>they were launching nukes from submarines for *decades* before GPS was around.
Yep, previously they used NAVSAT. GPS was first introduced into the Trident's IGS on the D5, though launch platforms had already shifted by then. In the updated navigation sub-system finalised last month, the IGS is being spun off to Draper and will no longer include integral GPS as this is provided by a new discrete Boeing unit. But you know all this, cos you're like an expert and not just reading wikipedia or something.