4-Bearing method

The 4-beraing method (also known as Speiss TMA), as the name suggests, is a method of acquiring firing data using only four bearings. It was developed after WWII in order for submarines to be able to target other submarines while both are fully submerged. In principle this method can be used in WP to gather firing data using only the hydrophone, however this is a slow and error prone process due to the difficulty in acquiring accurate bearings with the hydrophone, especially for larger convoys. Although it was developed for use with the hydrophone, it's also possible to use this method with visual bearings as well.

For this method to work your target has to travel on a steady course and speed during the entire data gathering process. It also does not work well if the bearing changes very slowly, such as when the target is moving straight away from, or towards you. There are many variations of the 4-bearing method, but the basic principle is that you gather 4 bearings at a fixed time interval. In it's simplest form you could just sit still in one spot for the first 3, then move to a different spot for the 4th. However it can also be done on the move. If the target is moving at a constant speed and course, then we know that it has to have traveled an equal distance between the first and second bearing, and the second and third bearing. Therefore we can use the map tools to draw a line from the midpoint, starting at some arbitrary point on bearing line two, and find a direction where the ends of the line is just touching both bearing lines one and three. Assuming we have done everything precisely and correctly, this line represents the course of the target. Using this information a "ghost bearing line" can be constructed. Extend the course line by the length between bearing lines one and two, or two and three (these should be the same). Trace a line from the end of this line to where you would have been at the fourth reading if you were to continue at the current course and speed. Change your course and/or speed before taking the fourth bearing. Where the fourth bearing line crosses the "ghost bearing line" should be the current location of the target. You can no move the course line so that it intersects their current location, and you can measure its distance traveled between measurements, from which their speed can be calculated.