The ADF/NDB navigation system is one of the oldest air navigation systems still in use today. It works from the most simple radio navigation concept: a ground-based radio transmitter (the NDB) sends an omnidirectional signal to an aircraft loop antenna. The result is a cockpit instrument (the ADF) that displays the aircraft position relative to an NDB station, allowing a pilot to "home" to a station or track a course from a station.
The Automatic Direction Finder (ADF) is the cockpit instrument that displays the relative direction to the pilot. Automatic direction finder instruments receive low and medium frequency radio waves from ground-based stations, including nondirectional beacons and instrument landing system beacons. They can even receive commercial radio broadcast stations.
The ADF receives radio signals with two antennas: a loop antenna and a sense antenna. The loop antenna determines the strength of the signal it receives from the ground station to determine the direction of the station, and the sense antenna determines whether the aircraft is moving toward or away from the station.
The non-directional beacon (NDB) is a ground station that emits a constant signal in every direction, also known as an omnidirectional beacon. An NDB signal operated on a frequency between 190-535 KHz does not offer information on the direction of the signal, just the strength of it.
Signals move over the ground, following the curvature of the earth. NDB stations are classified into four groups based on the beacon range in nautical miles.
- Compass locator—15
- Medium Homing—25
- High Homing—75
Aircraft flying close to the ground and the NDB stations will get a reliable signal despite the signal still being prone to the following errors:
- Ionosphere error: Specifically during periods of sunset and sunrise, the ionosphere reflects NDB signals back to earth, causing fluctuations in the ADF needle.
- Electrical interference: In areas of high electrical activity, such as a thunderstorm, the ADF needle will deflect toward the source of electrical activity, causing erroneous readings.
- Terrain errors: Mountains or steep cliffs can cause bending or reflecting of signals. Pilots should disregard erroneous readings in these areas.
- Bank error: When an aircraft is in a turn, the loop antenna position is compromised, causing the ADF instrument to be off balance.
Pilots have found the ADF/NDB system to be reliable in determining position, but for a simple instrument, an ADF can be very complicated to use. To begin, a pilot selects and identifies the appropriate frequency for the NDB station on his ADF selector.
The ADF instrument is typically a fixed-card bearing indicator with an arrow that points in the direction of the beacon. Tracking to an NDB station in an aircraft can be done by "homing," which is simply pointing the aircraft in the direction of the arrow.
With wind conditions at altitudes, the homing method rarely produces a straight-line to the station. Instead, it creates more of an arc pattern, making homing a rather inefficient method, especially over long distances.
Instead of homing, pilots are taught to "track" to a station using wind correction angles and relative bearing calculations. If a pilot is headed directly to the station, the arrow will point to the top of the bearing indicator, at 0 degrees. Here's where it gets tricky, while the bearing indicator points to 0 degrees, the aircraft's actual heading will usually be different. A pilot must understand the differences between relative bearing, magnetic bearing, and magnetic heading to properly utilize the ADF system.
In addition to constantly calculating new magnetic headings based on relative magnetic bearing, if we introduce timing into the equation—in an effort to estimate time en route, for example—there is even more calculating required.
Here is where many pilots fall behind. Calculating magnetic headings is one thing, but calculating new magnetic headings while accounting for wind, airspeed, and time en route can be a large workload, especially for a beginning pilot.
The workload associated with the ADF/NDB system can be laborious and many pilots have stopped using it. With new technologies like GPS and WAAS so readily available, the ADF/NDB system is becoming an antiquity, and some have already been decommissioned by the Federal Aviation Administration.