Teachers trying to monitor students on a field trip may soon get help from a smartphone if one of the youngsters wanders off, a South Korean researcher says. Hyukjae Jang at the Korea Advanced Institute of Science and Technology has developed RubberBand, a smartphone app that can tell if a child has wandered away from the group and is possibly lost. A sensor that can be attached to each child\'s backpack or clothing broadcasts a radio signal to all the other children\'s sensors once every second, and the smartphone app records the signal strength between every pair of children in the group to determine their locations relative to each other. If a child, or a few children together, wander away from the main group the app plays a sound to alert the teacher and can even display photographs of the children involved. \"This kind of work is invaluable even if we are able to prevent just one child from going missing,\" NewScientist.com quoted Jang as saying. He is to present the system at the UbiComp conference in Pittsburgh in September.