Surface Acoustic Wave (SAW) tags are tags using the properties of piezoelectric materials to encode an unique ID in the signal reflected by a tag.
This works converting the electromagnetic pulse picked up by the antenna into an acoustic wave, propagating along the surface of the tag. Appropriately spaced obstacles (encoding the unique ID) reflect back the wave to the antenna, where it is re-converted into a train of pulses which is picked back from the reader.
Being chip-less, they are readonly and they do not offer anticollision, but can withstand extremely harsh conditions (such the ones experienced in the Space Shuttle).
