  A passive underwater acoustic marker is presented and its feasibility for
underwater recognition, positioning, and navigation is proved by a numerical
method and experimental results. These markers are composed of acrylic elastic
objects designed by backscattering strong peaks associated with the subsonic
Rayleigh wave resonance of a polymer target excited by a broadband pulse, and
having a unique acoustic signature for a selected frequency band, akin to
acoustic barcodes. Therefore, the backscattering response of markers can be
regulated by changing the geometry of elastic objects. These acoustic barcodes
naturally operate in a wider frequency band, and have a longer lifetime and
lower cost, compared with active acoustic markers.
