G.D.O.

From The Stargate Omnipedia

GDO.jpg
Unofficially, a "Garage Door Opener." The device transmits an encoded signal from an off-world team through the Stargate's wormhole to be received by the S.G.C. computer, identifying the SG unit or ally. Upon signal confirmation base personnel will then open the iris covering the Stargate, allowing the inbound traveler(s) to enter the base safely.

A G.D.O. and iris code (a sequence of numbers that must be manually entered into the G.D.O.) is assigned to every member of every SG team, and to a small number of alien allies including the Tok'ra and Jaffa Master Bra'tac. The codes can be altered if one of the devices is compromised.

KEY EPISODES

Serpent's Song - The Tok'ra are issued their own G.D.O. and iris code.
Unnatural Selection - Probing O'Neill's mind, the advanced Replicators uncover his G.D.O. code.