Navigation Fundamentals
VOR, NDB, DME, and GPS navigation — concepts, terminology, and practical reference for pilots.
Modern aviation uses a combination of conventional ground-based navaids (VOR, NDB, DME) and satellite-based systems (GPS/GNSS). Understanding each system's capabilities and limitations is essential for both VFR and IFR operations.
Heading, Track, and Bearing
Key Definitions
| Term | Definition | Reference |
|---|---|---|
| True North (TN) | Geographic North Pole direction | Fixed, geographic reference |
| Magnetic North (MN) | Direction of Earth's magnetic field | Varies by location and time |
| True Heading (TH) | Aircraft nose direction, referenced to True North | Calculated from TC + or − WCA |
| Magnetic Heading (MH) | Aircraft nose direction, referenced to Magnetic North | TH ± magnetic variation |
| Compass Heading (CH) | Reading on compass (affected by deviation) | MH ± compass deviation |
| True Track (TT) | Intended path over ground, referenced to True North | Flight planning reference |
| Magnetic Track (MT) | Intended path over ground, referenced to Magnetic North | MH with zero wind |
| Ground Track | Actual path flown over ground (wind corrected) | ATC/radar reference |
The conversion chain: True ➜ (±Variation) ➜ Magnetic ➜ (±Deviation) ➜ Compass. Mnemonic: "True Virgins Make Dull Company" (True Variation Magnetic Deviation Compass). West variations ADD, East SUBTRACT (WAEA) when converting True to Magnetic.
VOR (VHF Omnidirectional Range)
VOR is a ground-based radio navigation system operating in the VHF band (108–118 MHz). It provides magnetic bearing information to/from the station. VORs form the backbone of most instrument airways worldwide.
| VOR Term | Explanation |
|---|---|
| Radial | A bearing FROM the VOR station (magnetic). Radial 090 points East from the station. |
| Course | A bearing TO the VOR station = reciprocal of radial (radial − 180°) |
| CDI (Course Deviation Indicator) | Needle showing whether aircraft is left/right of selected course |
| OBS (Omni Bearing Selector) | Knob to select the desired course/radial |
| Full-scale deflection | Typically ±10° from course for CDI (some instruments ±5°) |
| TO flag | Aircraft is heading generally toward the station |
| FROM flag | Aircraft is moving generally away from the station |
| Cone of Confusion | Area directly overhead VOR where signal is unreliable (~30° cone) |
VOR Types
| Type | Full Name | Frequency | Range |
|---|---|---|---|
| TVOR | Terminal VOR | 108–112 MHz | 25 nm |
| BVOR | Branch VOR | 112–118 MHz | 40 nm |
| HVOR | High-altitude VOR | 112–118 MHz | 130+ nm (above FL180) |
| DVOR | Doppler VOR | 108–118 MHz | Higher accuracy, resistant to terrain |
NDB (Non-Directional Beacon)
NDB is a ground-based medium-frequency (190–1750 kHz) radio beacon. Unlike VOR, it transmits in all directions without providing bearing information directly — the aircraft's ADF (Automatic Direction Finder) receiver shows the direction to the station.
| NDB Term | Explanation |
|---|---|
| Bearing | Direction to/from the NDB in degrees |
| Relative bearing | Bearing to station relative to aircraft nose (0° = straight ahead) |
| QDM | Magnetic heading to steer to reach the station (zero wind) |
| QDR | Magnetic bearing from the station (radial equivalent) |
| ADF | Automatic Direction Finder — cockpit instrument showing relative bearing to NDB |
| Magnetic bearing to station | Magnetic heading + relative bearing (if <360°) else − 360° |
NDB/ADF systems are subject to errors from: night effect (atmospheric distortion at night), coastal refraction (bending over coastlines), mountain effect, and thunderstorm interference (ADF may point toward electrical storms). Use with awareness of limitations.
DME (Distance Measuring Equipment)
DME provides slant range distance from an aircraft to a ground station. It works by the aircraft interrogating the ground station and measuring signal round-trip time. Usually co-located with VOR (VOR/DME) or ILS (ILS/DME).
- Measures slant distance, not ground distance (difference negligible above ~30 nm or above ~3,000 ft AGL)
- Frequency paired with VOR — selecting the VOR frequency automatically tunes DME
- Readout: nautical miles
- Can also display groundspeed and time to station
- Multiple DMEs can be used for position fixing (DME/DME)
GPS / GNSS
| Term | Meaning |
|---|---|
| GNSS | Global Navigation Satellite System (umbrella term) |
| GPS | US system (24+ satellites), most common in aviation |
| GLONASS | Russian system |
| Galileo | European system |
| SBAS | Satellite-Based Augmentation System (WAAS in USA, EGNOS in Europe) — improves accuracy to ~3 m |
| RNP | Required Navigation Performance — performance-based nav with on-board monitoring and alerting |
| RNAV | Area Navigation — enables routes not tied to ground-based navaids |
| LPV | Localizer Performance with Vertical guidance — precision-like approaches using GPS + SBAS |
GPS approaches include LNAV (lateral only), LP (lateral performance), LNAV/VNAV (lateral + baro-vertical), and LPV (lateral + SBAS vertical). LPV minimums can be equivalent to ILS CAT I. Always check current NOTAMs for GPS/SBAS outages before flight.
Dead Reckoning (DR)
Dead reckoning is the process of estimating current position based on a known previous position, heading, speed, and elapsed time — without reference to external fixes. Essential for situational awareness when primary navaids are unavailable.
Navigation Calculations
| Calculation | Formula | Notes |
|---|---|---|
| Wind Correction Angle (WCA) | WCA = arcsin(crosswind component / TAS) | Use flight computer or calculator |
| Ground Speed | GS = TAS ± headwind/tailwind | + tailwind, − headwind |
| Time to fix | Time = Distance ÷ Ground Speed | In hours; ×60 for minutes |
| Distance | Distance = GS × Time | GS in knots, time in hours = nm |
| Fuel required | Fuel = fuel flow × time | Always add required reserves |