Aircraft find their way with ground-based radio aids, satellites, or instruments that sense their own motion. The FAA names three radio navigation systems for VFR flying: VOR, NDB and GPS. Larger aircraft add distance measuring equipment and inertial systems. This guide explains how each one works, what its limits are, and how far you can trust it.
Key Takeaways
- VOR gives a magnetic bearing to or from a ground station on VHF frequencies of 108.0 to 117.95 MHz. It is line-of-sight, so range grows with altitude.
- DME measures slant range to a VORTAC or VOR/DME in nautical miles, so the reading is affected by altitude.
- An ADF tunes to a nondirectional beacon. Low and medium frequencies are not limited to line-of-sight but are vulnerable to lightning static.
- GPS needs at least four satellites for a three-dimensional position, and receivers with RAIM need at least five to check their own integrity.
Navigation Systems Compared
Aircraft navigation systems (FAA Pilot's Handbook, Chapter 16, and Maintenance Technician Handbook, Chapter 11)| System | How it works | Main limit |
|---|
| VOR | A ground station transmits radials, lines of magnetic bearing extending outward like the spokes of a wheel, on 108.0 to 117.95 MHz. The aircraft shows its bearing to or from the station. | Line-of-sight. Reception range at 1,000 feet AGL is about 40 to 45 miles. |
|---|
| DME | A UHF system paired with a VOR/DME or VORTAC that measures the slant range distance to the station. The receiver tunes the paired frequency automatically. | Reads slant range, not ground distance, and groundspeed and time-to-station are accurate only when tracking directly to or from the station. |
|---|
| ADF and NDB | The ADF receiver tunes a nondirectional radio beacon, normally in the 200 to 415 kHz band. The needle shows the relative bearing to the station. | Static from lightning and other electrical disturbances causes needle deviations and signal fades. |
|---|
| GPS | A satellite system. The receiver measures its distance to several satellites to find its position anywhere in the world. | No integrity assurance without RAIM, and a poor antenna view can lose the signal. |
|---|
| VOR/DME RNAV | The computer in the aircraft electronically relocates a VORTAC as a waypoint, defined by a radial and a distance, so the pilot can fly direct between almost any two points. | Needs both VOR and DME signals. If the VOR has no DME, RNAV mode does not work. |
|---|
| INS and IRS | A self-contained system that works out position from accelerations, starting from a known point. Two or more accelerometers, one referenced to north and one to east. | Used on some large aircraft for long-range navigation. Needs no ground signals. |
|---|
VOR
The VOR is the long-serving workhorse. The same indicator behaves the same way whether the aid is a plain VOR, a VOR/DME, or a VORTAC, which combines a VOR with military TACAN equipment. A radial is a line of magnetic bearing, numbered from 001, which is 1 degree east of magnetic north, around to 360. The FAA calls radial alignment accuracy excellent, generally within plus or minus 1 degree.
VORs are classed by use, and the useful range depends on class and altitude:
VOR and VORTAC usable altitudes and radius (FAA Pilot's Handbook)| Class | Altitudes | Distance |
|---|
| T (Terminal) | 12,000 feet and below | 25 miles |
|---|
| L (Low altitude) | Below 18,000 feet | 40 miles |
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| H (High altitude) | Below 14,500 feet | 40 miles |
|---|
| H (High altitude) | 14,500 to 17,999 feet, conterminous 48 states only | 100 miles |
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| H (High altitude) | 18,000 feet to FL450 | 130 miles |
|---|
| H (High altitude) | FL450 to 60,000 feet | 100 miles |
|---|
DME
Because DME measures the direct distance between the aircraft and the station, altitude affects the reading. The FAA's example is that passing directly over a VORTAC at 6,076 feet AGL would show about 1.0 nautical mile. A VOR radial alone gives only a line of position, but a radial plus DME distance pinpoints the aircraft on that line. Most receivers also show groundspeed and time-to-station.
ADF and NDB
An NDB has one advantage over a VOR: low and medium frequencies are not line-of-sight, so the signal follows the Earth's curvature and can be received at any altitude within range. Flying to the station takes two techniques. Homing means keeping the needle on zero, which in a crosswind curves the path and drifts the aircraft downwind. Tracking means applying a wind correction to hold a straight line to the station. The FAA says the ADF can still be a valuable aid with proper precautions, though it is not as popular as the VOR.
GPS and WAAS
The FAA describes GPS as a space-based system with worldwide coverage. The baseline constellation is 24 satellites in six orbital planes. A receiver needs at least four satellites for an accurate three-dimensional position. Selective Availability, which deliberately degraded accuracy, was discontinued on May 1, 2000.
- RAIM. Receiver autonomous integrity monitoring checks whether a satellite is giving corrupted data. It needs at least five satellites in view, or four plus a barometric altimeter. To isolate and remove a bad satellite, it needs six, or five plus the barometric input. Without RAIM, the pilot has no assurance of the accuracy of the position.
- VFR receivers. The FAA warns that many VFR GPS receivers and all hand-held units lack RAIM alerting, so a systematic cross-check with other navigation is needed.
- Databases. A current database is required for IFR but not VFR. An outdated moving map can lead pilots into airspace they meant to avoid.
- Altitude. GPS-derived altitude should not be relied on for aircraft altitude, because the vertical error can be large and no integrity is provided.
- WAAS. The Wide Area Augmentation System refines GPS positions using correction data sent from geostationary satellites to a WAAS-enabled receiver.
Inertial Systems
An INS or IRS needs no input radio signal from the ground. It derives attitude, velocity and direction from measurements of the aircraft's accelerations from a known starting point, updated continuously through calculations on the accelerometers' readings. The FAA says the IRS designation is generally reserved for the more modern systems.
Using More Than One
The FAA tells beginning pilots to use radio navigation to supplement pilotage, navigating by visual reference to the ground, which protects against disorientation if a radio fails. It also says a prudent pilot never places complete reliance on one method of navigation when others are available to cross-check. For how these aids are used on instrument approaches, see our guide to types of flight approaches.
Frequently Asked Questions
What are the main types of aircraft navigation systems?
The FAA names three radio systems for VFR navigation: VOR, nondirectional beacon (NDB) with ADF, and GPS. Distance measuring equipment, VOR/DME RNAV and inertial navigation systems are also used.
How far can a VOR be received?
VOR signals are line-of-sight. The FAA says reception range at 1,000 feet AGL is generally 40 to 45 miles, and increases with altitude. Usable range depends on the station class.
What is the difference between VOR and GPS?
A VOR is a ground station that gives a magnetic bearing to or from it. GPS is satellite-based and gives position anywhere in the world, but needs enough satellites and, for integrity checking, RAIM.
How many satellites does GPS need?
At least four for an accurate three-dimensional position. RAIM needs five in view, or four with a barometric altimeter, to detect an integrity problem.
What does DME measure?
The slant range distance in nautical miles between the aircraft and a VOR/DME or VORTAC, which means the reading is affected by altitude.
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