A Perimeter Scan displays all fleets within scanner range, and various additional data depending on distance.

Any fleet or colony can perform a perimeter scan.

Colonies scan everything in orbit around their planet or in colonies on the same planet.

Fleets at a colony or in orbit around it can see the same as a colony plus a distance throughout the system determined by its scanner level. The scan will not reveal fleets docked at a colony on another planet.

Fleets with a scanner level of 0 can only see fleets at their current location, and only the number of vessels, not their tonnages. If the fleet is docked at a colony, then it can only see other fleets docked at the colony.

Fleets with a scanner level of 1 or greater can see fleets as far away as their scanner level multiplied by 150,000 km from inside a system, or multiplied by 3 million km across interstellar space.

Scout Command Frigates have a cloaking ability that cuts their visibility at least in half.