Sure it is. The same way it was legal to track and report on CIA "extraordinary rendition" flights using publicly available information.

What is not protected is actual interference or obstruction, and first amendment protections can be lost if the system’s design, stated purpose, or predictable use crosses from observation and reporting into intimidation or operational coordination that materially interferes or obstructs.

Given how these systems are already being used, and the likely intent behind building one, that's a real risk if you're not careful.