mode-s-demodulator

A JavaScript module for demodulating and decoding Mode S / ADS-B messages from aviation aircrafts.

Mode S is an aviation transponder interrogation mode used by Secondary Surveillance Radar (SSR) and Automatic Dependent Surveillance-Broadcast (ADS-B) systems.

For an example of this module in use, see AirplaneJS.

Installation

npm install mode-s-demodulator --save

Usage

const Demodulator = require('mode-s-demodulator')

const demodulator = new Demodulator()

// data contains raw IQ samples from an RTL-SDR device
demodulator.process(data, data.length, function (message) {
  // got new Mode S message from an airplane
  console.log(message)
})

Tip: Use together with the rtl-sdr module to get raw IQ samples that can be processed by this module.

API

Demodulator.UNIT_FEET

A constant indicating that the unit used to encode message.altitude is feet. Check against message.unit.

message is an object given as the first argument to the demodulator.process or demodulator.detectMessage callback.

Demodulator.UNIT_METERS

A constant indicating that the unit used to encode message.altitude is meters. Check against message.unit.

message is an object given as the first argument to the demodulator.process or demodulator.detectMessage callback.

demodulator = new Demodulator([options])

Initialize a new demodulator object.

Arguments:

• options - An optional options object

The available options are:

• fixErrors - Set to false to disable automatic error correction (default: true)
• aggressive - Set to true to use an aggressive error correction algorithm (default: false)
• checkCrc - Set to false to disable checksum validation (default: true)
• crcOnly - Set to true to only validate the checksum of the Mode S messages without trying to decode them any further than necessary in order to validate the checksum (default: false)
• mag - An optional pre-initialized magnitude Uint16Array used by the process function. If not provided it's expected that process will always be called with the same amount of data

demodulator.process(data, size, onMsg)

A convenience function that takes care of everything related to processing the provided data.

It handles initializing a magnitute array (if not provided to the Demodulator constructor) and calling computeMagnitudeVector and detectMessage respectively.

Arguments:

• data - A buffer object containing raw IQ samples
• size - The size of the data buffer
• onMsg - Called for each new message detected in data. Will be called with the message as the only argument.

demodulator.computeMagnitudeVector(data, mag, size[, signedInt])

Calculate the magnitude of each sample in the signal.

Arguments:

• data - A buffer object containing raw IQ samples
• mag - An Uint16Array to which the magnitude of each sample will be written. Since each IQ sample in data consists of two bytes, the array should be at least half the size of the data buffer
• size - The size of the data buffer
• signedInt - Optional boolean indicating if the IQ samples are encoded as signed integers (default: false)

demodulator.detectMessage(mag, size, onMsg)

Detect Mode S messages in the magnitute array.

Arguments:

• mag - The mag array computed using computeMagnitudeVector()
• size - The size of the mag array
• onMsg - Called for each new message detected in mag. Will be called with the message as the only argument.

Acknowledgement

This project is a JavaScript port of the popular dump1090 project by Salvatore Sanfilippo. It modularizes the code into separate functions and removes all non-essentials, so that only the demodulation and decoding logic is left.

License

MIT