Originally Posted by D driver
what I could think was that the receiver is required to combine all the different orbital elements transmitted and thus orbital position alone would not be of much use without orbit alignment etc. and thus would not be entirely correct to term it as position ?
That's gitting closer to the principles, but there's more to consider also.
Originally Posted by D driver
@WeeJeem is that what you mean? If not mind explaining a bit preferably with references?
The most relevant document is probably
this one here, specifically Appendix II, but whether it's a help, well, YMMV...
Soooo, bearing this in mind, i'll give it my best shot to explain in "Brian Cox" terms, but I'd ask others who also know the system to help out here when I flag a bit
Firstly, the SVs are
not geostationary. They are tazzing about in various directions at an orbital altitude of about 20,000km, at an orbital speed of about 14,000km/hour.
Secondly, here's the tricky bit that "just is": according to Special Relativity, time is
relative to an observer. So, for example, two events that occur simultaneously for one observer may not be simultaneous for another observer, i.e the concept of "now" is relative.
Why is this so important? Well, because it means, for example, that if an SV the broadcasts a message saying "I am at point X,Y,Z and the time is HH:MM:SS", unfortunately the only observers who can use that information are i) the SV itself and ii) any other observer who happens to be in the same inertial reference frame.
If we recall what we said earlier about the SVs " tazzing about in various directions", then clearly any (all!) SVs cannot be sharing the same inertial reference frame with our GPS receiver, so "now" is not the same for the SV and the receiver.
Which makes it kinda tricky to calculate "time of flight" when we can't agree on the start time to use to calculate the time of flight.
What most certainly
can be done is to measure time differences between arrivals of multiple signals ("time-difference of arrivals", aka TDOA) and then iteratively solve a simultaneous set of multilateral hyperboloid equations to derive the receiver's position - not triangles, not even spheres, but intersecting hyperboloids
The other little sting in the tail is that seemingly innocuous little word "iteratively" - what needs to be done is to make a first iteration of the equations that provides an initial solution, then use that first solution to refine the calculation to derive a second, more accurate solution, and use that to refine the calculation, and so on, and so on.
Take a look at Figure 20-3 in the linked document, and you'll at least get a feel for a) what a gps receiver is "responsible" for, and b) you'll see a couple of the refining iteration loops on the right hand side (clock correction, tropospheric model, etc).
Hope this helps a bit - time for a cuppa, methinks
