ATPL EASA Radio Navigation Practice Exam 2025 – All-in-One Guide to Master Your Certification!

Ground waves propagate along the surface of the Earth, which is why this answer is the correct choice. Ground waves are a type of radio wave that follow the contours of the Earth, allowing them to travel considerable distances, especially at lower frequencies. This propagation is influenced by the Earth's surface and can be affected by factors such as terrain and obstacles.

The propagation of ground waves occurs primarily at frequencies below about 30 MHz, where they can effectively bend around obstacles and follow the curvature of the Earth. This characteristic allows them to be utilized in various communication systems, particularly in situations where line-of-sight transmission is not feasible.

Other choices, while discussing different wave propagation methods, do not explain ground wave behavior accurately. For instance, radio waves that travel through the ionosphere concern skywaves, which can reflect off the ionosphere for long-distance communication, but this does not apply to ground waves. Similarly, stating that ground waves travel in straight lines through the atmosphere or in circular patterns around the planet does not reflect their actual nature and method of propagation.