gdo
The device used to open the iris on the Stargate. By entering a sequence of numbers into the keypad, the device transmits the identification code, or IDC, through the Stargate via radio waves to inform Stargate Command that there are friendly targets incoming.

Upon receiving this signal, the SGC has the choice of either sending a 'go' or 'hold' code back to the GDO. The former indicates that travel to Earth is safe, while the latter indicates that the arriving SG team should hold their position due to some difficulty.

The GDO was referred to earlier as simply a "transmitter". The acronym GDO stands for "Garage Door Opener".
Teams returning through the Stargate to Earth use their GDO to identify themselves as friendly.