Definition - What does Sonde mean?
A sonde is located in the bottom hole assembly (BHA). It transmits a signal that allows personnel on the surface to track the location of the BHA and the drill string as they ride along the design pathway. While the sonde does not steer the directional drilling string, the information it provides allows the driller to turn the string to its intended path.
Trenchlesspedia explains Sonde
A sonde is a tracking instrument. As the drill head, the BHA, moves along the design pathway; you can follow its progress through the radio signal it transmits to a receiver on the surface. Using the sonde to track the BHA's progress allows installation accuracy of one inch or less. During the first few feet of directional drilling, the steerable drill string will almost certainly wander from the intended path; however, after that, the sonde's transmissions, when coordinated with the drilling pathway plan, can be used to return the drill string to its intended path.