A sonde is a small, portable device used in conjunction with a drain camera or underground locator to help locate pipes, drains, and other buried utilities. It is typically attached to the end of the camera cable and emits a detectable signal that can be traced with a receiver. Sondes are essential for pinpointing the exact location of a camera or pipe in hard-to-reach or deep areas, such as underground or within walls. They are commonly used in drainage and sewer inspections to improve the accuracy of locating and mapping subsurface infrastructure.