I'm not familiar with "TDOA" - what is it?

My assumption is that the system knows how far away the cell phone is from
each tower. So:
One cell tower gives you a sphere on which the cell phone could lie.
Two towers give you a "box" where the cell phone could be, of which two
points are on the surface of the earth.
Three towers give you one deterministic point on the earth.
A fourth tower gives you a deterministic point in all space, including
altitude.

I can't remember if the third gives you altitude or not, but I'm pretty sure
that two towers give you two points, both of which are on the surface of the
earth and you have no way of knowing which is which, hence the need for a
third tower, satelite, or whatever.

Am I missing some bit of data that allows you to eliminate one of the two
points in a two-tower system?

--Ian


> Three - why does one need three when using 2 and TDOA
> will work for position on a geometrical plane? Well,
> solving for 'elevation' I suppose is a goal/reason for
> using three ...

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