Fine Guidance Sensor (FGS) for the Hubble Space Telescope is a system of three instruments used for pointing the telescope in space, and also for astrometry and its related sciences. Each FGS uses a combination of optics and electronics to provide for pointing the telescope at a certain location in the sky. There are three Hubble FGS, and they have been upgraded over the lifetime of the telescope by manned Space Shuttle missions. The instruments can support pointing of 2 milli-arc seconds (units of degree). The three FGS are part of the Hubble Space Telescope's Pointing Control System, aka PCS. The FGS function in combination with the Hubble main computer and gyroscopes, with the FGS providing data to the computer as sensors which enables the HST to track astronomical targets.