Passive acoustic homing guidance tracks the target’s
noise signature, and has the advantage of not
providing early warning to the target. Modern designs,
unlike their predecessors will use sophisticated signal
processing to reject noise generating decoys.
Active acoustic homing guidance uses a high
frequency sonar system in the nose of the torpedo
to produce range, angle and velocity measurements
of the target. The characteristic acoustic signature
will alert the target to the approaching torpedo.
A scheme described in some publications is semiactive
acoustic homing, where the launch platform
‘paints’ the target with a sonar signal, which the
torpedo seeker homes in on to impact.
All these schemes can be combined in a modern
guidance package design, as the volume of the
guidance package is large enough to provide
significant internal computing capacity.
Most modern submarine launched torpedos are
‘wire guided’, in the sense that the torpedo after
launch is tethered to the launch platform by a cable
which is used to transmit steering commands to
the outbound torpedo. At some point the torpedo
is commanded to switch over to its internal seeker,
and the cable is cut. This approach permits the
torpedo to be guided over a long distance using
steering commands generated from a passive
sonar in the launch platform. The “wire” may be
a metal cable or in some designs, an optical fibre,
the latter providing enormous bandwidth compared
to metal cables.
An alternative to conventional homing guidance is
that of wake homing guidance, employed especially
in Soviet Type 53 series heavyweight torpedoes,
intended for attacking capital ships and large
transports. A wake-homing guidance system looks
for the wake of its target, and when it crosses the
wake, it will turn back toward the wake. Repeated
zig-zag turns will eventually converge with the
direction of the wake, and drive the torpedo into
the stern of its target. Wake homing is considered
to be especially difficult to defeat, since there
are no easy ways of producing decoy wakes of
comparable size to that produced by a large ship.
The long term outlook for torpedo guidance evolution
is increasing sophistication and countermeasures
resistance, as volume and power supply are not
significant constraints for contemporary signal
and data processing hardware, and the basic
computing technologies involved follow exponential
growth laws.
More interesting will be developments in propulsion
technologies, especially battery driven systems, as
that area of technology will see significant growth
in coming years. Supercavitating torpedoes may
also see significant growth, as China and Iran
both appear to have gained access to Russian
technology.