Before the advent of GPS (Global Positioning System), measuring the speed of a submarine involved various techniques and instruments. Here are a few methods commonly used:
Dead Reckoning: Dead reckoning is a navigation technique that estimates the submarine's speed and position based on previously known positions and the course and speed at which it has been traveling. By keeping track of the elapsed time, direction, and estimated speed, the submarine's position can be approximated. However, dead reckoning is subject to cumulative errors and requires frequent updates from external references.
Doppler Log: A Doppler log, also known as a Doppler speed log or a Doppler sonar, is an instrument that uses the Doppler effect to measure the speed of a submarine. It works by emitting sound waves, which bounce off the seabed or particles in the water and return to the submarine. By measuring the change in frequency of the returning waves due to the submarine's motion, the Doppler log can determine the speed relative to the seabed or the water particles.
Hydrophones and Sonar: Hydrophones are underwater microphones used for listening to sounds in the water. Submarines can use hydrophones to detect and measure the relative speed of other vessels or objects in the vicinity. Sonar systems, which emit sound waves and listen for their echoes, can also be used for measuring the speed of the submarine based on the Doppler effect.
Torpedo Data Computer: Submarines equipped with a torpedo data computer could use it to calculate their own speed. By tracking the motion of a fired torpedo, which had known speed properties, and analyzing the relative positions and time intervals, the submarine's speed could be estimated.
It's important to note that these methods provided estimates of speed rather than precise measurements. With the advancement of technology, including the development and widespread use of GPS, modern submarines have more accurate and efficient means of determining their speed and position.