Signature radius refers to the size of the signal that can be used to detect a ship or any other object in space using Ship sensors. A smaller signature radius makes a ship harder to hit with turrets due to tracking effects, take less damage from missiles, and take longer to lock.
Signature radius is proportional to the physical size of the ship. However some modules can increase or decrease the signature radius of a ship. For example, a Microwarp Drive will increase the signature radius, as will Shield rigs. Some implants will decrease the signature radius.
Signature radius applies to any object in space, not just ships. This includes drones, wrecks, Jet Cans, Player Owned Stations, or stations.
Tracking and Damage Effects
Turrets and missiles work differently in regards to signature radius.
All turrets have a tracking number which represents the amount of radians per second it can rotate to keep lined up with a target. A 1 radian angle is equal to an arc of 1 radius in length. So a turret with 0.1/sec tracking is locked on a target orbitting 10,000m away it can pace the target if it travels 1/10 of a radius or less; or 0.1*10,000m = 1000m per second or slower. If the signature radius of the target is equal to or greater than the turret's scan resolution, the turret will track at the tracking speed listed in its info page. If the signature radius is smaller than the scan resolution of the striking turret, tracking speed will be reduced in proportion to the ratio of the signature radius to the scan resolution.
Missiles always hit their target so long as they reach their target before they run out of fuel (flight time is indicated on the missile's info page). Instead of tracking, each missile has an explosion radius. If the target's signature radius is equal to or greater than the explosion radius of the missile, the target will take full damage from the missile. If the target's signature radius is smaller than the explosion radius, the damage will be reduced in proportion to the ratio of the signature radius to the explosion radius. Damage in either case will further be reduced by the ratio of the missile's explosion velocity to the target's velocity, if the target's velocity is greater than the missile's explosion velocity.
Signature Radii by Ship Type
Minmatar ships have the lowest signature radii and Caldari ships have the highest.
The signature radius of a Strategic Cruiser is based on its defensive subsystem.