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VOR Service Volumes in the GPS Era: What Pilots Need to Know About the Legacy and New Volumes

For most of us, VOR navigation feels like legacy equipment in a GPS-first cockpit. But the FAA still treats VOR as an important backup, especially through the VOR Minimum Operational Network, or MON. The MON is designed to let pilots continue navigating during a GNSS disruption by flying VOR-to-VOR or proceeding to a MON airport with a conventional approach that does not require GPS, DME, ADF, or surveillance. The FAA’s goal is nearly continuous VOR signal coverage at 5,000 feet AGL across the continental U.S. outside the Western U.S. Mountainous Area, with at least one MON airport within 100 NM.


Before comparing the old and new volumes, it helps to define what a service volume actually is. The FAA publishes Standard Service Volumes, or SSVs, for most NAVAIDs. In plain English, that is the three-dimensional space where the FAA assures adequate signal strength, course quality, and protection from interference from nearby facilities on similar frequencies. One important caveat: the published SSV does not guarantee freedom from terrain or obstruction blockage.


Another detail pilots should keep straight: VOR service volumes are defined in feet Above the Transmitter Height, or ATH, while the MON coverage concept is usually described at 5,000 feet AGL. Those are not the same reference. If you are briefing a route or thinking about whether a given VOR should be reliable, that distinction matters.



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The legacy VOR service volumes

For years, pilots learned three original VOR classes: Terminal, Low, and High. Those legacy service volumes are still part of the system.


  • Terminal (T): from 1,000 feet ATH up to and including 12,000 feet ATH, out to 25 NM.

  • Low (L): from 1,000 feet ATH up to and including 18,000 feet ATH, out to 40 NM.

  • High (H): from 1,000 feet ATH up to and including 14,500 feet ATH, out to 40 NM; from 14,500 feet ATH up to and including 60,000 feet ATH, out to 100 NM; and from 18,000 feet ATH up to and including 45,000 feet ATH, out to 130 NM.


That structure worked well when the VOR network was denser and conventional navigation was the primary system. But it was not ideal for the FAA’s newer backup-navigation concept, especially the need to support VOR use around 5,000 feet AGL during a GPS outage. The FAA explicitly notes that the MON requires use of VORs at 5,000 feet AGL, which is beyond the original SSV ranges, and that this is one reason additional service volumes were created.


The new VOR service volumes

To support the MON, the FAA created two new VOR-specific service volumes: VOR Low (VL) and VOR High (VH). These are geometrically larger than the legacy low and high volumes and are being applied to retained MON VORs. No avionics changes are required to use the new service volumes.


  • VOR Low (VL): from 1,000 feet ATH up to but not including 5,000 feet ATH, out to 40 NM; from 5,000 feet ATH up to but not including 18,000 feet ATH, out to 70 NM.

  • VOR High (VH): from 1,000 feet ATH up to but not including 5,000 feet ATH, out to 40 NM; from 5,000 feet ATH up to but not including 14,500 feet ATH, out to 70 NM; from 14,500 feet ATH up to and including 60,000 feet ATH, out to 100 NM; and from 18,000 feet ATH up to and including 45,000 feet ATH, out to 130 NM.


The practical takeaway is simple: at the altitudes that matter most for MON planning, the new VOR volumes extend useful protected range from 40 NM to 70 NM. That larger protected footprint is what lets the FAA keep fewer VORs while still preserving a conventional backup network.


What this means in the cockpit

For pilots, the biggest trap is assuming every VOR behaves like the old textbook examples. That is no longer true. Some retained facilities now carry the newer VL or VH classifications, while others remain legacy T, L, or H. The AIM also notes that co-located components may no longer share the same service volume, so a VOR/DME or VORTAC may not have matching classifications across all components.


The second takeaway is that VOR is now a backup system, not the FAA’s primary navigation strategy. MON navigation will not be as efficient as the PBN route structure. But in a GPS outage, it is designed to give you something that matters more than efficiency: a workable path to continue through the outage area or get to a safe landing using conventional navigation.


The third takeaway is a training one: pilots should stay proficient. FAA outreach for the MON specifically emphasizes maintaining VOR and ILS proficiency and understanding what equipment may become unavailable during a GPS disruption. In other words, the MON only helps if the pilot can still tune, identify, intercept, track, and brief a conventional approach without leaning on magenta-line habits.


Bottom line

Legacy VOR service volumes gave us the familiar T, L, and H classes. The FAA’s newer MON-driven model adds VL and VH, mainly to expand protected VOR coverage where it counts most for backup navigation. For pilots, the headline difference is easy to remember: the new volumes preserve the same basic lower-altitude structure near the station, but they expand protected range to 70 NM in the key mid-altitude bands that support conventional navigation during GPS disruptions. That makes VOR less of a relic than many pilots assume. It is no longer the primary system, but it is still very much part of the safety net.



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