[time-nuts] Satelles PNT from Iridium satellites

[Hal Murray]

time-nuts at lists.febo.com said:
> On more broader sense, was GPS originally designed to provide timing
> service?  Or is it a byproduct of needing to measure location and speed, thus
> it needed a constant signal, and that using it to sync reference signal is
> just an ancillary and after-thought use cases?

I don't think that early GPS advertised that you could get accurate time, but 
it's fundamental to the way it works.

Suppose you know the time and the orbits of all the satellites and the 
satellites send a signal saying "my time is xxx".  The difference in time 
between when the satellite sends it's signal and when you hear it tells you 
the distance from that satellite.  With 3 satellites, you have 3 equations and 
3 unknows, X, Y, and Z, or their polar equivalents.  If you don't know the 
time, you need a 4th satellite to give you a 4th equation.

You can work out the 2D version easily by hand.  The distance-from-satellite 
turns into a circle.  2 circles will intersect in 2 places.  (ignoring the 
none or just touch cases)

In addition to the time, the GPS satellites also send their orbit parameters.


-- 
These are my opinions.  I hate spam.