Which methods should periodically validate GNSS positions according to NAVDORM?

The correct answer is based on the requirement for validating Global Navigation Satellite System (GNSS) positions to ensure navigational accuracy and safety. Periodic validation of GNSS positions is crucial because GNSS signals can be affected by various factors such as atmospheric conditions, obstructions, and multipath errors.

Visual and radar methods are effective for this validation because they provide additional reference points that can confirm the accuracy of GNSS data. Visual observations allow navigators to cross-check their positions against recognizable landmarks, while radar can be used to detect other vessels or navigational aids in the environment. By combining these methods, navigators can increase confidence in their GNSS-derived positions, particularly in challenging or congested navigational areas.

Other options may involve different technology or instruments, but they do not represent the ideal pairing for periodic validation as effectively as visual and radar methods do. Techniques like GPS and sonar or using an echosounder and visual might not provide the comprehensive situational awareness required for validating GNSS positions in all navigational scenarios. Hence, visual and radar serve as a robust method for ensuring accuracy and reliability in navigation.