Page 8 - Volume 15 Number 2

Figure 1: This late Oct. 2020 FAA graphic shows the VOR MON network, as well as the MON Airport network on the left. The geographic makeup of VORs to be decom- missioned and the types and numbers of VORs to be retained to charted on the right. (Source: faa.gov)
VOR can also be co-located with other navigation equipment. Distance Measuring Equipment (DME), co-located with a VOR station, known as a VOR/DME, can exist in High, Low or Terminal VOR stations. Tactical Air Navigation (TACAN) can also be co-located with a VOR, known as a VORTAC. VORTACs including the VOR station itself, DME and TACAN azimuth information (which is used mainly by U.S. military aircraft).
VOR usage as a primary source of navigation has not had any one peak point in history. Airlines and high-end corporate aviation began to rely on it less as Inertial Navigation, OMEGA and LORAN systems became available. Yet, at the same time, those systems were too heavy, complex or expensive to see widespread use within GA, keeping VOR as GA’s main navigational tool. Eventually, OMEGA and LORAN systems become more common place in lower-end jets, turboprops and GA aircraft, relegating VOR to an enroute backup, while still remaining a primary system for terminal procedures (especially at smaller airports). GPS became popular in smaller aircraft as a means of VFR navigation first (it was not certified for IFR usage until later). Because of the high cost of the initial generation of IFR-certified GPS equipment, legacy airliners often flew on for decades using VOR and ADF, rather than the new- fangled GPS navigation. Yet, as we all know, GPS eventually eclipsed all other forms of navigation for terminal, short and long range. Meanwhile, VOR soldiered along, always the trusty backup, but all too often ignored by pilots and instructors alike.
Many speculated the VOR would soon go the way of the Dodo. The rapid development and expansion of GPS and Satellite Based Approach Systems (SBAS) has been “taking over aviation,” particularly since 2007 when WAAS GPS equipment first began to appear in the GA fleet. Fortunately, the FAA and the various aviation advocacy
Figure 2: Timetable of the VOR MON program. (Source: faa.gov)
But they were expensive to build and purchase and also expensive to maintain. Such VORs became known as High VORs and are usable both at low altitude and throughout the U.S. Flight Levels (18,000 to 60,000 feet). Thus, they were used to define the network of high altitude airways (Jet Airways). Low VORs were used as a cheaper solution to fill the geographic gaps between the cost prohibitive High VORs. They were used to
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supplement the VOR network, so that additional low altitude airways (Victor Airways) could be developed at less expense. Finally, Terminal VORs exist strictly to support terminal procedures (VOR approaches or to define mandatory intersections on other types of approaches). Terminal VORs are the least expensive, are not used in the Airway networks, and have the most limited range and altitude capabilities.
FEBRUARY 2021