Improving the availability of LDACS-based APNT with Air-to-Air ranging

The currently envisioned plan for Alternative Position, Navigation, and Timing (APNT) relies heavily on Distance Measuring Equipment (DME) for providing RNAV/RNP services. Alternative proposals include the use of L-Band Digital Aeronautical Communications System (LDACS) signals as a means of obtaining ranging measurements, relative to a network of ground stations. We extend the LDACS-based approach to include the Air-to-Air (A2A) Communications capability, which has been specified for future implementation. Using A2A distance measurements between aircraft, potentially extends the availability of APNT. This is particularly interesting for low-altitude users (e.g. non-precision approach, departure) in the vicinity of high-altitude (e.g. en-route) users.