# Why most RTLS using 4 anchors?

Hi there,

RTLS can be fully implemented with three anchors.
But why most RTLS using 4 anchors?

thanks.

Look at it from a purely mathematical point of view. You need at least one measurement for each unknown.
For a TDoA RTLS system there are 4 unknowns. Tag X,Y,Z and tag transmit time.
4 unknowns require 4 measurements which in turn requires 4 anchors.
This is exactly the same as GPS which requires 4 satellites to calculate a position.

For a TWR based system each range is measured independently and so transmit time doesn’t factor into things. This means a TWR system only needs 3 anchors. Assuming that is that the tag is stationary over the measurement period, given a short enough measurement period and low enough speeds this is normally a reasonable assumption.

Requirements can be reduced if you make more assumptions. e.g. if you assume a fixed height then you have one less unknown and so need one less anchor.

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My curiosity has been solved.
thanks a lot.