Loran is a ground-based radionavigation system, with signals that propagate along the surface of the Earth (groundwaves) and reflect off the ionosphere to return to the Earth's surface (skywaves). Groundwaves and skywaves impinge on a receiver, together with other noise present in the spectrum at frequencies around 100 kHz (such as noise from nearby power lines or lightning). In general, groundwaves are useful for navigation purposes, but skywaves contribute unwanted noise and distort groundwaves. In order to resolve the most useful groundwaves accurately and to reduce noise introduced by skywaves, less useful groundwaves, and other interference sources, a modern Loran receiver must track, separate, and process all these signals simultaneously to achieve the performance now possible.