A "classic" ranging system used since the 1960s uses radar to time a radio signal's travel to a target (in this case, the rocket) and back. This technique is accurate to approximately 1%. The accuracy of this technique is limited by the need to create a sharp "pulse" of radio so that the start of the signal can be accurately defined. There are both practical and theoretical limits to the sharpness of the pulse. In addition, the timing of the signals often introduced inaccuracies of its own until the introduction of high precision clocks.