A Brief Summary of Four Keynote Addresses

The principles of operation of satellite navigation systems1 may be outlined as follows. Earth-orbiting satellites, carrying extremely accurate atomic clocks, broadcast time information at the L1 frequency (1.5754 GHz) and L2 frequency (1.2276 GHz). A sensitive receiver (e.g., aboard an aircraft) picking up these radio signals (~10−16 W received power) from four different Global Positioning System (GPS) satellites measures four pseudo-ranges and, by triangulation, calculates the aircraft’s position (in three dimensions) and its velocity. The performance of such a positioning system — with its accurate time reference — may be enhanced, or “augmented”, in various ways by using additional radio sources, either in space or on the ground.