Something like a GoPro that can do photo timelapses. Weather & condition ignorant, it's a set & forget. Grab a still of a clock with the seconds exposed on the first few frames & adjust accordingly.
What level of detail are you looking for - 5 fps is kind of wonky, either do 1 fps as a photo, or do low res video at 24fps. As these are just backup of the rf based system, what fraction of a second accuracy are you looking for?