Aircraft utilize HF communications when VHF (Line of Sight) communications is not sufficient. The primary usage of HF is for Trans-oceanic flights. Trans-oceanic flights communicate with ground stations via HF for position reports and other purposes. Another utilization of HF communications is for HFDL or High Frequency DataLink. Finally Military Aircraft (MILCRAFT) utilize HF for operational and training.
HFDL is a HF data link protocol, defined in ARINC spec 635-3.
It may be described as some sort of HF ACARS.
Transmissions on HF are in USB on a sub carrier of 1440 Hz with a symbol speed of 1800 baud.
Modulation is 2-PSK, 4-PSK or 8-PSK with effective bit rates of 300, 600, 1200 or 1800 bits/sec.
The HFDL service is operated by ARINC as GLOBALink service through a worldwide network of HF stations.
On board the aircraft, a pilot simply sets one of the HF radios to "DATA" after takeoff, and the HFDL seamlessly integrated into the flight management system.
The ACARS will use HF or VHF depending on what is available. The HF part of the system is usually taken out of "DATA" mode before landing to prevent inadvertent RF exposure to ground personnel, since the system will start to tune around and seek a connection to the network as HF conditions change.
HFDL signals are present whenever the HF bands are open, and are actually more robust than voice transmissions. The author has often gotten solid copy of HFDL transmissions while finding voice from the same geographic area to be a struggle. Even when conditions are marginal, a scan of the current HFDL frequencies will often yield readable data. With a suitable computer controlled radio, the HFDL nets can be followed up and down the spectrum with the diurnal cycle of the ionosphere.
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