A common problem in ground moving target indication (GMTI) radar is detecting a target with even a large radar cross section (RCS) when its line-ofsight velocity falls below the minimum detectable velocity (MDV) for that radar system. In a cooperative scenario, a target may employ a tagging device which can shift or spread its Doppler signature to become more detectable. In this paper we develop a bound on the minimum modulatable RCS of a passive (non-amplifying) device needed to be detectable as a function of radar and mode parameters, and compute this bound for specific GMTI and SAR radar modes.