Schematic layout of the CCDs in the focal plane of Gaia. Due to the satellite spin, a source enters the focal plane from the left in the along-scan (AL) direction. All sources brighter than G = 20 mag are detected by one of the skymappers (SM1 or SM2, depending on the field of view) and then tracked over the subsequent CCDs dedicated to astrometry (AF1–9), photometry (BP and RP), and radial-velocity determination (RVS1–3). In addition there are special CCDs for interferometric Basic-Angle Monitoring (BAM), and for the initial mirror alignment using Wavefront Sensors (WFS).