Night Effect in ADF Navigation

What Is Night Effect in ADF Navigation?

Night effect occurs when sky waves—radio waves reflected from the ionosphere—reach the aircraft's ADF receiver along with the usual ground waves from the NDB. During daylight, the ionosphere absorbs most of these frequencies, but at night, reduced solar radiation allows sky waves to travel further and return to Earth, especially at dawn and dusk.

How Does Night Effect Cause Errors?

• Ground waves travel along the Earth's surface and are vertically polarized, providing accurate bearings.
• Sky waves, reflected from the ionosphere, arrive from above at steep angles and with different polarization.
• When both reach the ADF receiver, their phase and polarization differences cause the ADF needle to oscillate or 'hunt,' making it difficult to obtain a stable bearing.
• This effect is most pronounced during twilight periods and at ranges greater than approximately 200 NM from the NDB, where sky waves can return to the aircraft's position.

Operational Implications

• The ADF needle may show rapid, small swings rather than a steady indication, reducing confidence in the bearing.
• Pilots may notice fading or cyclic changes in signal strength when using the BFO (beat frequency oscillator) function, confirming sky wave interference.
• Short-range NDBs (within ~200 NM) are less affected, as sky wave geometry prevents significant interference.
• Night effect can also allow interference from distant NDBs on the same frequency, further complicating navigation.

Key Points for ADF Procedures

• Always identify the NDB before use, especially at night.
• Be cautious interpreting ADF bearings during dawn, dusk, or night—expect possible needle fluctuations.
• For critical navigation (e.g., approaches), prefer VHF aids over ADF when night effect is suspected.