Locator chip implant
From The Stargate Omnipedia
Homing beacon given to Stargate Command and Atlantis personnel, allowing them to be beamed out of unfriendly territory by the Asgard transporters installed on Earth's battle cruisers.
KEY EPISODES
Off the Grid - SG-1 is the first unit to be given locator chip implants.
The Scourge - Because of the planet's unusual magnetic activity, SG-1 is unable to be detected on the surface of Gamma Site by the Odyssey.