I have 2 ships. For each of these ships, I have their speed over ground, course over ground (relative to north), and current location (x,y).
How do I calculate their relative velocity?
The questions relates to the first step in the projection algorithm described in this paper
It would be better to know if you know about taking scalar projections. Effectively their relative speed will be the difference of their cosine projections. But please elaborate what prerequisite knowledge you have in this field so as to answer your problem better.