Pentaids, sensor probes and ECM drones.

Pentaids (Penetration Aids) are basically 500mm missile modified by having their warhead replaced with jammers. They have the ability to spoof targeting systems so as to make a single missile appear to be multiple missiles or a single flight of missiles appear as multiple flights. The purpose of Pentaids is to cause anti-missile missiles to expend themselves uselessly against targets which do not exists.
Pentaids are fired from standard missile launchers and expend themselves quickly, that is they are meant to travel at standard missile accelerations (1000Gs) and have a duration no longer than the missiles they are based upon.
They are very effective against the terminal guidance systems of anti-missile missiles, but somewhat less effective against ship based sensor platforms.

Sensor Probes are modified 250 mm missiles. The warheads are removed and smaller engines are installed. Some retain their normal energy cells, others are outfitted with Nuclear Power Units (NPUs) which give them longer endurance. For some purposes their laser communicators can be replaced with standard encrypted EM communicators, An advanced compact robot computer is installed. The sensor packages used are very much weaker than those available to a spacecraft, but the purpose is to get the probe in close without detection. To this purpose they have the ability to operate independently, following very complex program guidelines to reduce the likelihood they would be compromised.
Most probes use sensor packages optimized for a specific purpose. All have some level of PESA capability, but beyond that they may be either primarily passive platforms or primarily active platforms.
Large numbers of probes can be deploy to create a passive sensor net. Active probes can be combined with passive probes and passive shipboard sensors systems to create pseudo-passive sensor nets. These nets operate very much the way the human eye does, or the way sonar buoys do. The active probes send out an EM wave pulse which reflects off of targets. The reflected waves are picked up by passive sensors disclosing information about the targets, just as the human eye sees objects illuminated by the sun or another light source. The enemy can attempt to take out the broadcasting active sources, but cannot detect the passive receivers.

Drones are also based on 500mm missiles, but not only have their warheads been removed, but they have smaller engines and NPU (Nuclear Power Units) rather than Energy Cells. This allows them greater endurance. Drones are used to spoof enemy sensors by jamming, distortion, or projection. A drone can make a Battlerider appear to be a Dreadnought, or making a destroyer seem to be a whole squadron of destroyers.