Abstract: A tracking system of the invention comprises a fixed part (30) and a consumable sensor capsule (1) the location of which is tracked in real time as it moves through the GI tract. The fixed part emits (31) acoustic signals, and the capsule receives these signals and in turn generates, after a set time delay, a response which is received by the fixed part and a computation is made of the distance between the capsule and the fixed part based on the time of flight and the intervening organs as modelled in the system's processor. The response is transmitted after a pre-set time delay and so is a simulated echo. Multiple receivers (36) are located at positions on a belt (40) chosen so that interference by bone is minimised, and so that the tracking procedure is ambulatory. The capsule has sensors (12, 13) which transmit data via an RF antenna incorporated in the capsule casing.