🔍 Key Concepts

• HF vs VHF vs satellite frequencies and how they propagate • How the ionosphere affects HF radio signals (fading, skip, atmospheric noise) • Which systems are designed to minimize static and fading (satellite links, VHF FM)

💭 Think About

• Group each option by what frequency range it mainly uses (HF, VHF, satellite/L-band) and recall which of those is most affected by ionospheric conditions and atmospheric noise. • Ask yourself which service is known for long‑range over‑the‑horizon communication using the ionosphere, and which are more short‑range, line‑of‑sight, or satellite‑based. • Consider which modulation types (e.g., VHF FM, satellite digital links) are relatively clear of static compared to classic long‑range radiotelex services.

✅ Before You Answer

• Identify which option operates primarily in the HF band. • Confirm which systems (among the choices) are VHF line‑of‑sight and therefore less prone to ionospheric fading. • Verify which listed service is a form of radiotelex/data over HF, traditionally known to suffer from fading and static.