spotter

"It's probably that cow-boy spotter of Norcross's, but he can't get away," Hatch was gritting—meaning Tarbell, probably.

He was still below the alleged spotter with its straw-stuffed observer when Tam dived for him.

Not the least dangerous of the many services the airplane is performing is that of the artillery “spotter.”

Or he may have to report the results of a mock bombardment, and thus prepare himself for the duties of the artillery “spotter.”

We were watching the top of the hill and saw the little L4 spotter planes flying low and directing the artillery fire.