  Reconfigurable intelligent surfaces (RISs) are considered as an enabling
technology for the upcoming sixth generation of wireless systems, exhibiting
significant potential for radio localization and sensing. An RIS is usually
treated as an anchor point with known position and orientation when deployed to
offer user localization. However, it can also be attached to a user to enable
its localization in a semi-passive manner. In this paper, we consider a static
user equipped with an RIS and study the RIS localization problem (i.e., joint
three-dimensional position and orientation estimation), when operating in a
system comprising a single-antenna transmitter and multiple synchronized
single-antenna receivers with known locations. We present a multi-stage
estimator using time-of-arrival and spatial frequency measurements, and derive
the Cram\'er-Rao lower bounds for the estimated parameters to validate the
estimator's performance. Our simulation results demonstrate the efficiency of
the proposed RIS state estimation approach under various system operation
parameters.
