GPS has been in use for two decades for military and commercial positioning and navigation. In recent years, additional constellations have been added – GLONASS, Galileo and Beidou. These four Global Navigation Satellite Systems (GNSS) provide reliable, accurate worldwide coverage. Over the years, accuracy and reliability have evolved so that one-meter accuracy is routinely achievable, and downtime events are extremely rare. Through international cooperation, these GNSS systems share common frequency bands, and affordable, multi-constellation navigation can be accomplished with a single receiver. The various signals are spaced close enough together to make reception efficient, but not so close as to interfere with each other.
Reliance on GNSS is now commonplace. However, all these GNSS systems share a common vulnerability: their signals are very weak. GNSS satellites operate from Mid-Earth Orbit (MEO), approximately 20,000-25,000 km above the earth, to provide the best coverage and geometry for triangulation. As such, the transmitted signal is extremely weak upon arrival at the surface of the earth – so weak that it is weaker than the surrounding radio noise. Special signal processing techniques recover the GNSS signal from the background noise, but the weak signal strength at the user’s receivers makes GNSS navigation very susceptible to interference.