Other measures vessels can take to protect themselves against piracy are air-pressurised boat stopping systems which can fire a variety of vessel-disabling projectiles, implementing a high freewall and vessel boarding protection systems (e.g., hot water wall, electricity-charged water wall, automated fire monitor, slippery foam). Ships can also attempt to protect themselves using their Automatic Identification Systems (AIS). Every ship over 300 tons carries a transponder supplying both information about the ship itself and its movements. Any unexpected change in this information can attract attention. Previously this data could only be picked up if there was a nearby ship, thus rendering single ships vulnerable. However, special satellites have been launched recently that are now able to detect and retransmit this data. Large ships cannot therefore be hijacked without being detected. This can act as a deterrent to attempts to either hijack the entire ship or steal large portions of cargo with another ship since an escort can be sent more quickly than might otherwise have been the case.