Defense posts protect the planet from attacking starships and increase colonist fighting effectiveness against ground assault. Cost of building a defense post is 10 MegaCredits and one supply unit. The maximum number of defense posts you can build on a planet is limited by the number of colonists on the planet.
If a planet has 15 defense posts, the planet's industrial activity will be screened from long range sensor sweeps by enemy ships.
If a planet has 30 defense posts, and it is a victim of any of the Super Spy Deluxe or Super Spy Command missions, it has a chance to burn up 10 of the defense posts to emit an ion pulse that will decloak all cloaked ships in orbit.
Defense posts have no effect on what information orbiting enemy ships have access to. Orbiting enemy ships always have access to updated colonist and climate information, and they can also determine if there is a starbase present.
For more details on defense posts, please look here on the Details page.