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Posts from the ‘Radio technology’ Category

ACARS and transponder transmissions: receiving and decoding

March 23rd, 2014

Drs. Andor Demarteau

In the light of news surrounding the missing Malaysia Airlines aircraft (Flight MH370), and all the speculation surrounding the Aircraft Communications Addressing and Reporting System (ACARS), I decided to have a look around to see how easy/difficult it is to receive and decode the transmissions with readily-available hardware and/or software.

My experimentation led me to a particular smartphone app which advertised the ability of decoding ACARS transmissions from the raw audio received through a typical hand scanner. Results varied depending on the method used for audio input to the app (for processing), either via the scanner’s speaker or through a direct audio cable input.

This effectively gave me a full ACARS decoder for all transmissions by aircraft within a radius of between several tens to several hundreds of kilometres, depending on the flight level of the particular plane.

(Thought: Makes for even-more interesting plane-spotting sessions!)

Interestingly, there are also desktop-based software (for Linux, Mac and Windows systems) that offer similar capabilities.

How ACARS is transmitted

There are three available methods used for the transmitting of ACARS data.

For everything over land and close to airports, the VHF (very high frequency) band is used. This is the same frequency range used by air traffic control audio feeds (108.000 MHz to 136.975 MHz). In most cases where flights do not fly over vast empty land masses or major oceans, or the North or South Pole, this is the cheapest and preferred way. It is also the type of transmissions that can easily be received as discussed above.

Next, there is the satellite-based ACARS transmission, using either the much-discussed Inmarsat or Iridium network. The latter has slightly-better coverage over the North and South Poles, and was only enabled for ACARS transmission back in 2007.

The third type, introduced in 1995, is a network of globally-spread HF (high frequency) ground stations. This is similar to the VHF method discussed above, but with the difference of radio transmissions over HF travelling greater distances (dependent on the frequencies used and time-of-day).

ADS-B and ‘Mode S’ transponder transmissions

The other type of transmission that was highly speculated on was those from the transponder.

Transponders can be set to several modes. But for commercial aviation, ‘Mode S’ and the more-advanced ADS-B (Automatic dependent surveillance-broadcast) modes are most widely adopted for use.

For receiving these transmissions, there are software packages available (for major computing operating systems) that can decode, process and plot the received data on a Google Maps display. Web sites like FlightRadar24.com and Plane Finder use this type of data to plot aircraft positions and flight tracks.

With this in mind, it is not hard to see why, if done deliberately, the ACARS system and transponder on MH370 were disabled. There are possibly more “stations” listening (for example: users with handheld scanners) than just the official ground stations assigned to the handling of ACARS transmissions. Same goes for ADS-B, which is used by secondary radar and the TCAS (Traffic Collision Avoidance System).

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