An Alternative Positioning Navigation and Timing concept based on Diverse Ranging

Abstract

An Alternative Positioning Navigation and Timing concept, called Diverse Ranging (DivR) is proposed for sustaining NextGen Performance Based Navigation and Automatic Dependent Surveillance - Broadcast during Global Navigation Satellite System outages. The system consists of a network of ground stations providing navigation signaling services to aircraft. The service is provided by using ground stations to selectively reply to the spontaneous broadcast messages of a small subset of aircraft. These new messages allow avionics to calculate positions in two modes - the Direct-Reply (DR) mode and Non-Reply (NR) mode. The DR mode is used by aircraft receiving addressed replies from the ground stations and is based on observed round-trip range measurements. The NR mode is used by aircraft receiving ground station replies that are addressed to other aircraft, and the processing is based on pseudorange and echoed pseudorange measurements. Timing signal broadcasts are also sent by the ground stations, which are synchronized using aircrafts' position broadcasts. The following analyses were conducted in order to characterize the performance of DivR: (1) nominal error overbounding and a preliminary Fault Modes and Effects Analysis, (2) initial integrity and continuity risk allocations based on Targeted Level of Safety Fault Tree Analysis, (3) theoretical derivations of the Navigation System Error (NSE) and Horizontal Protection Level (HPL) performance bounds, and (4) a terminal case study based on Washington Dulles International Airport for which the NSE, nominal-condition HPL, and spectrum impact were analyzed. The results show that DivR meets the required navigation accuracy and integrity requirements under nominal conditions for terminal operations in both low and high interference environments, with 99% availability and a 1-sec update interval. Further analyses are required to evaluate the performance under faulted conditions and evaluate time to alert and continuity performance. Timing service synchronization accuracy is expected to be sub-microsecond.