Abstract: A measuring while drilling tool is disclosed. The preferred embodiment incorporates an outer tubular member adapted to be connected in a drill string above the drill bit. The apparatus utilizes a mud pulser sub extending through the hollow tubular member and anchored in the drill string. The apparatus includes a sensor module and a battery module operatively connected to form a single tool. The apparatus utilizes mud flow pressure in the drill string to move a mud pulser sub through constriction rings for creating a sequence of mud pulses detectable by surface located receiving means.