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Okay, from the start there is a problem. You say the Gartagen's developed this in YE 26, but the did not have an encounter with another species until YE 33.
Also what sensors is it going to try to 'confuse' Different sensors use differing tech and look for different things.
How are you going to falsify LADAR information without outright changing your ship's shape or manipulating the photon stream? Subspace Mass sensors detect mass so you'd probably have to generate a gravity field to mimic the mass to begin with. Thermal pretty much means you're going to have to generate more or less ambient heat around the ship. How does a "passive field." do this exactly?
You do not say what the passive fields purpose is. I presume it is meant to absorb any active scan that it encounters.
This means your stealth system must then have a variety of systems to create the false images.
Subspace Mass Sensor would not be an active sensor. The other ship would be scanning for gravitational forces. So as Soresu said you would need to have the ability to create a gravity field to give the appearance of more mass.
Thermal sensors how would you create the heat signature of a vessel that is a different size and shape?
RADAR works off of bouncing an RF signal off an object with a sweeping beam. A passive field is not going to be able to match that effect. A programmable radar could match the RF Frequency and PRF of the incoming radar, but it could not create a different shape because the only time the other ship would get a signal from it is when both dishes point straight at each other.
If you are going for Stealth, expand the idea of the Passive field, rather than trying to make this Stealth and Camo