I still dont get your magnetic variation thing and how navigation is possible using that.
The magnetic longitudes are defined as a line joining all the points on the surface if earth with same variation, and these lines are never straight as geographical longitudes but have a constant rate of drift for each area for which navigational charts are available and are very accutate as they are produced for use by ships and aircrafts.
You dont need your own satellite just for knowing magnetic variation. But how even accurate magnatic variation cam give you position, that is beyond me as there are no magnetic latitudes.
A missile can cover oceanic span just by using INS and when making landfall, can correct position scene matching. For example looking for a hill , light house , a land contour or other prominent coastal feature.
Once above that known landmark. Then navigate inland by TERCOM. and INS.
That is more useful than yout method of variation.
I have been a navigator myself. But if i am mussing something, correct me.