It is not appropriate to discuss the details of the TERCOM system here; however, I believe the following insights will help you to understand the relevancy of the cruise missile defense that you propose:

-	While TERCOM navigation depends upon the existence of unique terrain, TERCOM data gathering and processing, both for mapping and fix taking, integrates data over relatively large areas and tends to smooth out anomolous terrain features.  The existance of balloons such as you propose would not influence the mapping or the fix taking process.
-	TERCOM fix information is used to update the cruise missile inertial navigation system.  After an update the accuracy of the navigation system degrades gracefully with time.  Therefore, the final update of the navigation system could occur, depending upon the accuracy desired, at substantial distances and at any azimuth desired from the target.

It is my conclusion that the defense that you propose would not pose a threat to the cruise missile system that the United States plans to deploy.

Sincerely, John H. Brownley Captain, USN Military Assistant Offensive Systems