When tag is outside of the location area (border made by positions of anchors), it still can appear inside the localization area. This behavior is based on the TDOA principle where distance between tag and anchors are unknown, only the arrival time of the signal to anchor is known, not a flight time. So even when tag is out of the location area, location engine can wrongly put it into its border. This problem is not unambiguously solvable, it is only possible to predict whether the tag is inside or outside of the envelope. User can use this feature to draw exits from localized area and to suppress the behavior. If exits are drawn, the following algorithm is used:
- Calculate position from x anchors.
- Find user defined exits, near the envelope (created from x anchors). Exits can be 2 meters near envelope.
- Relocate these exits to the edge of the envelope (this projection is implemented because there are not always exits to the room directly on the anchor envelope)
- Estimate the distance from the tag position to all such exits.
- Find out if the tag is at least 0.1 meter around the anchors envelope.
- If yes, the tag left the room, mark this tag as a sign of leaving the room and discard position.
For each next position algorithm checks if tag is still on the border (0.1 meter) of the envelope. If yes and the tag is still marked as leaving, the position of the tag will be discarded. Flag "leaving" will be removed from tag, if last five positions are not on the border of envelope.