11.6.2 Ionospheric Scatter Communications

Ionospheric scatter exploits scattering from irregularities in the ionosphere at upper HF and low VHF frequencies (approximately 30–100 MHz). Unlike classical sky-wave refraction, ionospheric scatter does not rely on specular layer reflection but on random inhomogeneities in electron density that redirect a small portion of the transmitted energy toward the receiving station. The received signal is therefore weak and highly variable, resulting from the vector sum of many scattered components.

Single-hop ranges of 1,000–2,000 km are possible, but very high transmit powers (typically ≥10 kW) are required to overcome severe path loss, Doppler spread, fading, and multipath effects. As with troposcatter systems, large high-gain antennas and diversity techniques are necessary to maintain link reliability.

Because ionospheric scatter channels exhibit significant Doppler spread, delay spread, and slow fading, achievable data rates are relatively low. Early military systems typically supported 75–600 bps, with later implementations achieving 1.2–4.8 kbps and, under favorable conditions, up to about 9.6 kbps. Modern digital techniques using adaptive modulation, interleaving, and strong forward error correction can improve reliability, but practical data rates generally remain below 10 kbps. Consequently, ionospheric scatter has primarily been used for low-rate command, control, and telemetry applications rather than broadband communication.

Although ionospheric scatter was employed in several early military long-range communication networks, it has largely been superseded by satellite communications and modern troposcatter systems, which provide substantially higher capacity and more predictable performance.