An indirect radar echo is caused by a reflection of the main lobe of the radar beam off the observer's vessel. Which of the following is NOT a characteristic of indirect echoes?

The reasoning for selecting the choice that states indirect echoes always appear on a bearing of 90° from the true bearing of the contact lies in the nature of how indirect radar echoes are formed. An indirect echo originates from a reflection off the observer's own vessel, creating a signal that does not directly represent the true position of the object being tracked.

In reality, indirect echoes can be observed in various positions relative to the true bearing, not strictly at a consistent 90°. This inconsistency is due to the complexity of radar beam reflections, vessel orientation, and the maritime environment, which can significantly affect the way an echo is perceived and displayed. Therefore, stating that indirect echoes always appear at a bearing of 90° is not accurate and highlights why this choice is not characteristic of indirect echoes.

In contrast, the other characteristics—such as their bearing being almost constant despite changes in true bearing, their usual occurrence in shadow sectors, and the fact that their plotted movements often seem abnormal—all accurately describe the behavior of indirect echoes in radar systems. These traits stem from the unique way indirect echoes are created and processed by radar systems, further clarifying why they don't conform to a predictable pattern of being at a 90° bearing.