Jonas consults his handheld GPS receiver and determines that the sub is 13 miles off course. The GPS signal can't be received underwater, much less inside of a steel can. Subs can only receive VLF (Very Low Frequency) signals underwater and then only an antenna outside of the sub's hull.
When Jonas gets the operational plan from the safe, it is classified "Top Secret-No FORN." "No FORN" stands for "No Foreign Nationals" meaning that the information in the plan cannot be shared with non-US personnel. He hands the plan to the Korean skipper for him to look over. Additionally, the cover of the plan is in Korean. A "No FORN" plan would only need to be in English since foreign nationals are not allowed access.