Patent ID: 6549119
Filing Date: 2003-04-15
Classification: G06K

Abstract:
An identification system comprising a plurality of transponders and interrogation means, wherein each of said transponders includes means for responding to an interrogation signal by transmitting a response signal representing information stored in the respective transponder, each transponder being capable of operating in one of two modes, a first mode in which the response signal is at a first power level and a second mode in which the response signal is at a second, lower power level greater than zero, means for initially setting each said transponder in the first mode, and means for switching from the first mode to the second mode in response to a mode switching signal, and wherein the information stored in each transponder is obtainable from the respective response signal irrespective of the power thereof being at the first or the second level, and wherein the interrogation means includes(a) a first interrogator station including means for generating said interrogation signal and means for detecting said response signal at said first power level, and means for generating said mode switching signal for subsequently switching said transponders into said second mode; (b) a second interrogator station, including respective means for generating said interrogation signal, and means for detecting said response signal at said first power level but incapable of detecting said response signal at said second power level; and c) a third interrogator station, including respective means for generating said interrogation signal, and means for detecting said response signal at said second power level as well as at said first power level and thereby obtaining the stored information from transponders which have previously been switched into said second mode.