What is GNSS about …

For general information to the beginner I want to stress in this post the fundamentals.
I feel like they are often forgotten, and this is the most fundamental concept; ” It is all a matter of time!” With that phrase I mean to say that GNSS position is only possible because we measure the time ‘t’ that a signal generated in point X (the source) takes to travel to point Y (the receiver) and this time ‘t’ times the speed of light (300000 km/s ; 187500 mi/s) gives us a distance between points X and Y.

As long as we have 4 independent distances and we know the positions of all the sources we can calculate the 3D position of point Y. This is important; we need 4 distances because the emitter and receiver clocks are never synchronised properly by having 4 measurements we can solve the 3D + dT (receiver clock correction).

This is even more important; we must know the positions of the emitters and their clock corrections to a high degree of accuracy at the moment of the signal being emitted so that we can calculate the position of Y correctly. The GNSS constellations emitt each satellite’s positions and “clock bias” as part of the ‘Navigation Message’ encoded in the timing signal. It is also possible to use external sources for that information such as the IGS (IGS, etc).

In future blogs I will explain some of those fundamentals elements in more detail.

Happy Positioning!

Ignacio Romero

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