Telemetra knows where it is at all times. It knows this because it knows where it isn’t. By subtracting where it is from where it isn’t, or where it isn’t from where it is (whichever is greater), it obtains a difference, or parallax. The ranging subsystem uses parallax to generate corrective estimates to drive the reading from a distance where it is to a distance where it isn’t, and arriving at a distance where it wasn’t, it now is. Consequently, the distance where it is, is now the distance that it wasn’t, and it follows that the distance that it was, is now the distance that it isn’t. In the event that the distance that it is in is not the distance that it wasn’t, the system has acquired a variation, the variation being the difference between where the target is, and where it wasn’t. If variation is considered to be a significant factor, it too may be corrected by the EKF. However, the iPhone must also know where it was. The Telemetra ranging computer scenario works as follows. Because a variation has modified some of the information the iPhone has obtained, it is not sure just how far it is. However, it is sure how far it isn’t, within reason, and it knows where it was. It now subtracts where it should be from where it wasn’t, or vice-versa, and by differentiating this from the algebraic sum of where it shouldn’t be, and where it was, it is able to obtain the deviation and its variation, which is called the error (±).

The First Computational Rangefinder.
Distance, with the margin to trust it.