ADF/NDB Navigation

AST 241

Professor Barnhart

NDB Background

• Oldest form of radio navigation still in use• Operate in the LF/MF range (190 – 535 kHz- or

between 100 and 10,000 meters)• Still in use because of its simplicity and low cost

to set up and operate + decent accuracy• Little used for enroute navigation in the US but

still used for approaches• Used for enroute nav. In much of the world

NDB background

• NDB’s are also useful at ranges up to 600 N.M.

• Ground equipment consists of a simple a.m. radio station

• Aircraft receiver is little more complicated than an AM radio.

Principles of NDB Navigation

• Transmitters vary in power from 25 watts for Locator Outer Markers (LOM’s) on an ILS approach to as much as 2000 watts.

• The range of the most powerful transmitters is listed as 75 miles but is usually much greater.

• 4 types of NDB’s; Compas locators, Approach aids (25 NM), Enroute beacon, High power beacons- used in some costal areas- largely obsolete

Aircraft Equipment

• The Automatic Direction Finder (ADF) consists of:– AM receiver– Sense Antenna– Loop Antenna (directional antenna)– Indicator (fixed or movable card)

Aircraft Equipment Ctn.

• The Automatic in ADF comes from the fact that the directional antenna is rotated electronically rather than manually by rotating the antenna or turning the aircraft to make signal strength determinations.

ADF Operation

• There is a switch on the receiver marked:– OFF/ADF/ANT/BFO– ANT- gives maximum receiver sensitivity and

should be used for tuning and identifying (3 letter coded I.D. except for LOM’s which have 2 letter I.D.’s)

– BFO (ContinuousWave) position is used for better identifying unmodulated signals which are used in other parts of the world.

ADF Operation Ctn.

• After identifying station be sure to place selector switch in the “ADF” mode as the indicator will not display any bearing information in the ANT mode

• Leave volume up continuously when using the ADF as NDB frequencies can shift

• Most units have a “Test” function which swings the needle; don’t use if the needle doesn’t swing. This verifies proper signal reception.

ADF indicators

• Fixed card– With a fixed card there are two ways to

determine magnetic bearing to the station:• Turn the aircraft toward the station and note

heading• MH + RB = MB (to the station)

– MH = aircraft magnetic heading– RB = Relative bearing– MB = Magnetic Bearing (to the station)– (If the result is more than 360 then subtract 360

ADF Indicators

• Movable Card Indicators:– Set aircraft heading at top of the indicator and

read the relative bearing to the station.

NDB Navigation Techniques

• Homing:– Turing the aircraft such that the needle always

points directly toward the station- easy but inefficient as it does not compensate for crosswinds

– Results in a curved path to the station in a crosswind situation

NDB Navigation Techniques

• Tracking:– Establishing a wind correction angle that negates the

drift caused by the crosswind

– Principle: WHEN THE ANGLE OF DEFLECTION = THE ANGLE OF INTERCEPTION YOU’RE ON COURSE

– Or: when the angle formed by the aircraft heading and the desired course is the same as the angle between either the 0 or 180 mark on the indicator and the pointer, the aircraft is on course.

Tracking ctn.

• Tracking tip- When tracking inbound on an NDB bearing and you note you’re off course:– Note the angle of deflection, double it, and use that as

your intercept heading. When that angle of deflection is then noted on the indicator (from the nose or tail) the aircraft is back on course

– Establish an initial track by taking half of that intercept angle out

Tracking Ctn.

• Example:– You want to track inbound on the 360 bearing

to the station. There is a wind from the west.– While tracking (heading north) unaware of the

wind you note a left needle deflection of 10 degrees.

– Double that to 20 degrees for your intercept heading and turn the aircraft left to 340 degrees.

Tracking Ctn.

• As you turn to 340 you notice the needle move toward the top of the indicator.

• As you continue on 340 the needle will drift toward the right. When it indicates 20 degrees, the aircraft is back on course.

• Take half your calculated intercept heading and use that as your initial track heading (350 degrees). This results in a 010 degree indication on the indicator.

• Repeat the process until track correction and needle deflection are equal.

NDB/ADF Limitations

• Night Effect- – NDB’s emit three signals, among these are the ground

wave and the sky wave. Normally the ground wave is the strongest which is the one use for navigation.

– The skip zone is the difference or gap between the ground wave and the reflected sky wave.

– Light passing through the ionosphere causes height fluctuations within it varying the angle of the reflected sky wave and possibly causing signal confusion during this time between the two signals. The needle may wander. Resolves shortly after sunrise or sunset.

Fading

• Usually occurs at night when ground wave and sky wave interact going in and out of “phase” causing the signals to be either canceled or reinforced as the atmosphere changes.

• Pilots will notice a rythmic swinging of the needle and a volume fluctuation of the identifier.

• Average the fluctuations and note the bearing when the signal seems strong.

Shoreline Effect

• Ground waves change direction as they pass from land to water and visa versa; they are bent slightly.

• Pilots should note potential bearing indication errors when flying in the vicinity of costal areas. NDB’s used primarily for oceanic navigation have been designed to minimize this error.

Other sources of Error

• Terrain- mountains, areas with high concentrations of iron

• Interference- between two stations may cause the ADF to oscillate between one station and the other- indicated by receiving two simultaneous I.D. codes. Usually resolves itself with proximity.

Thunderstorms

• Lightening can create signal disturbances causing the needle to momentarily swing in the direction of the storm. As such ADF is sometimes referred to as a “Poor man’s Storm Scope.”

• Pilots should not rely on ADF indications in the vicinity of thunderstorms.

ADF Accuracy

• Variable- not possible to determine. As such, back up ADF indications whenever possible.

The End

Questions?