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An Aircraft Communication Receiver

Gary Schafer, April 2024

Commercial aircraft have multiple forms of communication. Commercial pilots need to talk to multiple groups (ground control, the airline, the airport), and passengers need to communicate with people on the ground. Each has a specific function. The earliest form of communication was analog audio allowing pilots to talk to ground control and each other. This form of communication is based on amplitude modulation (AM). AM, analogous to ice cream, comes in different "flavors". These flavors are based on two conditions of the AM signal. These are the carrier condition and the sidebands. Aircraft communication is based on full carrier and double sideband, giving it the general description of AM-DSB-FC.

Aircraft audio communications as seen on GQRX spectral trace (top) and spectrogram (bottom). This shows the full carrier (the thin spike at the center) and the double sidebands (the mirror-imaged spectra on either side of the carrier).

An aircraft audio communications receiver has three requirements. These are:

  1. demodulator: I looked at differentamplitude demodulators a few years ago. Aircraft audio communications are AM-DSB-FC. This requires a noncoherent demodulator. For this circuit, we'll use the complex magnitude detector. That is a noncoherent demodulator that works well on full carrier AM signals.
  2. automatic gain control (AGC): a big problem with AM is that the power of the received signal directly affects the amplitude of the demodulated signal. An automatic gain control (AGC) circuit helps with this by increasing the signal level such that it has a certain, defined amplitude. This only works within limits as it is increasing both the signal and noise amplitude (unless it directly affects the first amplifier at the output of the antenna... may have to look at that in a future post). Regardless, this is still a requirement.
  3. squelch: Unless you're someone who enjoys the soft, soothing sound of static, a squelch circuit is also a requirement. The squelch looks at the absolute amplitude of the signal within a defined bandwidth. So long as the signal level is below the threshold, it will either not pass any samples, or the samples will be set to 0.

Here's what I put together after a bit of trying out different blocks.

This is the Gnu Radio flowgraph for receiving aircraft audio communications. This flowgraph uses a "Range" block to adjust the center frequency over the range of 118 - 137 MHz, which is the range for such communications. The "Power Squelch" block and another "Range" block adjust the power level that activates the rest of the circuit. The settings for the I've found that the "Feed Forward AGC" works best in this circuit. The last part is the "Complex to Mag" block performs the AM demodulation, and the "DC Blocker" block removes the leftover DC offset from the "Complex to Mag" block.
Display of the flowgraph when running. The "Wide Spectrum" shows all of the signals within the "stare" bandwidth of the SDR (roughly 2.4 MHz), while the "Narrow Spectrum" shows only the signal being demodulated.

The result was pretty good. Using my antenna in the attic, I'm able to recover the audio from ground control from Baltimore-Washington International (BWI), which is several miles away.

This is the demodulated audio from the nearby airport ground control.

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