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10
THE SBT AND THE TDC
 
What the SBT is

SBT stands for submarine bathythermograph, which is a device for recording the variation of temperature with depth. The recording unit hangs from the bulkhead of the control room near the ladder leading to the conning tower. As the submarine changes depth, the bathythermograph traces on a smoked card the curve that represents the thermal gradient of the water.

 
Illustration showing SBT in submarine and blister outside submarine.
 
Reading the SBT records

1. If, during a dive, the temperature of the water increases with depth, the chart records a positive thermal gradient.

2. If the temperature decreases with depth, the curve shows a negative thermal gradient.

3. If the temperature remains the same with change in depth, the curve shows you are in isothermal water. Isothermal means uniform temperature.

In many cases, the chart may reveal a combination of two or more of these conditions at different levels.

Isothermal
Illustration showing isothermal card.
Negative Gradient
Illustration showing negative gradient card.
Positive Gradient
Illustration showing positive gradient card.
 
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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 sonar contact.

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.

 
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General view of the TDC
Photo of TDC.
 
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The value of the sonar operator to the TDC

By this time it should be apparent why the sonar operator is so important to the TDC. The essential information for TDC comes from various sources:

1. Own ship's course is registered AUTOMATICALLY from the master gyrocompass in the control room.

2. Own ship's speed is also fed AUTOMATICALLY, from the submarine's "log" (actually a speedometer at the keel).

3. Target's course is either estimated by PERISCOPE observation of the angle at which the target ship is traveling, or by plotting from other data. Since this information is only approximate, it is modified as the problem progresses.

4. Target's speed is estimated principally from the turn count obtained by the SONAR operator, or from the periscope identification of the target. This information also is approximate. Therefore, it too is corrected as the problem demands.

5. Target's range can be accurately determined by SONAR single-ping echo-ranging. However, this is used only to supplement estimates from periscope observation,

6. Target's relative bearings can be determined spottily by brief periscope observation. But for a continuous flow of relative bearings, the TDC operator depends on SONAR.

This is why the sonar operator must give bearings continuously after contact. If he fails to do so, TDC operations are seriously handicapped.

A continuous flow of accurate sonar bearings is important for TDC operation and for a successful torpedo attack.

 
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Version 1.11, 19 Oct 07