What type of signals does the ADF use to locate the NDB?

The ADF, or Automatic Direction Finder, uses radio frequency signals to locate Non-Directional Beacons (NDBs). NDBs transmit a continuous radio signal that can be detected by the ADF equipment aboard an aircraft. The ADF measures the direction of the incoming signal relative to the aircraft's heading, allowing pilots to determine their bearings to the NDB.

Radio frequency signals are especially suited for this application because they can travel long distances and penetrate various atmospheric conditions, making them reliable for navigation. This aspect of radio signals ensures that pilots can receive directional information even at considerable distances from the NDB.

In contrast, GPS signals, while also used for navigation, rely on satellite technology and function differently than radio beacons. Visual signals are not applicable in this context, as they are predominantly used for ground-based navigation aids rather than airborne systems. Similarly, infrared signals are not relevant to the operation of ADFs or NDBs, as they pertain to different technologies and applications. Thus, radio frequency signals stand out as the correct choice for how the ADF locates NDBs.