Combat Equipment (FFE)
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An indispensable device which gives you a 360 degree view of your immediate surroundings, allowing you to know where your enemies are even when they are out of view.
Radar Mapper
Not a necessary device, but a good one. You get quite a bit of money from a single tonne of cargo space. You do get some information about your opponent, but it rarely changes your fighting strategy.
A must have for small/medium ships that can't withstand many missile hits. The naval one is preferred, but it is a bit expensive.