When Asgard transport beam technology was installed on the Prometheus, it operated without Asgard sensors, and individuals or objects to be transported had to be tagged in advance by a portable locator beacon that was usually worn on the arm, wrist, or ankle. During the mission to Tegalus, however, Daniel was taken prisoner and his locator beacon was confiscated, making rescue by beaming impossible. To prevent this situation from happening in the future, the SGC developed a new locator chip implant. SG-1 was the first to be implanted with the subcutaneous transmitters before the mission to P6G-452, and since then, members of SG teams and the Odyssey crew have been routinely implanted.
The subcutaneous transmitters operate in subspace, and they send a unique signal that can be located, traced, or targeted by the SGC. The Odyssey's sensors can detect and pinpoint the location of an implanted individual to instantly beam him away from among a sea of other life signs. The SGC has also used the transmitters to locate team members who are missing. When Athena captured Vala, she specifically had the transmitter removed from her upper left arm so that she could not be traced, but Mitchell's transmitter enabled the team to find him in a motel after he had been captured. The SGC had also implanted Vala's father, Jacek, with a subcutaneous transmitter without his knowledge, which made it possible to track and locate him near the SGC after he had removed his ankle bracelet and tried to escape.
The technology can be susceptible to interference, however. The radioactivity in the ionosphere above the Gamma Site caused interference that prevented Odyssey from getting a lock on the transmitter signal until a boost to the power of the communication array enabled Odyssey to triangulate the team's position within a target area. Similarly, when Carter performed an EVA at the supergate, her proximity to the active event horizon of the gate made it impossible for the sensors to lock onto her position. The transmitters also cannot penetrate a ship's defensive shields, making it impossible to get a lock on a target and beam an individual away in a battle situation when a ship's shields have been activated.
Ba'al had also acquired the Asgard transport beam and locator beacon technologies, and each of his many clones had been implanted with a subcutaneous transmitter. These transmitters enabled SG-1 to locate and capture several dozen of the Ba'al clones using the tracking system on Ba'al's own alkesh. SG-1 was confidant that the clones being detained at the SGC could not be beamed away because jamming devices had been installed two years earlier after the theft of the stargate, preventing anyone from locking on to a signal from within the base. However, the multiple transmitters in close proximity served to amplify the signal enough to be picked up through the jamming screen, and the Ba'al clones were beamed away from the base, as Ba'al had planned.
Cross Reference: Asgard Transport Beam, Ba'al, Locator Beacon, Odyssey
Episode Reference: Off the Grid, The Scourge, Camelot, Flesh and Blood, Insiders, Counterstrike, Memento Mori, Company of Thieves, Family Ties, Dominion