Chapter 10: Radio Waves and Propagation

The spectrum at sea

VLF and LF (3–300 kHz) follow the Earth's curvature as groundwaves over thousands of miles and are used for timing and eLoran. MF (300 kHz–3 MHz) gives reliable groundwave by day out to a few hundred miles; it carries DGPS beacons and NAVTEX at 518 kHz. HF (3–30 MHz) relies on ionospheric skywave for long range, varying with time of day and solar activity. VHF (156–174 MHz) and above are line-of-sight only, which sets the range of bridge-to-bridge radio, AIS and radar.

• Groundwave — stable, predictable, lower frequencies
• Skywave — long range but variable delay and fading
• Line of sight — range limited by antenna heights

Propagation errors

Skywave contamination of a groundwave signal produces cycle slip and timing error, worst at night. Multipath — reflections from the sea surface, superstructure or nearby land — corrupts both radio fixes and GNSS pseudoranges near large steel structures. The atmosphere refracts radio waves slightly downward, giving a radar horizon about 15% beyond the geometric horizon; ducting can extend it enormously and subrefraction can shorten it.