Before the widespread use of GPS (Global Positioning System), submarines relied on several methods to measure their speed and navigate underwater. Here are some techniques used to determine submarine speed:
Dead Reckoning: Dead reckoning involves estimating the submarine's position based on a known starting point, its speed, and the time elapsed. The speed would be determined by measuring the revolutions per minute (RPM) of the submarine's propellers and correlating it with the known speed characteristics of the vessel.
Log and Knotmeter: Submarines often employed a device known as a "log" or "knotmeter." This instrument uses a propeller or impeller to measure the speed of the water passing by the submarine. By correlating the water speed with the submarine's movement, the speed of the vessel could be determined.
Doppler Log: Doppler logs were utilized to measure the speed of the submarine relative to the surrounding water. These instruments emit sound waves or sonar pulses and measure the frequency shift of the echoes reflected back from the water. This shift in frequency is used to calculate the speed of the submarine.
Pressure Hull Pitometer Log: This system measured the pressure difference between the bow and stern of the submarine to estimate the speed. It utilized pressure sensors on the hull, which detected changes caused by the flow of water around the vessel. By measuring the pressure difference, the speed of the submarine could be determined.
These methods provided estimates of the submarine's speed and were often used in combination to improve accuracy. However, it's important to note that these techniques had limitations and were not as precise as the satellite-based GPS systems available today. GPS revolutionized navigation by providing highly accurate and continuous positioning information, making it an invaluable tool for modern submarines and other vessels.