This work describes an actively controlled frequency selective surface (FSS) to implement ultrawideband radio identification (RFID) tags. The tag exploits the change in the radar cross section (RCS) of the FSS, which is loaded with switching PIN diodes to modulate the backscattered time-domain response of the tag to an input ultra-wide band pulse. The basic operation theory of the system is explained. An experimental setup based on a ultrawideband radar working as a reader is proposed to measure the modulated radar cross section of the tags. As a proof of concept, a battery-assisted or semipassive tag has been developed.