Sub-wave communication

A sub-wave communication is a sentient piece of software that when used in a marginaly low signal strength was undetectable by Dalek scanners and was created by the Mr Copper Foundation. The wave was then further developed by Harriet Jones.

The Sub-wave communication system was created for the purpose of seeking out anyone who came into contact with The Doctor in recent times and would possibly be able to help find him again. In the episode The Stolen Earth it was used then to contact Martha Jones, Torchwood 3 (specifically Captain Jack Harkness) and Sarah Jane Smith through video conferencing hardware.

The System also managed to find Rose Tyler who was currently at the time in Donna Noble's house with her mother and Grandfather, but was only able to watch as the Laptop owned by Donnas Grandfather had no webcam because they were "Naughty". Going on this assumption, the residual atron energy absorbed by space and time travel (which all the doctors companions would have) must act as a receiver and transmitter to the nearest video device.

Later in the episode The Stolen Earth the sub-wave was then used to combine the entire Earth's phone systems to communicate with the Doctor. This was assisted by the use of the rift in time and space. The Doctor then was able to hone in on Earth's location. However the signal was then strong enough that the Daleks were then able to trace the source of the transmission and then to where the transmission caddied after the discovery of Harriet,Torchwood 3. (DW: The Stolen Earth)