15.loran-and-adf. LORAN and ADF Navigation
Although both LORAN-C and Automatic Direction Finder (ADF) systems have been largely superseded by GPS, the FAA Pilot's Handbook of Aeronautical Knowledge retains coverage of these systems because legacy receivers remain in service and the underlying navigation principles still appear on knowledge tests and in oral exams.
LORAN-C (Long Range Navigation)
LORAN-C was a ground-based, low-frequency hyperbolic navigation system operating at 100 kHz. The U.S. Coast Guard terminated North American LORAN-C transmissions in 2010, but the principles remain instructive.
A LORAN chain consisted of one master station (M) and two to four secondary stations designated W, X, Y, and Z. Each chain was identified by a unique Group Repetition Interval (GRI) — the time in microseconds between master pulses (e.g., GRI 9960 for the Northeast U.S. chain).
LORAN-C used time-difference (TD) measurements:
- The receiver measured the difference in arrival time between the master pulse and each secondary pulse.
- Each TD value plotted as a hyperbolic line of position (LOP) on a LORAN chart.
- The intersection of two or more LOPs produced a fix.
LORAN-C accuracy was typically 0.25 nautical miles (absolute) and approximately 50 feet repeatable, making it suitable for en route navigation but not for precision approaches. Reception was affected by:
- Ground wave propagation (reliable to ~1,200 NM).
- Sky wave contamination at night (reduced accuracy).
- Precipitation static (P-static), thunderstorms, and terrain.
LORAN receivers typically displayed information as latitude/longitude, bearing and distance to waypoint, ground speed, and cross-track error — a presentation very similar to today's GPS.
Automatic Direction Finder (ADF)
The ADF is an airborne low/medium-frequency receiver that automatically points to a selected ground station. It operates in the 190–535 kHz band, receiving signals from Non-Directional Beacons (NDBs) and, on some receivers, commercial AM broadcast stations (540–1700 kHz).
Components
- Loop antenna — directionally sensitive; produces a sharp signal null.
- Sense antenna — non-directional; resolves the 180° ambiguity of the loop.
- Receiver — tunable to NDB frequencies.
- Indicator — fixed-card, movable-card (heading-driven), or Radio Magnetic Indicator (RMI).
Indicator Types
- Fixed-card (relative bearing) indicator: the card is fixed with 0° at the top. The needle shows relative bearing (RB) — the angle from the aircraft's nose to the station, measured clockwise.
- Movable-card indicator: the pilot manually rotates the card to match magnetic heading; the needle then shows magnetic bearing TO the station directly.
- RMI: the card is automatically slaved to the heading system, so bearing TO the station is read directly under the needle head.
Key Bearing Formula
With a fixed-card indicator, magnetic bearing to the station (MB) is computed as:
MB = MH + RB
Where MH is magnetic heading and RB is relative bearing. If the result exceeds 360°, subtract 360°.
Example: MH = 040°, RB = 300°. MB = 040 + 300 = 340° to the station.
Homing vs. Tracking
- Homing keeps the needle on the nose by turning toward it. In a crosswind, this produces a curved ground track.
- Tracking establishes a wind correction angle (WCA) so the aircraft maintains a straight ground track to the station, with the needle deflected into the wind by the amount of the WCA.
Station Passage
Over an NDB, the needle becomes erratic and then swings to point behind the aircraft (near 180° relative bearing). This needle reversal marks station passage.
Limitations and Errors
ADF is susceptible to several errors not present in VOR or GPS:
- Thunderstorm effect — the needle deflects toward lightning rather than the station.
- Night effect — sky waves bouncing off the ionosphere cause needle wander, especially near sunrise/sunset.
- Terrain effect — mountainous terrain and coastal refraction (shoreline effect) bend the signal.
- Electrical interference and precipitation static.
- Bank error — banking the aircraft tilts the loop antenna and skews indications.
NDB Identification
NDBs transmit a continuous Morse code identifier (typically three letters). The pilot must monitor this ident audibly to confirm the correct station is being received; unlike VOR, an NDB has no automatic failure flag, so loss of the ident is the primary integrity check.
Why Study These Systems Today
ADF receivers remain installed in many older aircraft and are still found on a small number of NDB approach charts. Understanding relative bearings, station orientation, and the limitations of low-frequency navigation reinforces the situational-awareness habits required for any form of pilotage or radio navigation.