Murray Leinster 
Operation Terror (Serapis Classics) [EPUB ebook] 

On the morning the radar reported something odd out in space, Lockley awoke at about twenty minutes to eight. That was usual. He’d slept in a sleeping bag on a mountain-flank with other mountains all around. That was not unprecedented. He was there to make a base line measurement for a detailed map of the Boulder Lake National Park, whose facilities were now being built. Measuring a base line, even with the newest of electronic apparatus, was more or less a commonplace job for Lockley. This morning, though, he woke and realized gloomily that he’d dreamed about Jill Holmes again, which was becoming a habit he ought to break. He’d only met her four times and she was going to marry somebody else. He had to stop…