Complex sentence: Sometimes information would have to be plotted that came from an external source, not normally part of the reporting chain; a fighter calling an SOS would report this through his radio on an emergency frequency, which would be picked up by the closest listening post.
Split into simpler sentences: Sometimes information would have to be plotted that came from an external source, not normally part of the reporting chain. A fighter calling an SOS would report this through his radio on an emergency frequency, which would be picked up by the closest listening post.