at 78 decibels, and would appear on the typical narrowband receiver at a distance of 30 kilometers. But the nearest broadband receiver was in the sonar dome of the destroyers behind the row of cruisers. The three aircraft carriers had no sonar systems, leaving that equipment to the cruiser, destroyer, and frigate hulls. The second row of warships, the Aegis cruisers, had bow-mounted sonar domes configured for active pinging sonar rather than passive listening, and were capable of streaming a Dynacorp T-65 and T-148 towed sonar arrays, but the towed arrays were fragile and required clear sea miles astern, making the array unusable while steaming in a tight formation.

The cruisers also were not using their active sonar domes, since the active sonar would interfere with the passive sonar searches of the 6881 submarines ahead. In the third row behind the cruisers were the Aegis destroyers, which carried bow-mounted active sonars, all disengaged, and towed sonar arrays including the T-65, T-148, and T-22, all of which had been retracted and stowed for later use outside the battle formation. In addition, the destroyers carried a Seahawk V patrol helicopter with a dipping sonar transducer capable of active or passive sonar. The frigates behind the destroyers were similarly equipped, though the towed array systems varied.