GPS, Global Positioning System. We all know what it is and how it works. Only problem is the denial from FE'ers on the subject and simple shrugging off of the topic by going on about pseudolite crap. So here goes.
Everyone understands triangulation right? Knowing where something is by knowing it's distance from at least 3
points on a map, on a table, on the globe. On a 2d surface it's easy, we just draw a circle of specific radius around each point we know we are from. If your initial measurements were correct, you will get a point
where all three intersect.
This image shows this (note, the 'rounded' triangle in the middle is our point. In this example, our measurements taken are not perfectly accurate and so our point is rather an overlap of all three.)
In 3d space it isn't that much harder. If we know our distance from 3 satellites, and how high the satellites are orbiting, we now have 4 points (the fourth being our altitude of zero. Alternatively, we know our distance from 4 satellites to know our 4 points).
We need this fourth point because we added a dimension to work with. If we don't add a fourth, we are left with two points where all three spheres overlap, one that is higher than the satellites' orbit, and one at ground level (or at our altitude if we are using 4 satellites). We aren't out in space, therefore we must be at the location which gives us our altitude of zero (with 3 satellites) or our actual altitude (with 4 satellites or more).
What's that you said? "How do we know our distance from each satellite though?"
Well I'll tell you.
Each satellite knows the [atomic, yes, atomic] time and your receiver knows the time too [not quite atomic. This leads to errors. More expensive units just have more accurate clocks]. Your receiver sends out a "Hey satellites, I want this message back k?" message and the satellites in range will send it back. Your receiver figures out how long it took and therefore knows how far away it was [knowing the speed of light and Doppler effect on the specific radio frequency sent out etc]. To create redundancy [accuracy or backup], there should always be at least 7 (provided no obstructions) satellites in view.
Some smart guys sitting in computer labs in places around the USA monitor the satellites and make sure they are on path. Remember, a satellite will orbit the earth in a constant circle [sometimes oval] no matter how fast the earth is spinning. Therefore if we take into account the earth's rotation, each satellite makes a curved path along the ground (unless it orbits parallel to the earth's rotation)."But why can't pseudolites exist then?"
Well, that's because of the frequency used in GPS.
GPS actually uses 5 different frequencies. A couple are the actual navigation frequencies and the others are used for ionosphere research and even detection of nuclear detonation to ensure treaties are being adhered to. All of these frequencies lie between 1575.42 MHz -1176.45 MHz placing them in the UHF range. So?
Well, UHF relies on line of sight. You might not be able to see satellites at daytime due to the 'blueness' of the sky (refraction of visible wavebands) but they are there, and at night time, you can [sometimes] see them. Radio masts like those claimed to be pseudolites are not line of sight and cannot (repeat, cannot
) possibly give any indication of distance due to their waves bouncing off the ionosphere and therefore not travelling in a straight line. If we could stay in line of sight, sure it might work. But how many radio masts can you see in any direction right now? (I can’t see any, but my GPS still tells me where I am)But if the earth is actually flat, wouldn’t these radio waves just go straight?
Yea, but 'bendy light theory' bends all wavelengths equally [apparently] so no radio waves will reach you in a straight line [in FET]. Besides, in a valley you can still get GPS reception so the signals must be coming back from the sky. This leads us to the conclusion that satellites must
exist because they are the only possible way for a GPS to work. And as I'm sure we can all figure out, for satellites to exist, they need to be suspended in orbit around a spherical earth. Nail in the coffin.
Tl;dr this if you want, FE’ers. It’ll only harm your integrity. Feel free to dispute anything I have said.