Global Navigation Satellite Systems (GNSS) — Page 307, Lesson 300

Let’s begin with the big picture, because this chapter is about a fundamental shift in navigation. I want to walk you through the introduction to Global Navigation Satellite Systems, or GNSS. The story starts in the 1950s. That’s when the development of space-based navigation systems began, with the establishment of the USA Transit system. So Transit was the pioneer. The current generation of systems started development in the 1970s, and the next generation is already under development right now. The grand intention is that GNSS will eventually replace all terrestrial radio navigation facilities — that is, all the ground-based radio aids we use today. However, despite USA assertions that this replacement is imminent, it is unlikely to be achieved in the foreseeable future. So keep that in mind: the plan is total replacement, but the reality is that it won't happen soon. Now, what has GNSS brought us? A new dimension of accuracy. Precision is measured in metres. And when special differential techniques are used, the potential is for accuracies substantially less than one metre. So we're talking about metre-level precision as standard, and sub-metre precision with differential techniques. At present, there are two operational global navigation satellite systems. Let me name them precisely. The first is the NAVSTAR Global Positioning System, commonly called GPS, operated by the USA. The second is the Global Orbiting Navigation Satellite System, GLONASS, operated by Russia. GLONASS had serious problems following the disintegration of the USSR in 1989/1990, but it is now fully operational. Beyond those two, there are enhancements and planned systems. There's Local Area Differential GNSS, abbreviated LADGNSS, which provides improved accuracy and integrity to aircraft making airfield approaches. Then there's Wide Area Differential GNSS, WADGNSS, of which the European Geostationary Navigation Overlay System — EGNOS — is the European contribution to a global augmentation system. EGNOS provides integrity and improved accuracy. Then we have the European Galileo system, which is under development. It's intended to provide a limited service from 2014/2015 and be fully operational by 2020. Now, the principal reason the Europeans are developing their own system is one of internal security, since access to the full GPS or GLONASS facilities is outside European control. That's a key point: it's about control and security, not just technology. China is also developing its own system, known as Compass or Beidou 2, expected to be fully operational by 2020. Now, this chapter will study GPS, LADGNSS, and EGNOS in detail. But bear in mind that GLONASS and Galileo operate on similar principles to GPS, although there are differences in implementation. So if you understand GPS deeply, you understand the family. Let me now move to satellite orbits. This is where the physics comes in. Johannes Kepler's laws quantified the mathematics of planetary orbits, and these laws apply equally to the orbits of satellites. Using these laws, and given a starting point, the satellites — which we call space vehicles, abbreviated SVs — calculate their positions at all points in their orbits. And here's a key term: the SVs' orbital position is known as ephemeris. So ephemeris is the precise position of a satellite in its orbit at any given time, calculated using Kepler's laws from a known starting point. Let me show you the satellite constellation so you can see how these space vehicles are arranged. So to summarise what we've covered: GNSS is the umbrella term, with GPS and GLONASS as the two operational systems, LADGNSS and WADGNSS/EGNOS as differential enhancements, and Galileo and Compass/Beidou 2 as the new systems under development. And the foundation of it all is Kepler's laws, which let each space vehicle compute its ephemeris — its orbital position — at every moment. That's the core idea we'll build on as we study GPS in detail.