In the United States Armed Forces, the joint precision approach and landing system (JPALS) is an all-weather system for precision guidance of landing aircraft. It is based on real-time differential correction of the Global Positioning System (GPS) signal, augmented with a local area correction message, and transmitted to the user via secure means. It is used on terrestrial airfields as well as the US Navy's amphibious assault ships and aircraft carriers (hull classifications LH and CVN, respectively).
The onboard receiver compares the current GPS-derived position with the local correction signal, deriving a highly accurate three-dimensional position capable of being used for all-weather approaches via an Instrument Landing System-style display. While JPALS is similar to Local Area Augmentation System, but intended primarily for use by the military, some elements of JPALS may eventually see their way into civilian use to help protect high-value civilian operations against unauthorized signal alteration.