How VOR Navigation Works: Tracking Radials, VOR Checks, Service Volumes, and the MON
- Nathan Hodell
- Aug 31, 2025
- 9 min read
Updated: Aug 5
GPS has made VOR navigation feel almost old-fashioned, but the VOR is far from obsolete — it remains the backbone of the FAA's backup navigation system, it's tested thoroughly on every instrument checkride, and on the day GPS fails (jamming, receiver failure, satellite issues), the pilot who can confidently track a radial is the one who gets home. Understanding VORs isn't just about knowing what they are; it's about being able to intercept and track a course, perform the accuracy checks the regulations require, and understand the system's limitations.
This post covers VOR navigation in practical depth: how to intercept and track a radial, the required VOR accuracy checks for IFR flight, the service volumes that determine reception range, the cone of confusion and station passage, and the Minimum Operational Network that keeps VORs relevant.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
What a VOR Is and How It Works
A VOR (VHF Omnidirectional Range) is a ground-based radio navigation station transmitting in the VHF band (108.0–117.95 MHz). It allows aircraft to determine their bearing (radial) from the station and to navigate along selected courses.
The two-signal principle:
The VOR transmits two signals:
Reference phase signal: Omnidirectional (same in all directions)
Variable phase signal: Rotating, creating a phase difference that varies with direction
The aircraft receiver compares the phase difference between these two signals. The phase difference is unique to each direction from the station, allowing the receiver to determine which radial the aircraft is on.
Radials:
A radial is a magnetic bearing FROM the VOR station. There are 360 radials (one per degree). The aircraft is always on some radial from the VOR. The VOR's 360° radial points magnetic north from the station; the 090° radial points magnetic east, and so on.
Why this is better than older systems:
Compared to NDBs (non-directional beacons), VORs are:
More precise (typically ±1°)
Easier to interpret
Less affected by certain types of interference
The OBS allows selecting any course directly
The Cockpit Instruments
VOR navigation uses specific instruments:
The OBS (Omni-Bearing Selector):
Knob used to select the desired course/radial
Rotates the course card
Sets the course you want to fly
The CDI (Course Deviation Indicator):
Vertical needle showing deviation from the selected course
Needle left: course is to your left
Needle right: course is to your right
Centered: you're on course
Each dot typically represents 2° of deviation
The TO/FROM indicator:
Shows whether the selected course takes you TO or FROM the station
TO: flying the course will take you toward the VOR
FROM: flying the course will take you away from the VOR
Essential for proper orientation
The HSI (Horizontal Situation Indicator):
Combines the heading indicator with VOR display
More intuitive presentation
Shows aircraft heading and course relationship
Reduces interpretation errors
How to Intercept a Radial
Intercepting a radial is a fundamental VOR skill. Here's the process:
To intercept and track a course:
Tune and identify the VOR:
Set the frequency
Listen for the Morse code identifier (confirm you have the right station)
This step is mandatory — never navigate without identifying
Determine your position:
Set the OBS to center the needle with a FROM indication
This tells you your current radial
Now you know where you are relative to the station
Set the desired course:
Set the OBS to the radial/course you want to fly
Note which way the needle deflects
The needle shows which way to turn to intercept
Turn to an intercept heading:
Turn toward the needle (needle right, turn right)
Use an intercept angle (typically 30-45°)
Larger angle = faster intercept but more overshoot risk
Intercept the course:
As you approach the course, the needle moves toward center
When the needle centers, turn to the course heading
You're now on the radial
The key principle:
"Turn toward the needle" — if the CDI needle is deflected right, turn right to intercept. The needle shows where the course is relative to you.
How to Track a Radial
Once on a radial, tracking it requires wind correction:
Tracking with wind:
Fly the course heading initially:
Set heading to match the course
Watch the CDI
Detect drift:
If wind pushes you off course, the needle deflects
Needle right: you've drifted left of course
Needle left: you've drifted right of course
Apply a correction:
Turn toward the needle to return to course
Use a correction angle (start with ~20°)
The needle will move back toward center
Re-center and establish a wind correction angle:
When the needle centers, reduce the correction
Establish a heading that holds the course
This is your wind correction angle (WCA)
Bracket the course:
Make progressively smaller corrections
Find the heading that keeps the needle centered
This is "bracketing" — converging on the right WCA
The tracking skill:
Good VOR tracking is about finding the wind correction angle that holds the course. It takes practice to make smooth, progressively smaller corrections rather than chasing the needle.
The Reverse Sensing Trap (Briefly)
One critical VOR pitfall: reverse sensing. If you set the OBS to a course opposite your direction of flight (e.g., flying TO the station with a FROM course set, or vice versa), the CDI needle gives backwards indications — turning toward the needle takes you further off course.
The fix: ensure your OBS setting roughly matches your direction of flight. When flying TO a station, the course should show TO; when flying FROM, it should show FROM. With an HSI, reverse sensing is eliminated because the instrument accounts for aircraft heading.
(For a full treatment of reverse sensing, see our dedicated post on the topic.)
VOR Service Volumes
VORs have standardized service volumes defining their usable range. This affects which VORs you can use at what distances and altitudes. The three standard service volumes (legacy):
Terminal (T):
From 1,000 feet AGL up to 12,000 feet AGL
Within 25 NM
For terminal area navigation
Low (L):
From 1,000 feet AGL up to 18,000 feet AGL
Within 40 NM
For low-altitude enroute navigation
High (H):
Multiple ranges by altitude
Up to 130 NM at high altitudes
From 1,000 feet AGL up to 60,000 feet
For high-altitude navigation
The new expanded service volumes:
With the VOR MON program, the FAA introduced two new service volumes:
VOR Low (VL): Expanded low-altitude range (up to 70 NM at certain altitudes)
VOR High (VH): Expanded high-altitude range
These expanded volumes support navigation with fewer VORs in the network.
Why service volumes matter:
They define the guaranteed usable range
Beyond the service volume, signals may be unreliable
Using a VOR beyond its service volume risks erroneous indications
Flight planning must account for service volumes
The Cone of Confusion and Station Passage
Directly over a VOR, navigation becomes unreliable:
The cone of confusion:
A cone-shaped area directly above the VOR
The signal becomes unreliable here
The CDI may fluctuate
The TO/FROM indicator may flip back and forth
This is normal and expected
Identifying station passage:
The TO/FROM indicator flips from TO to FROM
This is the definitive indication of station passage
The CDI may swing during passage
After passage, the indicator shows FROM
The size of the cone:
Wider at higher altitudes
Higher altitude = more time in the cone
At cruise altitude, the cone can take noticeable time to cross
Practical implications:
Expect needle fluctuations over the station
Use the TO/FROM flip to identify passage
Don't chase the needle in the cone of confusion
Resume normal tracking after passage
Required VOR Checks for IFR Flight
For IFR operations, the regulations require periodic VOR accuracy checks. This is heavily tested.
The 30-day requirement (FAR 91.171):
VOR equipment must be checked within 30 days before IFR flight
The check verifies accuracy
Must be logged (date, place, bearing error, signature)
The acceptable check methods and tolerances:
VOT (VOR Test Facility):
A test signal at certain airports
Set OBS to 0°, needle centers with FROM (or 180° with TO)
Tolerance: ±4°
Ground checkpoint:
Designated ground checkpoint at an airport
Published radial
Tolerance: ±4°
Airborne checkpoint:
Designated airborne checkpoint
Published radial
Tolerance: ±6°
Dual VOR check:
Compare two VOR receivers
Tune both to the same VOR
Difference between them: maximum 4°
Tolerance: 4° between the two
Airway checkpoint (over a known point):
Select a published airway radial over a known landmark
Tolerance: ±6°
The logging requirement:
Each VOR check must be logged with:
Date
Place
Bearing error
Signature
The memory aid for tolerances:
Ground-based checks (VOT, ground checkpoint): ±4°
Airborne checks: ±6°
Dual VOR: 4° between them
Identifying a VOR
Always identify a VOR before using it:
The Morse code identifier:
Each VOR transmits a Morse code identifier
Listen to confirm the correct station
The identifier is on the chart
Why identification matters:
Confirms you tuned the right frequency
A VOR under maintenance may transmit a "TEST" code or no identifier
Never navigate by an unidentified VOR
An unidentified VOR may be unreliable
The voice identifier:
Some VORs also have voice identification
May include the station name
Confirms the station
Maintenance indication:
A VOR under maintenance removes its identifier
If you can't identify it, don't use it
The lack of identifier is the warning
VOR/DME and VORTAC
VORs are often combined with distance-measuring equipment:
VOR/DME:
VOR (azimuth) + DME (distance)
Provides both direction and distance
DME gives slant-range distance to the station
VORTAC:
VOR + TACAN (military system)
Civilian aircraft use the VOR and DME components
Military aircraft use TACAN
Common facility type
DME considerations:
DME measures slant range (direct distance to station)
Not ground distance
The difference is significant when close and high
Directly over a high station, DME shows your altitude (in NM)
The slant range error:
DME measures the hypotenuse, not the ground distance
Most significant when close to the station at high altitude
Negligible at greater distances
Example: directly over a VOR at 6,000 feet, DME reads ~1 NM (your altitude)
The VOR Minimum Operational Network (MON)
The FAA is reducing the VOR network but maintaining a backup:
The MON concept:
As GPS became primary, many VORs became redundant
The FAA is decommissioning some VORs
But maintaining a Minimum Operational Network (MON)
The MON provides backup navigation if GPS fails
What the MON provides:
Coverage at 5,000 feet AGL and above across the continental U.S.
Ability to navigate to a "MON airport" with an instrument approach
Backup in case of GPS outage
No location more than 100 NM from a MON airport
The MON airports:
Designated airports with VOR/ILS approaches
Reachable using only VOR navigation
Provide a safe landing option during GPS outage
Don't require GPS for the approach
Why this matters:
GPS can fail (jamming, interference, receiver failure)
The MON ensures a backup exists
Pilots should maintain VOR skills
The MON is the safety net for GPS-dependent navigation
Why VOR Skills Still Matter
Despite GPS dominance, VOR proficiency remains important:
GPS vulnerabilities:
GPS jamming
GPS spoofing
Receiver failures
Satellite issues
Interference
When VOR saves the day:
GPS outage en route
Need to navigate to a MON airport
Backup when primary navigation fails
Required for certain operations
Training value:
VOR navigation teaches fundamental navigation concepts
Understanding radials, courses, and tracking
Situational awareness skills
Required for instrument rating
The professional standard:
A proficient instrument pilot can navigate by VOR confidently. GPS is primary, but VOR competence is the backup that matters when technology fails.
Common Misconceptions
"VORs are obsolete."No — they're being reduced but maintained as the MON backup network. They remain essential for GPS-out navigation.
"I don't need VOR checks if I have GPS."For IFR flight using VOR equipment, the 30-day check is still required (FAR 91.171).
"Reverse sensing means the VOR is broken."No — reverse sensing results from setting the OBS opposite your direction of flight. It's a setting issue, not equipment failure.
"DME shows ground distance."No — DME shows slant range (direct distance), which differs from ground distance, especially close and high.
"I don't need to identify the VOR if I tuned the right frequency."Always identify — a VOR under maintenance may transmit on its frequency but be unreliable. The Morse identifier confirms it's usable.
On the Written Test and Checkride
VORs appear consistently on tests, especially instrument. The most commonly tested topics:
How a VOR works (reference and variable phase)
Radials (magnetic bearings FROM the station)
VOR checks and tolerances (±4° ground, ±6° airborne, 4° dual)
The 30-day IFR check requirement
Service volumes (T, L, H)
Cone of confusion and station passage
The MON
Quick Reference
VOR Basics:
VHF Omnidirectional Range
108.0–117.95 MHz
Provides radials (magnetic bearings FROM station)
Accuracy: ±1° typical
Cockpit Instruments:
OBS: Select desired course
CDI: Course deviation (each dot ~2°)
TO/FROM: Direction relative to station
HSI: Combined heading + VOR (eliminates reverse sensing)
Intercepting a Radial:
Tune and IDENTIFY
Determine position (center needle FROM)
Set desired course
Turn toward the needle (30-45° intercept)
Center needle, turn to course
Tracking:
Turn toward the needle to correct
Establish wind correction angle
Bracket with progressively smaller corrections
Service Volumes:
Terminal (T): 25 NM, 1,000-12,000 AGL
Low (L): 40 NM, 1,000-18,000 AGL
High (H): up to 130 NM by altitude
New: VL and VH (expanded for MON)
VOR Checks (FAR 91.171):
Check | Tolerance |
VOT | ±4° |
Ground checkpoint | ±4° |
Airborne checkpoint | ±6° |
Dual VOR | 4° between |
Airway over landmark | ±6° |
Required within 30 days for IFR
Log: date, place, bearing error, signature
Cone of Confusion:
Directly over the VOR
Unreliable signal, needle fluctuation
TO/FROM flip = station passage
Wider at higher altitude
DME:
Measures slant range (not ground distance)
Over station at altitude: reads ~your altitude in NM
The MON:
Minimum Operational Network
Backup if GPS fails
Coverage at 5,000 AGL+ across CONUS
No point more than 100 NM from a MON airport
Key Principle:
GPS is primary, but VOR competence is the backup that matters. Master intercepting, tracking, and the required checks.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.