The primary satellite functions discussed below – telemetry, tracking, and command (referred to as TT and C, or TT&C) – are operations functions associated with the satellite bus. The satellite bus provides the necessary support functions for the operation of the instruments, which are frequently called the satellite payload. The satellite bus provides power, attitude control (maintaining the desired orientation of the satellite, or satellite pointing), commanding and data handling, health-monitoring (both satellite and payload), station-keeping (i.e., maintaining the desired orbit and correcting for atmospheric drag, orbit precession, etc.), and maintaining the correct thermal environment (e.g., turning on heaters if necessary), amongst other functions.