Concept

The idea of integrating navigation functionality into the communications system LDACS is in many ways inspired by similar approaches in modern terrestrial mobile communications networks. Using trilateration or multilateration, a mobile receiver is able to determine its position if it can perform ranging, i.e., determine its distance, to a sufficient number of signal sources at known locations, e.g. to the base stations.  The cellular organization of the LDACS communications network allows exploiting this principle for aircraft position determination. Figure Cellular Concept
At cruising altitudes an aircraft typically “sees” multiple LDACS ground stations within its radio horizon. Since the forward link transmissions are synchronized and transmission signals from different ground stations are separated in frequency, an aircraft can perform independent range measurements to all visible ground stations. At least four range measurements are required to solve the navigation equations to obtain the aircraft position in x-, y-, and z-coordinates as well as the clock offset between the synchronized LDACS ground station clocks and the local clock at the airborne receiver. The number of range measurements might be reduced if additional information from other sensors can be obtained. For example, the altitude (z-coordinate) might be taken from the barometric height sensor which is readily available at any aircraft. In fact, due to relatively small variations of station altitudes with respect to the aircraft altitude, the estimation of the latter might be very inaccurate - a well-known problem for all ground-based navigation systems. The navigation functionality of LDACS is foreseen to act as back-up system in the case of disruptions of  the global satellite navigation systems (GNSS). As the GNSS signals are characterized by very low received power, the disruptions might occasionally occur due to unintentional interference, or even intentional jamming. In aeronautics, such a back-up system is commonly referred to as APNT (Alternative Positioning, Navigation, and Timing). The theoretical limits on ranging accuracy obtainable with  LDACS signals indicate that the position accuracy achievable with the LDACS APNT service is well below the required limits, which indicates the suitability of LDACS as a future APNT system.