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

Let’s start at the very beginning of GNSS — Global Navigation Satellite Systems. I want you to hold onto that full name, because it’s the umbrella term for everything we’re about to study. The story begins in the 1950s, with the USA’s Transit system — that was the first space-based navigation system. The current generation of systems started development in the 1970s, and the next generation is already being developed. Now, here’s a key point for your professional mindset: it’s intended that GNSS will eventually replace all terrestrial radio navigation facilities. But despite US assertions that this is imminent, it’s unlikely to happen in the foreseeable future. So don’t think of GNSS as something that wipes out everything else tomorrow — it’s the future, but terrestrial aids still matter. What GNSS has brought is a new dimension of accuracy. We’re talking precision measured in metres. And when special differential techniques are used — we’ll get to those — the potential is for accuracies substantially less than one metre. That’s a huge leap from older systems. Now, let’s map the players. At present there are two operational global navigation satellite systems, plus enhancements under development, plus a planned European system. First, the NAVSTAR Global Positioning System — GPS — operated by the USA. Second, the Global Orbiting Navigation Satellite System — GLONASS — operated by Russia. GLONASS had serious problems after the disintegration of the USSR in 1989/1990, but it is now fully operational. Then we have the enhancements. Local area differential GNSS — LADGNSS — provides improved accuracy and integrity to aircraft making airfield approaches. And wide area differential GNSS — WADGNSS — of which the European Geostationary Navigation Overlay System, EGNOS, is the European contribution to a global augmentation system, providing integrity and improved accuracy. Then there’s the planned European system: Galileo. It’s under development, intended to provide a limited service from 2014/2015 and be fully operational by 2020. And here’s the crucial reason the Europeans are building their own system — internal security. Because access to the full GPS or GLONASS facilities is outside European control. China is also developing its own system, known as Compass or Beidou 2, expected fully operational by 2020. Now, for this chapter, we’ll study GPS, LADGNSS, and EGNOS in detail. But bear in mind — GLONASS and Galileo operate on similar principles to GPS, although there are differences in implementation. So if you master GPS, you’ve got the core of all of them. Let’s move to satellite orbits. This rests on Johannes Kepler’s laws, which quantified the mathematics of planetary orbits — and those same laws apply equally to the orbits of satellites. Using these laws, and given a starting point, the satellites — which we call space vehicles, or SVs — calculate their positions at all points in their orbits. And here’s a term you must know: the SVs’ orbital position is known as ephemeris. That’s the word for where a satellite is in its orbit at any given time. Let me show you the GPS constellation so you can see what we mean by the satellites in orbit. So, to tie it together: GNSS is the family, GPS and GLONASS are the two operational members, LADGNSS and EGNOS are the differential enhancements, Galileo and Compass are the future systems, and every one of them relies on satellites — space vehicles — whose positions in orbit, the ephemeris, are calculated using Kepler’s laws. That’s the foundation.