Tuning the HFDL Network

A general Description of the HFDL Network

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.

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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:

  1. Transmissions on HF are in USB on a sub carrier of 1440 Hz with a symbol speed of 1800 baud.
  2. Modulation is 2-PSK, 4-PSK or 8-PSK with effective bit rates of 300, 600, 1200 or 1800 bits/sec.
  3. Several ground stations provide global network coverage and system status updates.

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.


Software for receiving High Frequency Data Links:


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:

  1. PC-HFDL - Version 2.01 written by Charles Brain G4GUO.
  2. PC-HFDL - Version 1.01 (free) written by Charles Brain G4GUO.
  3. HOKA Software

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
02 Molokai, HI 21937 21928 17934 17919 13324 13312 13276 11348 11312 10081 8936 8912 6565 6559 5514 5463 4687 3434 3019 2947
03 Reykjavik, ISL 17985 15025 11184 8977 6712 5720 3900 3116
04 Riverhead, NY 21934 21931 17952 17934 17919 13276 11387 11354 11315 10027 8912 8885 8831 6661 6652 6646 5652 5523 3428 3410
05 Auckland, NZL 21949 17916 13351 11327 10084 8921 6535 5583 3404 3016
06 Hat Yai, THA 21949 17928 13270 10066 8825 6535 5655 4687 3470
07 Shannon, IRL 11384 10081 8942 8843 6532 5547 3455 2998
08 Johannesburg, AFS 21949 13321 8834 4681 3016
09 Barrow, AK 21937 21928 17934 17919 11354 10093 10027 8936 8927 6646 5544 5538 5529 4687 4654 3497 3007 2992 2944
11 Albrook PAN 21940 17901 10063 6589 5589 2902
13 Santa Cruz, BOL 21997 21988 21973 21946 17916 13315 11318 8957 6628 4660 3467 2983
14 Krasnoyarsk, RUS 21990 17912 13321 10087 8886 6596 5622 4679 2905 2878
15 Al Muharraq, BHR 21982 17967 13354 11312 10075 8885 5544 2986
16 Agana, GUM 17919 13312 11306 11288 8927 6652 5451
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!





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