VORTAC is a type of aircraft navigation system transmitter station. It is really an acronym for two abbreviations, VOR and TACAN. [http://en.wikipedia.org/wiki/VHF_omnidirectional_range VOR] is an abbreviation for VHF Omnidirectional Range, sometimes referred to as an “Omni”. It is a civilian, ground based Radio Frequency (RF) transmission that is received by aircraft, and gives the pilot his magnetic radial direction (azimuth) from or to the VOR station. [http://en.wikipedia.org/wiki/TACAN TACAN] (Tactical Air Navigation System) is a very similar system designed for military use, transmitting in the UHF frequency range. It has enhanced features, such as [http://en.wikipedia.org/wiki/Distance_Measuring_Equipment DME] (Distance Measuring Equipment), that were transferred over to the civilian navigation world.
So – a VORTAC is a ground based navigation transmission station, combining both the civilian VOR and military TACAN functions.The horizontal locations of VOR, TACAN and the “combo” VORTAC stations had to be surveyed accurately for proper integration into the area’s air navigation system. You may find them listed or used as azimuth stations or intersection stations when looking at NGS Benchmark datasheets. It is not recommended to approach one closely, both because of RF radiation concerns, and because of security concerns. They are normally well fenced anyway. At some future time, they might all be obsolete, being replaced by much more accurate GPS navigation, but that could be many years away.