A space-time attenuator is a device that blocks electricity, GPS signals, and consciousness transfer within a certain radius - effectively making the user invisible to observing entities in the future. The effect is achieved with deuterium oxide, slowing the movement of neutrons.
As of the end of season three, four space-time attenuators have been used. Ellis used one that protected his farm within a 100-yard radius. Vincent Ingram outfitted his personal residence with a space-time attenuator built into the heating system. The Faction used a portable device to prevent the Director from seeing them while they tried to assassinate Anna Hamilton, the future 53rd President of the United States.
The fourth STA was created by a non-traveler FBI task force that had been monitoring traveler activity around the globe. The STA shielded one of their operations rooms at headquarters, which was used by the FBI Director and Agent Yates to prevent Grant MacLaren from communicating with his team while they tried to interrogate MacLaren regarding the three nuclear devices that had been detonated in London, Moscow, and Shanghai.