Downhole telemetry

Signals transmitted in real-time (while drilling) from an instrument located near the bottom of the drill string to a receiving monitor on the surface (a surface-readout). Downhole telemetry may be transmitted via mud pulses (much as sonar signals are transmitted through water), through electro-mechanical methods (radio transmission) or via a wire within the drillpipe. Frequently, downhole telemetry is employed in determining the drift angle and direction of a deviated wellbore. The signal from the downhole tool to the surface is called an uplink. (See measurement while drilling)