Introduction

The monitoring of air band communications is a hobby that has become more and more popular over the last 10 years, especially in the UK. In common with the rest of the communications field, there are changes in process in this area that involve greater use of digital techniques.

During peak air traffic periods, there are over 1000 commercial flights simultaneously in the skies over North America alone. The control of these flights is the responsibility of air traffic control centres. This control generates hundreds of simultaneous voice messages on VHF frequencies, to which have to be added the flight management messages from airline operations centres. Much of the voice contact traffic is used for simply describing routine aircraft manoeuvres such as push back, takeoff time, landing time, gate arrival, aircraft performance, fuel consumption and position reports. Most messages also require read back for confirmation thus doubling the load on the voice channels. As Flight Engineers were eliminated from the flight decks of many aircraft, this reporting load fell on pilots, and a method had to be found to ease the work load and the demand on available frequencies.

The solution was developed by a private company in America called Aeronautical Radio Inc (ARINC) during the mid 1970s. However, it took nearly 20 years for the technology and price of available computer equipment to catch up and make the system viable for commercial operations.

This solution is called ACARS (Aircraft Communications Addressing and Reporting System). It is basically a network of several hundred ground radio stations, mostly situated in North America and Europe, which enable aircraft to operate as airborne computer terminals linked to them by VHF radio. Those who are familiar with amateur packet radio will recognise the similarity between this and ACARS. Information is automatically collected from sensors on board the aircraft and transferred over the radio link to ACARS ground stations, whence it is relayed to a central processing computer for distribution to users such as airlines via ARINC's electronic switching system. Currently in North America alone, over 2 million ACARS messages are processed every week! In addition to automatic data messages, the system is becoming widely used for transmission of weather information, fault reporting, and any other text messages that may be required between aircraft and ground.

What equipment do I need?

ACARS messages in Europe are transmitted in short data bursts on a single frequency of 131.725 MHz. The mode used is AM, and messages are simplex (ie. both air and ground stations are on the same frequency). Unless you live within 10 miles or so of a ground station, you will only hear the aircraft end of the link as the ground transmission are relatively low powered. However, most of the aircraft using the system are operating in airways or upper airways, and consequently provide strong signals over a very wide area. Any reasonable AM airband radio or scanner should be capable of receiving ACARS, although it may be necessary to use an outside aerial for best results. The squelch control on the radio must be turned completely off, as otherwise the transmission burst will be nearly over before the squelch opens!

You will then need a suitable decoder plus a display device. ACARS is a very specialised and high speed data mode, and only decoders that have been specially designed for it will be able to function. Currently the only devices available are three general purpose data decoding models manufactured by Universal Radio in the USA, and the AIRMASTER package (DOS or Windows version). The AIRMASTER includes a small interface in a DB25 type package which plugs into and takes its' power from an IBM compatible PC, which also runs the special decoding software supplied.

What messages will I hear?

With aircraft operating at 35000 feet and above, you will expect to hear them up to around 400 miles away. ACARS messages are sent immediately after aircraft departure, during high altitude flight, and during approach to land. Whilst not all commercial aircraft are ACARS equipped, it is now standard fitment on all new Boeing and Airbus deliveries, and is rapidly becoming a standard feature with all major airlines. Part of the ACARS message header specifies the flight number and the aircraft callsign and registration number, so it is very simple to build up a data base of users.

Future Developments

ACARS was primarily designed for use with VHF AM. It has also been used experimentally via HF and satellite links, and it is likely that there will be future expansion in this area. Various manufacturers of GPS equipment are also bringing out economy airborne avionics that use ACARS in conjunction with differential GPS (Global Positioning System) facilities to enable smaller aircraft to have the benefits of precision satellite based landing and navigation aids that until now have been confined to airlines because of the high cost.

Adding ACARS monitoring capability to your receiving station will open the door to a whole new world of digital aircraft communications.

Further reading

Those wishing to study this subject in more depth will find a chapter in the "Worldwide Aeronautical Communications Frequency Directory", and a fuller description in "Understanding ACARS". Both these books are published by Universal Radio Research in the USA. In addition, ARINC themselves have a very comprehensive list of publications on the subject, most of which are highly technical and start at around £50 pounds each, but nevertheless provide full engineering descriptions of every aspect of the system.

The AIRMASTER package costs £89.95 Sterling including UK Delivery within the EC and oustide the EC Community the price is £76.55 plus Delivery.

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