I assume this a sort of Drone (HG, p40) and hence need no hardpoints?
A good sensor operator with a good sensor would roll something like:To determine if a vessel is real or a decoy requires a Difficult (10+) Electronics (Sensors) check (1 round, INT).
2D +6[skill] +6[sensor] +1[command centre] = 2D + 13 and always succeed, hence never be fooled by the decoy.
I assume we cannot use the ship's sensor or ECM DM on this task?Alternatively, the decoy can be used to distract a salvo of missiles by providing their sensors with a more attractive target. A T18 can distract one salvo before it is destroyed. To determine how many missiles from a given salvo are pulled off target, roll 3D add the Effect of an Average (8+) Electronics (sensors) check, multiplying the result by 5% of the salvo.
If so it becomes: 5% × ( 3D + 2D + 6[skill] - 8[difficulty] ) ≈ 5% × 15,5 ≈ 75% of a salvo...
There is no limitation on how many decoys we can use against each salvo, so if we use two decoys against a salvo it is basically automatically neutered.
Use in squadron combat:
If we have two squadrons (10 ships each) fighting each other one side with missile ships.
Each missile ship targets each enemy ships for 100 salvoes per round.
Each missile ships carries missiles for 12 rounds for a total of 1200 salvoes.
To stop them completely we would need 2400 decoys or 200 Dt decoys, spread out over the squadron that is 20 Dt per ship.
Trivial cost to be virtually immune to missiles...
Multimode Decoys are way to bad at impersonating ships and way too good at fooling missiles!