NDB and ADF - ATPL 062 — Radio Navigation

Ground direction finding put the measurement on the ground; this lesson moves it into the aircraft. The pair carries its jobs in its names:

NDB — stands for non-directional radio beacon, and it is the ground part of the system: a transmitter that radiates equally in all directions;
ADF — stands for automatic direction-finding equipment, and it is the airborne part: the receiver whose needle points at the beacon.

Between them they are the oldest airborne radio aid still flying, and the exam loves them for the same reason instructors do: everything about them, from the arithmetic to the errors, is honest physics.

The beacon

The NDB operates in the LF and MF frequency bands, and the band assigned to aeronautical NDBs by ICAO (International Civil Aviation Organization) Annex 10 is 190-1,750 kHz. Those bands are a deliberate choice: as the propagation lesson taught, LF and MF have a surface wave, so the beacon reaches beyond the horizon without caring about aircraft altitude. One member of the family has its own name: a locator beacon is an LF/MF NDB used as an aid to final approach, usually with a range of 10-25 NM, planted on an approach path to give the procedure a marker the ADF can point at. The band has other tenants worth knowing about: certain commercial radio stations transmit within the frequency band of the NDB, one more reason the ident is checked rather than assumed. And per ICAO Annex 10, an NDB station has an automatic ground monitoring system: the ground side watches the beacon's health for the maintenance operation. That monitoring never reaches the cockpit, which is exactly why the no-failure-warning rule later in this lesson still stands: the ground knows, the needle does not say.

In navigation the NDB is used wherever a bearing is enough: as an en-route waypoint to steer at or from, as the anchor of holding patterns, and as the guidance for non-precision NDB approaches, the aircraft flying inbound and outbound bearings across it. Before any of that, the station must be identified: the procedure to identify an NDB is to tune it, listen for its Morse code identifier, and confirm it matches the chart before using the needle. The emission classes from the propagation lesson matter here: an NDB station emits a N0N/A1A or a N0N/A2A signal, the bare carrier paired with one of the two Morse ident schemes. The BFO (beat frequency oscillator) is the receiver circuit that mixes a local oscillation with the carrier to make an otherwise silent keyed carrier audible as a tone: to identify a N0N/A1A NDB, the BFO circuit of the receiver has to be activated, while a N0N/A2A beacon transmits its Morse as an amplitude-modulated tone, audible without it. On modern aircraft the BFO is activated automatically, the receiver deciding for itself when the mixing is needed.

The beacon's propagation path follows straight from the propagation lesson: an NDB's waves travel primarily as the surface wave, following the Earth's surface day and night, which is what makes the beacon's range independent of the aircraft's altitude; and part of the energy leaves upward as sky waves, absorbed by the ionosphere's D layer by day and returned from the higher layers at night, which is the root of every night-time misbehaviour in this lesson's error catalogue.

The three faces of the ADF

The ADF measures one thing, the relative bearing: the angle from the aircraft's nose, clockwise, to the station. The indicator types commonly in use are the electronic display, the RMI, the fixed-card ADF (also called the radio compass), and the moving-card ADF; the electronic display draws whichever of the mechanical presentations the aircraft's screens choose to imitate, so it is the three mechanical faces the syllabus wants read:

the fixed card — 0 is locked at the top, so the needle reads the relative bearing and nothing more; every magnetic quantity must be computed from it;
the moving card — the pilot rotates the card by hand so the current heading sits under the top index; while the setting is correct, the needle head reads the QDM directly, but every heading change silently invalidates it until the card is reset;
the RMI (radio magnetic indicator) — the card is driven continuously by the slaved compass system, so the needle head always reads the QDM and the tail the QDR, with no hand-setting to forget.

The bearing arithmetic

On a fixed card the needle gives only the relative bearing, and the pilot does the arithmetic the RMI does mechanically. The rule is one addition:

The heading itself usually starts life as a compass reading, so the conversion chain from the compass course runs through both corrections: compass plus deviation gives magnetic, magnetic plus easterly variation (or minus westerly) gives true, and the same chain converts a compass bearing into a magnetic and a true bearing. One worked pass through the whole ladder:

Flying the needle

The in-flight ADF procedures the syllabus names all grow from one choice, what to do about wind:

Homing — the lazy option: keep the needle on the nose and fly at the station. In any crosswind the aircraft drifts, the needle is re-centred again and again, and the path becomes a curve that bows downwind, arriving at the station from the downwind side, longer and slower than the straight line;
Tracking — does the work instead: the pilot holds a wind correction angle into the wind and keeps the needle displaced by exactly that angle, so the aircraft rides the straight track to the station.

Intercepting an inbound QDM — turn to a heading that cuts the desired track at a deliberate angle, typically 30 to 90 degrees depending on the distance to run, watch the needle head fall toward the inbound QDM as the track approaches, and roll out onto it with wind correction as it arrives;
Intercepting an outbound QDR — the same geometry flown away from the station, read on the needle tail: cut toward the required QDR and watch the tail walk onto it, the head pointing back at the station throughout;
Changing from one QDM or QDR to another — the same interception flown from a line already held: leave the old track, cut across, and capture the new one;
Station passage — directly overhead the beacon lies the cone of confusion: a funnel-shaped region above the station, widening with altitude, in which the bearings are erratic and momentarily unusable; approaching it the needle grows restless, then swings from the nose to the tail; passage is taken when the swing settles behind the wing line;
The abeam point — the station is abeam when the needle points 090 or 270 relative, square off the wingtip.

Limits and errors

The NDB's simplicity comes with a long list of honest weaknesses, each one syllabus material:

Power and range — the relationship is direct: more transmitter power, more range, the surface wave reaching further before it fades;
Fading — interference between the beacon's own sky wave and ground (surface) wave produces the fading the propagation lesson defined, the signal strength swelling and sinking as the two arrivals drift in and out of step;
Night/twilight effect — sky waves and ground waves arriving at the ADF receiver with a difference of phase and polarisation introduce bearing errors: the beacon's own returned sky wave, strongest around dawn and dusk, disagrees with its surface wave at the receiver and the needle wanders;
Coastal refraction — as a radio wave travelling over land crosses the coast, the wave speeds up over the water and the wave front bends, so a bearing taken across a coastline is pulled away from the true line toward the coast;
Station interference at night — interference from other NDBs on the same frequency may occur at night, by sky-wave contamination: after dark the ionosphere returns distant beacons' sky waves into your beacon's coverage, and the needle averages strangers into its answer;
Mountain effect — diffraction of the radio waves in mountainous terrain bends the arriving signal around the relief, so the needle points where the wave came from, not where the beacon is;
Thunderstorm static — static radiation from a cumulonimbus cloud can interfere with the radio wave and influence the ADF bearing indication, the needle wandering toward the cell;
Dip error — the bank angle of the aircraft causes a dip error: with the antenna's loop element banked along with the aircraft, the geometry of the measurement tilts, and the bearing indication is disturbed for as long as the wings are not level;
No failure warning — there is no warning indication of NDB failure: no flag drops; a dead or dying beacon can leave a plausible-looking needle, which is why the ident is monitored, not checked once;
The accuracy required of the pilot — per ICAO Doc 8168, the required bearing must be flown within plus or minus 5 degrees to be considered established during approach.

What the exam asks

The frequency numbers, the three displays, the bearing arithmetic and the error catalogue carry the marks. The overview, following the learning objectives:

Define NDB and ADF and their ground/airborne split; state the commercial-station neighbours and the Annex 10 automatic ground monitoring

State the N0N/A1A and N0N/A2A emissions, the BFO's function and when it is needed, and its automatic activation on modern aircraft; describe the NDB's propagation path

State the NDB's bands (LF/MF) and the ICAO Annex 10 assignment 190-1,750 kHz; define the locator beacon

Describe the use of NDBs for navigation and the identification procedure

Interpret RMI, fixed-card and moving-card ADF displays and the information they give

Calculate the true bearing from compass heading and relative bearing; convert compass bearings to magnetic and true

Name the indicator types in use

Interpret the cone of confusion

Describe homing and tracking with wind, QDM/QDR interception and changes, station passage and the abeam point

Explain power vs range, sky/ground-wave fading, the plus-or-minus-5-degrees established criterion, and the absence of a failure warning

Define the night/twilight effect and explain coastal refraction; state the night-time same-frequency interference and describe the mountain, thunderstorm-static and bank (dip) errors