Which of the following systems is most susceptible to fading or static interference?

The HF SITOR (Selectieve Automatic Radio Transmission System), also known as NBDP (Narrow Band Direct Printing), operates in the High Frequency (HF) band, which is known for its longer range and ability to bounce off the ionosphere. However, this same characteristic makes it particularly vulnerable to fading and static interference. Fading occurs because the ionosphere can reflect radio signals in an unpredictable manner due to various atmospheric conditions, leading to signal degradation.

HF signals are more susceptible to natural interference from weather conditions, such as thunderstorms, and can also experience static from electrical discharges and other environmental factors. This makes HF communication less reliable compared to systems that operate in more stable frequency ranges.

In contrast, other systems like Inmarsat, which operates in the geostationary satellites range, typically experience much less interference due to their frequency and the nature of satellite communications. DSC on channel 70 and VHF ARQ operate in frequency bands that are generally less prone to the types of atmospheric disturbances that affect HF frequencies.

Thus, the characteristics of HF SITOR make it the system most susceptible to fading or static interference among the given choices.