Under an amendment to the Airport & Airways Act of 1987, Notice to the FAA is required for any proposed structure within 1 mile of any visual or electromagnetic facility. The VORs above are the structural backbone of today's enroute navigational system. Although GPS is rapidly becoming the preferred method of navigation, we can expect VORs to remain in service for many years to come.
Often you may see obstructions within the 1 mile radius and think you are "good to go"; however, the FAA also is required to look at the "cumulative effect". Often an/any obstruction which is to be constructed can affect airborne radio receivers by either shadowing or scattering the navigational signal. If the obstruction is a radio transmitter (the proposed construction) and in the same bandwidth as a VOR or Instrument Landing System (ILS), it can create an overload in the airborne receiver--thus become a hazard to air navigation. With a complete aeronautical study by TowerSAFE International, often these problems can be identified early enough in the project's timeline to make necessary adjustments.