Which of these categories does NOT include vehicles?

The category that does not include vehicles is Ground to Ground identification. This category primarily focuses on the identification of different types of ground forces, such as infantry and other personnel. It is concerned with recognizing and classifying ground targets based on their characteristics, capabilities, and missions, rather than their vehicular status.

In contrast, the other categories all involve vehicles in some capacity. Personnel refers to individuals or troops, which may operate vehicles but does not itself categorize the vehicles. Obstacles typically include items that can hinder vehicle movement, such as barricades, mines, or rough terrain that affects vehicular navigation. Artillery, while primarily associated with weapons systems, often includes self-propelled artillery vehicles that play significant roles in direct fire engagements.

Thus, Ground to Ground identification clearly stands apart as it does not encompass vehicles in its scope.