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

Let's begin with the coordinate system, because everything about GNSS hangs off this one idea. GNSS uses an earth-referenced, three-dimensional Cartesian coordinate system. That means we have three axes — X, Y, and Z — at right angles to each other, and the origin, the zero point of all three, sits at the centre of the earth. So every position is defined by three coordinates measured from the earth's centre. That's the fundamental reference frame. Now, because these systems are global, every country and every user needs to agree on one common model of the earth's shape. The model chosen for GPS is the World Geodetic Survey of 1984 — WGS84. All GPS terrestrial positions are defined on this model and referenced to that Cartesian coordinate system. So when a GPS receiver gives you a position, it's giving you coordinates on the WGS84 ellipsoid. Other systems use their own models. Galileo uses the European Terrestrial Reference System 1989, ETRS89. GLONASS, the Russian system, uses Parameters of the Earth 1990, PZ90. Now, WGS84 is the ICAO standard for aeronautical positions. But here's the key point: all these models are mathematical models, regular shapes known as ellipsoids. Because they're all mathematical, converting from one to another — say ETRS89 to WGS84 — is a relatively simple mathematical process. And note that this transformation is built into GPS receivers available in the UK, for example, to work with Ordnance Survey maps. Now, let's be clear about what an ellipsoid can and cannot do. An ellipsoid is a regular mathematical shape, so it cannot perfectly represent the earth, and it cannot represent geographical features like mountains or land depressions. Here's the subtle part: mean sea level does not necessarily coincide with the surface of the ellipsoid. Why? Because the distance of mean sea level from the centre of the earth depends on gravitational forces, and those forces vary both locally and globally. So the sea surface bulges and dips relative to the smooth ellipsoid. For WGS84, the maximum variation between mean sea level and the ellipsoid surface is approximately 50 metres. That's a real number to remember — about 50 metres. This has a direct operational consequence. The vertical information from any system referenced to this model cannot be used in isolation for vertical positioning. You can't trust the altitude alone. There are exceptions: when you're in medium or high level cruise with all aircraft using the GNSS reference, and in LADGNSS applications — Local Area Differential GNSS — where the vertical error is removed. So in those two cases, the vertical data is usable. Otherwise, treat the vertical with caution. 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. Colorado Springs is the master control. The User Segment is the receivers — the equipment you and I use in the aircraft. Finally, GPS time. GPS time is measured in weeks and seconds, starting from 00:00:00 on 06 January 1980 UTC. So the clock started at midnight on that date. An epoch is 1024 weeks, after which the time restarts at zero. So every 1024 weeks, the week counter rolls over. GPS time is referenced to UTC, but it does not run in direct synchronization with it. That's why time correlation information is included in the satellite broadcast — the SV broadcast, where SV means space vehicle, the satellite. For example, in July 2000, the difference between GPS time and UTC was about 13 seconds. So the receiver needs that correction data to align GPS time with UTC. Let me pull this together. The coordinate system gives you the frame. The ellipsoid gives you the shape. The segments give you the architecture. And GPS time gives you the clock. All four are foundational to how GNSS actually computes a position.