Understanding the CDI, or course deflection indicator, is a critical part of navigating the skies. The CDI can take information from a variety of sources, including the GPS, VORs, and localizer. Glass CDIs will mark the source in words and color (magenta for GPS, green for NAV radios). The CDI can point to the next fix in a flight plan, or a course set by the pilot. In addition to showing where to fly, it can also show how far off the desired track the aircraft is. When setting up for an approach, CDIs can even automatically change the source when inbound on the final approach course.