Definition
The defined airspace around a VOR, VOR/DME, or VORTAC navigation aid within which the facility's signal is protected from interference and considered usable for navigation. These volumes are shaped as stacked cylinders of varying radius and altitude, and are categorized by class (Terminal, Low, High, VOR Low, VOR High) based on the facility's intended service range.
Plain English
An invisible cylinder of airspace around a ground-based navigation station where the signal is reliable. If you're inside the cylinder, you can trust the navaid. Outside it, the signal may not be dependable.
Context Anchor
Seen when planning direct instrument routes that depend on ground-based navigation signals instead of following published airways.
Derivation
The shape is literally a cylinder (a circular footprint extending up to a defined altitude), and 'service volume' means the volume of airspace in which the navaid provides usable service. The name describes the shape and the function together.
Why Pilots Care
Confirms that a chosen direct route stays inside reliable signal coverage and avoids areas where navigation accuracy may degrade.
Analogy
Think of an invisible can standing around the navigation station. If your aircraft is inside the can at the right altitude, the signal is expected to be usable; outside it, you should not count on it.
Intuition Check
Service does not mean maintenance here. It means the area where the navigation facility provides usable signal coverage.
Example Sentence 1
Before planning the direct leg, the pilot checked that both VORs had overlapping cylindrical service volumes at the planned cruise altitude.
Example Sentence 2
Above FL180 the flight left the cylindrical service volumes and required radar vectors to maintain navigation.