Complex sentence: At its busiest, each sector will have a radar controller.When it is quieter sectors are "bandboxed" with one controller operating multiple sectors, until at night there may only be one controller operating the whole bank.
Split into simpler sentences: At its busiest, each sector will have a radar controller. However, when it is quieter sectors are also "bandboxed" with one controller operating multiple sectors, until at night there may only be one controller operating the whole bank.