After looking up while on the bridge it turns out I did
not in fact have the SJ radar despite it being available in port and despite my "installing" it from the reknown menu. That was a bit of a let down but is obviously a small oversight. Once I'd returned from that mission I was given a new boat and installed SJ and it was actually installed this time and operating as per normal.
So... I guess in the old sub despite the SJ being available it wasn't able to be truly installed so to speak. It would have been nice if a pop-up had said "cannot install at this time" or something similar but no real harm done. Lesson learned for future campaigns.
