The first implementations of the Aircraft Communications Addressing and Reporting System (ACARS) saw deployment in the middle 1980s. ACARS greatly shrunk the gap between aircraft, flight crews, maintenance, and dispatch personnel. Instead of relying on phone patches on congested and scratchy voice radio circuits, messaging became something more akin to modern day text messaging. The original messages were limited to departure and arrival times, weather, and limited performance data.
Digital messaging between parties concerned with flight operations have since become more voluminous and sophisticated. Modern aircraft can automatically make routine engine performance observations, check systems status, note the present geographic position and weather, then downlink it all to the operator\'s maintenance and dispatch facilities. As we have recently seen with the loss of Air France 447, the ACARS / HFDL system dutifully reported a string of malfunctions before going inoperative.
Air traffic control is increasingly a user of data downlinked by aircraft, and not only uses Mode S transponder data, but also position reports and other items essential in the "Future Air Navigation System."
VHF ACARS has a limited range, and is not available over remote areas such as polar regions and over oceans. That is where High Frequency Data Link protocol becomes vital to long range aircraft operators. HFDL is defined in ARINC specification 635-3, and it is operated by ARINC as GLOBALink service through a worldwide network of HF stations. HFDL is actually a blanket term to describe signals of similar format and purpose:
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.
The following software may be used for reception of HFDL signals, and depending on the package, may enable automatic tuning of the various frequencies used by the network. One only needs to connect the receiver to the computer soundcard line or microphone input. For the more sophisticated control functions, the proper USB, ethernet, or parallel cables must be used between the computer and radio.
Software which can receive HFDL signals:
Note: PC-HFDL is by far the most popular decoding software, but the system tables must be updated to reflect the current network configuration. Users are advised to do a Google Search for the latest"pchfdl.dat" and "pchfdl.txt" files. There is plenty of configuration information on the internet for the software. PC-HFDL also works well in virtual machines and in Linux (under WINE). Other software is much less well supported.
Table 34 of the ARINC HFDL System
|Node ID||Xmtr Name||1||2||3||4||5||6||7||8||9||10||11||12||13||14||15||16||17||18||19||20|
|01||San Francisco CA||21934||17919||13276||11327||10081||8927||6559||5508||4672||2947|
|06||Hat Yai, THA||21949||17928||13270||10066||8825||6535||5655||4687||3470|
|13||Santa Cruz, BOL||21997||21988||21973||21946||17916||13315||11318||8957||6628||4660||3467||2983|
|15||Al Muharraq, BHR||21982||17967||13354||11312||10075||8885||5544||2986|
|17||Telde, Gran Canaria, CNR||21955||17928||13303||11348||8948||6529||5589||2905|
For the most up-to-date support, see the Yahoo! HFDL Forum.
As this is being written, the author is in Asia, using a Web controlled software defined radio located in the Netherlands to monitor North Atlantic HFDL traffic in the 10 MHz aero band. The digital audio is being piped directly to PC-HFDL, and the data is squeaky clean. That is mighty slick listening!