I am not an expert, but here goes.
There's two kinds of sensors.
Active (grav sensors) and passive (infrared).
You choose how big of a sensor, and what resolution you want, very similar to active sensors on ships.
If you fire a bunch of missiles at a ship, and it blows up while some missiles are still en-route, they will hunt for other targets using these sensors. You can also experiment with firing missiles at waypoints before having an active firing solution.
PROTIP: using active sensors on your missiles will alert the enemy to your position if they already don't know you are there, which is why I use IR missiles only on my fighters. I build fighters with thermal reduction engines and they use the active sensors of my carriers to lock on their targets. I sneak in, launch the missiles before they even know I am there. If they had active sensors, they would go hot as soon as they launched, blowing stealth out of the water.