Subelement K: K – Aircraft— Topic 71: Automatic Direction Finding Equipment (ADF) & Transponders
Question 3-71K2
Element 3 (GROL)What is meant by the term “night effect” when using an aircraft’s Automatic Direction Finding (ADF) equipment? Night effect refers to the fact that:
Explanation
Non-Directional Beacons (NDBs) transmit in the Medium Frequency (MF) band. During daylight hours, the D-layer of the ionosphere absorbs sky wave signals in this frequency range, meaning the aircraft's Automatic Direction Finder (ADF) primarily receives stable ground waves.
At night, the D-layer dissipates. This allows MF signals to reflect off higher ionospheric layers (like the E and F layers) and return to Earth as sky waves. These reflected sky waves can interfere constructively or destructively with the direct ground wave signal arriving at the aircraft. This interference causes fluctuations in signal strength and erratic, unreliable bearing indications on the ADF, making navigation challenging. This phenomenon is known as the "night effect."
Option A is incorrect because NDBs are critical navigation aids and operate continuously. Option C is incorrect; radio wave speed is constant in a given medium regardless of time of day, and ADF receives NDB signals. Option D is incorrect as "night effect" is a radio wave propagation phenomenon, not related to antenna moisture.
Related Questions
3-70K6 On runway approach, an ILS Localizer shows:3-71K1 What is the frequency range of an aircraft’s Automatic Direction Finding (ADF) equipment?3-71K3 What are the transmit and receive frequencies of an aircraft’s mode C transponder operating in the Air Traffic Control RADAR Beacon System (ATCRBS)?3-71K4 In addition to duplicating the functions of a mode C transponder, an aircraft’s mode S transponder can also provide:3-71K5 What type of encoding is used in an aircraft’s mode C transponder transmission to a ground station of the Air Traffic Control RADAR Beacon System (ATCRBS)?