Scenario: You depart airport at 2000 local for a 4.6 hour flight to a remote island. Island has no divert. Divert is to return to origin. Any other divert results in newspapers and "international incident". Island has no NAVAID. Weather is only available by the controlling agency on the field, which cannot be contacted until inside of 200nm. Everything outside of that is non-radar, and due regard. The ASOS on the field has been known to be slightly "off", but the controllers on the field can give you accurate wx. You're flying an airplane that has dual GPS. You're 3.8 into the flight and lose one of the GPS's. What do you do and why? Acft type: DC-8 with CFM's. Fuel is no factor (Enough for RTB, and mins.) Ask if you need more info, obviously.