Patent ID: 7119687

Claim:
A system comprising: a plurality of tags, each tag including means for transmitting a triangulation signal and a position signal, the position signal indicative of a change of position of the respective tag in a time period since a respective reset event; a plurality of triangulation stations for receiving the triangulation signals from the tags; at least one antenna for receiving the position signals from the tags; and a computer coupled to the triangulation stations and to the at least one antenna, the computer programmed to switch between: (a) a first tag location algorithm in which the computer determines a current location of one of the tags using a differential time of arrival procedure based on the triangulation signal transmitted by said one of the tags and received by at least some of the triangulation stations; and (b) a second tag location algorithm in which the computer determines a current location of said one of the tags based on the position signal transmitted by said one of the tags and a previous known position of said one of the tags.