Name one tool used for depth measurement in navigation.

The echo sounder or sonar is an essential tool used for depth measurement in navigation. This instrument functions by emitting sound waves into the water and measuring the time it takes for the waves to bounce back after hitting the seabed. By calculating the time delay and knowing the speed of sound in water, the device can determine the depth below the surface accurately. Echo sounders are widely utilized in various maritime activities, including fishing, underwater mapping, and navigation, as they provide critical information about the underwater topography, helping to avoid obstacles and ensuring safe navigation.

The other tools listed serve different functions and are not primarily designed for depth measurement. A magnetometer is used for detecting magnetic fields and measuring magnetic anomalies, primarily for locating submerged objects or map magnetic variations. A GPS receiver determines a vessel's position using satellite signals but does not measure depth. A radar altimeter, while capable of measuring height above the surface, typically uses microwave radar signals for altitude measurement, and its application is more suited for aviation than for measuring underwater depth.