The value of SBT data to the sonar operator
Although the sonar operator may not read the SBT charts himself, he
can get their information from the communication officer. As you already
know, water temperature conditions affect the efficiency of sonar listening
and echo-ranging. Here are some of the points you can learn from SBT data:
1. You can have some idea how safe your boat is from detection by
enemy listening and echo-ranging. For instance, if it is below a layer of
sharp temperature gradient (a thermocline), there is less danger of enemy
2. You can judge whether conditions are good or bad for your own listening. Isothermal water is best for listening. Sharp negative gradients are
likely to be bad.
3. You can estimate the chances of getting a single-ping echo-range on an
enemy target. Again, isothermal water is good; sharp gradients bad.
What the TDC is
TDC stands for torpedo data computer. It is located in the conning
tower usually close to the WCA sonar stack. As shown in the photograph on
the opposite page, it has many dials. Six of these dials show six main types of
data: own ship's course, own ship's speed, target's course, target's speed, target's range, and target's bearings. The TDC takes in all these pieces of data
and shows on its face a solution, in terms of relative bearings. Since some of
the dial settings can be only approximate (like target's course and target's
speed), the TDC operator checks the TDC solution with the bearings reported by sonar. As long as the sonar bearings and the bearings of the TDC
solution agree, he assumes that his other settings are correct. But as soon as
they begin to differ, he tries changing the settings on some of his dials, until
the two bearings agree again.
If it will help any, you may think of the TDC as an oversized cash
register. The sum of the various bits of information pops up on the front
panel. If the sum proves incorrect (does not check with sonar bearings),
then one or more of the figures used in the computing must be inaccurate.
The TDC officer then changes the settings he judges are incorrect, thereby
changing the solution. An accurate solution gives the correct data needed
for firing. So upon the accuracy of the TDC depends the entire success of
the torpedo attack.