HF Data Modems

HF data modems carry digital traffic over high-frequency radio (3 to 30 MHz), using ionospheric skywave propagation for ranges of hundreds to thousands of nautical miles without satellites. Maritime use includes GMDSS HF radiotelex (NBDP) and email systems such as SailMail and Winlink Pactor for vessels in GMDSS Sea Area A4 (polar) or beyond economical satcom. Throughput is low, typically a few hundred bits to a few kilobits per second, and links degrade with propagation, but HF remains a low-cost, infrastructure-independent backup for long-range data in remote regions.