Credential tracking system

In a system for tracking credential units within a reception zone exposed to interrogating radiation emitted from a tracking location at a predetermined carrier frequency, said interrogating radiation being sequentially modulated at different modulation frequencies during periodic transmit intervals for detection by the credential units in the reception zone causing emissions therefrom of reply signals at different code frequencies during reception periods between said transmit intervals, the improvement comprising:

• clock controlled means in each of the credential units for limiting said emissions of the reply signals therefrom at the reception zone to a plurality of time slots between successive transmit intervals, internal coding means in each of the credential units for selecting the time slots to which said emissions of the reply signals are limited and means for timing reception of said reply signals within the selected time slots at the tracking location to identify the respective credential units being tracked thereat.

In a method of identifying responders according to internal coding thereof while located within a reception zone exposed to interrogating radiation sequentially modulated during periodic transmit intervals at different frequencies corresponding to said internal coding for detection by the responders causing emissions therefrom of reply signals at different code frequencies;

• the steps of:
• limiting said emissions of the reply signals from the responders at the reception zone to a plurality of time slots within reception periods between said periodic transmit intervals;
• selecting the time slots for the respective responders within which the reply signals are emitted in accordance with the internal coding thereof and receiving the reply signals within the selected time slots at a tracking location from which the interrogating radiation is transmitted to identify and track the respective transponders thereat.