Signal Structures for Satellite-Based Navigation: Past, Present, and Future

The past 15 years have witnessed tremendous developments of new and modernized satellite-based navigation (satnav) systems, both enabled by and stimulating the design of satnav signal structures with different characteristics from those originally used by GPS and GLONASS. This invited overview paper introduces a framework for describing satnav signal structures, then uses this framework to provide perspectives on past and present signal structures, and some conjectures about the future. Twelve characteristics are used to define the satnav signal structure framework, enabling an organized way to describe qualitative and quantitative characteristics of specific signals. Many of today's signal characteristics were identified, evaluated, and recommended in a technical report written more than 40 years ago for the Air Force's Project 621B, a program preceding GPS. Some of them were not implemented in the original GPS signals, likely because technology was judged not ready at that time to cost-effectively implement them. Other satnav signal characteristics that have been introduced more recently, expanding and enhancing the "tool kit" available to today's signal designers, are also described. While "it is difficult to make predictions, especially about the future," 1 some conjectures are made concerning future satnav signals.