Which source is prioritized first in the positioning source hierarchy?

The correct choice reflects the importance of positioning accuracy and reliability in maritime navigation. In the positioning source hierarchy, Differential GPS (DGPS) is prioritized first because it enhances the accuracy of standard GPS signals. DGPS achieves this by using ground-based reference stations that transmit correction data to the GPS receiver, thereby improving the precision of position fixes and mitigating error factors commonly associated with standard GPS.

This is critical for operations where navigating close to hazards or requiring high positional accuracy is necessary, such as during search and rescue missions or when operating in congested waterways. The corrected signals significantly reduce position errors, making DGPS a preferred source when high accuracy is paramount.

Visual/Radar LOPs, standard GPS, and celestial navigation all provide useful navigation data, but they generally do not match the precision and reliability offered by DGPS. Hence, while radar and visual navigation, as well as celestial methods, remain important in certain situations (like in the absence of satellite signals), they are secondary to the enhanced accuracy provided by DGPS in the hierarchy of positioning sources.