Describe the concept of "over-the-horizon" communications.

The concept of "over-the-horizon" communications is accurately described by the use of high-frequency systems to communicate beyond the visual horizon through ionospheric reflection. High-frequency (HF) radio waves, typically ranging from 3 to 30 MHz, can travel long distances by reflecting off the ionosphere, a layer of the Earth’s atmosphere filled with charged particles. This reflection allows for communication that exceeds the normal line-of-sight distance, enabling operators to connect over vast geographic areas.

This technique is particularly advantageous in military and remote operations, where direct line-of-sight communication is often not feasible due to obstacles such as terrain or distance. The use of ionospheric reflection facilitates reliable communication links even when the transmitting and receiving stations are not in direct view of each other. Furthermore, the performance of HF systems can vary based on factors such as time of day, frequency used, and solar activity, all of which influence the ionosphere's properties.

The other options do not capture the essence of over-the-horizon communications as effectively. Options involving low-frequency systems or direct satellite links pertain to different communication paradigms, and the use of relay stations, while helpful for extending range, does not specifically employ the ionospheric propagation methods that define