Understanding VOR in the Era of GPS

Understanding VOR in the Era of GPS

Understanding VOR in the Era of GPS

I already know what most readers are thinking, “Why should I care about VORs anymore?” Global Positioning System (GPS) has become the default form of navigation for all segments of general aviation (GA), while ILS and RNAV/GPS (with vertical guidance) approaches have become the norm at both large and small airports. Chances are, most of us can’t even remember the last time we flew an approach without some type of vertical guidance, much less a standard VOR approach. This is especially true when you are talking about flying such approaches in actual Instrument Meteorological Conditions (IMC) down to or near published approach minimums. So, I can appreciate your reluctance to read further, but stick with me here.

The Very High Frequency Omnidirectional Range (VOR) is far from dead (or even dying) and is a critical component to the U.S. navigation network on several levels. Not the least of which is ensuring the ability for instrument equipped aircraft to continue navigating (both in the enroute and terminal phases of flight) should the integrity of the current GPS system be compromised for any reason for any length of time. GPS signal outages are not uncommon. The military intentionally jams (blocks) specific GPS signals regularly for a variety of reasons related to national security and military training exercises. Occasionally, GPS satellites go offline as they reach the end of their lifespan or are undergoing remote updating or maintenance. Such outages are generally limited in scope and include a heads-up to pilots in the form of Notices to Airman (NOTAMs). But, because the GPS system is satellite based (rather than ground based, like VORs) it is more susceptible to a variety of less predictable outages, such as cosmic events or malicious enemy attack. Of course, failures of onboard GPS equipment, related wiring or antennas, etc. is always a possibility, as well.

There is little doubt that Performance Based Navigation (PBN), Required Navigation Performance (RNP), and a variety of other tongue-twisters related to GPS have totally changed the landscape of long, short and terminal range navigation. That has made the chores of navigation exponentially easier and safer for pilots. Nonetheless, GPS is not, and never will be, a failure-proof system. A ground based backup system for navigation is critical to maintaining and protecting the National Airspace System (NAS). Maintaining our knowledge and skills related to the use of that backup system is no less critical.    

The VOR Revolution

Like many prior navigational advances, the introduction of the VOR was truly revolutionary. It was not dependent upon visual conditions or low altitude flying, such as lighted airways and ground markings were. It was not dependent upon the pilot’s ability to constantly monitor and decipher audible signals, such as the AN Radio Range system was. VOR navigation was not limited to only a few specific courses, such as the Four Course Range. It was not susceptible to atmospheric interference such as Non-Direction Beacons (NDBs). Nor were the pilots using it nearly as prone to misinterpreting its information and creating dangerous navigation errors. VORs provide an infinite number of precise radials, broken into 360 one-degree segments, that the pilot can track to/from the facility with a high degree of accuracy thanks to easy to interpret cockpit equipment. Its only real limit is range, as dictated by the strength of the facility’s signal and the line-of-sight between said facility and the receiving aircraft. VOR navigation might seem quite antiquated now, compared to the precision and ease of today’s various forms of GPS navigation. Nonetheless, it was a massive leap forward in technology when it was introduced nearly 75 years ago. Additionally, it has proven to be highly dependable, making it the longest-standing navigation system in the U.S. National Airspace System (NAS), with no end date to that streak in sight.

VOR Expansion

Development of the VOR began in 1937, but it was not until 1946 (soon after World War II) that the first station became operational and well into the 1950s before their installations had become more widespread. While those early VOR stations were combination vacuum tube and mechanical devices, solid state technology began to take hold within the VOR network in the 1960s. It was after that point that VORs became common enough to be adopted as the world standard for air navigation. After the introduction of the VOR, variations on its concept soon developed. Different types of VORs emerged to support different types of navigation. Big and powerful VORs, which could be received 100-plus miles away were great for use as both enroute and terminal navigation facilities. 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 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.

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 groups better understood the necessity of maintaining multiple forms of navigational systems. Yes, VOR stations are costly, have range limitations and require maintenance. Thus, some will be allowed to fade into history. However, a Minimum Operational Network (MON) of them will remain for the foreseeable future.

VORs Evolution to MON

In late 2011, the FAA published its first notice for public comment related to its proposal to draw down the VOR network within the Continental U.S. (CONUS) to a MON. After the normal process of comment evaluation, proposals and notices was complete, a plan for transitioning to NextGen navigation systems was published by the FAA in July 2016. It included a plan for transitioning to PBN and for establishing a VOR MON. Phase I of that plan was to run from Fiscal Year (FY) 2016-2020. Before Phase I began, the FAA owned 957 VORs. Another 100 non-federal VORs were also in operation around the U.S. Some VORs were decommissioned during Phase I, but the primary goal during that time was to remove, replace or amend affected Instrument Flight Procedures (IFP), which would allow for more widespread VOR decommissioning during Phase II. By the time you read this, Phase II will have already begun and is currently scheduled to run through 2030. During Phase II, the federal VOR count will fall to 589, but without a significant loss of capability for the average user (see Figures 1 and 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 decommissioned and the types and numbers of VORs to be retained to charted on the right. (Source: faa.gov)

The creation of the VOR MON was a well thought out plan, allowing critical VORs to remain and for an expansion in service volume in many of those. Additionally, a network of Minimum Operational Network Airports (MONA, also known as “Safe Landing Airports”) was established to ensure that, in the event of a widespread GPS outage, all aircraft operating within CONUS would be within 100 nautical miles of an airport with a VOR or ILS approach procedure that does not require GPS, Radar Coverage, DME or ADF to legally fly. Of course, any suitable airport or approach procedure that is even closer at the time of total GPS signal loss may be utilized.

Figure 2: Timetable of the VOR MON program. (Source: faa.gov)

Should They Stay or Should They Go?

It is important for us, as pilots, to fully understand what defines the VOR MON network and what VOR capability changes have developed to facilitate it. First, what criteria was used for decommissioning versus retention decisions? Beyond those VORs used to support ILS, LOC or VOR approaches at Safe Landing Airports, additional factors were applied. Any VOR falling into the following categories will be retained:

  • VORs which support international oceanic routes
  • A sufficient network of VORs to provide coverage at/above 5,000 feet AGL
  • VORs in the Western U.S. Mountainous Area (WUSMA) which anchor Victor Airways through high elevation terrain
  • Any VOR required for military use
  • All VORs outside of CONUS

Additionally, no non-FAA VORs were considered for decommission as part of the VOR MON plan. Yet, those VORs are not part of the VOR MON network either. So, they can be thought of as VORs in addition to the VOR MON. However, since they are not FAA owned, they could be subject to decommission by their owner/operators at any time. Fortunately, all DME, TACAN and communication capabilities will be retained and reconfigured as necessary, even if the VOR they are co-located with is to be decommissioned. This will not only protect the integrity of DME and TACAN units, but also important communications services, such as Hazardous Inflight Weather Advisory Services (HIWAS) and Remote Communications Outlets (RCO).

Figure 3: Charting example (with descriptions) of a MON Airport on a NOAA
(government) Low Enroute IFR chart. (Source: faa.gov)


With the transition to VOR MON and the Safe Landing Airports networks, the FAA has begun implementing some charting changes related to both (specifically, refer to Figure 3). To facilitate the reduction in VOR numbers within CONUS, while retaining a VOR network with consistent reception capabilities at/above 5,000 feet AGL, two new Standard Service Volumes (SSV) are being added to VORs on the retention list. While these new SSVs do not require major equipment changes, they do represent a change in the SSVs many of us have learned and memorized during initial and recurrent instrument training. Specifically, for High VORs, a 70- nautical-mile SSV will now exist between 5,000 feet and 14,500 feet Above Transmitter Height (ATH). For Low VORs, the same 70-nautical-mile SSV will extend from 5,000 feet up to 18,000 feet ATH. To see how these new SSVs integrate into the pre-existing SSVs for High and Low VORs, refer to Figure 4.

Figure 4: New VOR MON Standard Service Volumes (SSV) added to High and Low VORs are shown in green. Grey SSVs were pre-existing before the changes for the VOR MON project began and will remain on all VORs within the VOR MON network. (Source: faa.gov)

While total loss of GPS navigation signal without warning remains a rare occurrence, it is not at all beyond the realm of possibility. When that time comes (without warning or as predicted by NOTAM), will you be ready? Have you studied or practiced navigating solely by use of VOR (or VOR/DME) lately? When was the last time you shot an old-school VOR approach without vertical guidance? Or thought about DME slant range error, reverse sensing, or the VOR cone of silence? Can you still fly a DME Arc without any help from GPS distance information? Can you do it using a standard VOR Omni Bearing Selector (OBS), as well as via a Radio Magnetic Indicator (RMI) needle? Those skills are likely to be woefully unpracticed and unused in the era of GPS. Yet, they are not skills we can simply allow to entirely escape our grasp. Grab your favorite flight instructor, saddle up your favorite training aircraft or simulator and go pretend it’s 1980-something. It will only seem like a futile exercise until your first GPS signal loss on an IMC flight to a LIFR airport. Then you’ll likely be thankful for the VOR MON and MONA networks, as well as your own commitment to stay proficient in all forms of navigation that your aircraft is equipped for.

Copyright 2020-21, Matthew McDaniel
First publication rights granted to
Village Press for King Air Magazine.
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copyright holder.

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