Global Navigation Satellite Systems (GNSS) — Page 307, Lesson 302

Let’s start with the very foundation of GNSS — the coordinate system. GNSS uses an earth-referenced, three-dimensional Cartesian coordinate system. That means we define every position using three axes — X, Y, and Z — with the origin, the zero point, at the centre of the earth. So a position is given as a set of three coordinates, like X1, Y1, Z1 for one point and X2, Y2, Z2 for another. This is a global, earth-centred frame, and it’s the backbone of everything we do with satellite navigation. Now, because these systems are global, we need a common model of the earth itself. The earth isn’t a perfect sphere, so we use a mathematical model called an ellipsoid — a regular, smooth shape that approximates the earth. For GPS, the model chosen is the World Geodetic Survey of 1984, which we call WGS84. Every GPS terrestrial position is defined on this model and referenced to that Cartesian coordinate system. WGS84 is also the ICAO standard for aeronautical positions, so it’s the one we use in aviation. But other systems use different models. Galileo uses the European Terrestrial Reference System 1989, or ETRS89. The Russian system, GLONASS, uses a model called Parameters of the Earth 1990, or PZ90. Now, here’s the key point: since all of these are mathematical models, converting from one to another — say from ETRS89 to WGS84 — is a relatively simple mathematical process. And in fact, GPS receivers available in the UK incorporate this transformation as a feature, because the UK’s Ordnance Survey maps use a different model. Now, there’s an important limitation. The ellipsoid cannot be a perfect representation of the earth, and it cannot represent geographical features like mountains or land depressions. Mean sea level — the average level of the sea — depends on gravitational forces, which vary both locally and globally. So mean sea level will not necessarily coincide with the surface of the ellipsoid. For WGS84, the maximum variation between mean sea level and the ellipsoid surface is approximately 50 metres. That means the vertical information from any system referenced to this model cannot be used in isolation for vertical positioning — except in two cases: when you’re in medium or high level cruise with all aircraft using the GNSS reference, and in LADGNSS applications, where the vertical error is removed. So for precise vertical work, you can’t just trust the ellipsoid height on its own. Now let’s move to the GPS segments. GPS comprises three segments: the Space Segment, the Control Segment, and the User Segment. The Space Segment is the satellites themselves. The Control Segment is the ground infrastructure — and I want you to note the monitor stations: Hawaii, Kwajalein, Colorado Springs, Ascension Island, and Diego Garcia. These monitor the satellites. The User Segment is the receivers — the equipment you and I use in the aircraft. Finally, let’s talk about GPS time. GPS time is measured in weeks and seconds, starting from 00:00:00 on 06 January 1980 UTC. There’s a concept called an epoch — that’s a period of 1024 weeks, after which the time restarts at zero. GPS time is referenced to UTC, but it does not run in direct synchronization with it. So the satellite broadcast includes time correlation information to let receivers correct for the difference. For example, in July 2000, the difference was about 13 seconds. So GPS time and UTC drift apart, and the system tells you how to reconcile them. That’s the foundation — the coordinate system, the earth models, the three segments, and GPS time. Let’s look at the figures to see this visually. shows the Cartesian coordinate system with the X, Y, Z axes and the two points. shows the GPS satellite constellation. And illustrates why the use of two frequencies is important — we’ll get into that shortly.