gr_ALS162_Receiver

This is a basic ALS162 time signal receiver for GNURadio, containing:

1. signal demodulation and detection of the ALS162 signal with an SDR using GNURadio (and Python modules)
2. a simple live decoder of received bits provided by the GNURadio ALS162 receiver
3. and additional tools for testing the receiver (especially, if no SDR hardware is available):

• a simulated ALS162 transmitter
• a simulated ALS162 channel (AWGN)

The French time signal ALS162 (ALS = Allouis transmitter, 162 = frequency: 162 kHz) was also formerly known as TéléDiffusion de France (TDF):

• https://en.wikipedia.org/wiki/ALS162_time_signal

Overview

The flowgraph is provided in the examples folder:

• ALS162_Transmitter.grc
  • only for simulation
• ALS162_Channel.grc
  • only for simulation
• ALS162_Receiver_ExtDetection.grc
  • for SDR reception and for simulation, incl. position symbols

Supplementary tools are provided in the python folder:

• DecodeALS162.py decodes the received bits from a specified ZMQ server upon receiving them. It shows the current time, date, weekday, etc. at each new minute.

Requirements

The ALS162 receiver was tested with:

• gnuradio & GNURadio Companion 3.10.1.1 (Linux)
• Radioconda 2023.02.24 & gnuradio & GNURadio Companion 3.10.5.1 (Windows)
• Python 3.10.6
  • PyQt5 5.15.7
  • pyzmq 23.1.0
  • gnuradio-osmosdr 0.2.0
• An SDR receiver capable of receiving in the range of at least 1 kHz - 1 MHz, e.g. an Airspy Discovery HF+ is configured and used for this project.
• An antenna that provides a sufficiently clear ALS162 signal, e.g. a simple YouLoop loop antenna was used for this project. Indoor reception should probably be possible if you are close enough (<1000 km) to the ALS162 transmitter in Allouis, France. You should mount the antenna close to a window or outside.
• The user might also need some antenna cables and adapters to connect the SDR with the antenna.
• This project has been successfully tested in:
  • Ubuntu 22.04.2 LTS (recommended)
  • Windows 11

Instructions/Setup

Simulation

• Open all 3 flowcharts (ALS162_Transmitter.grc, ALS162_Channel.grc, ALS162_Receiver.grc) in GNURadio Companion.
• Generate the python files with the "Generate the flowgraph"-button.
• Open 4 separate terminals (e.g.: T1, T2, T3, T4).
  • Change to your cloned repository in all 4 terminals.
• T1: Go to /examples/ALS162_Transmitter/
• Run transmitter with: python3 ALS162_Transmitter.py
  • It should be run in the 1st step.
  • A GUI should appear.
  • The terminal should provide additional information continuously.
• T2: Go to /examples/ALS162_Channel/
• Run Channel with: python3 ALS162_Channel.py
  • It should be run in the 2nd step.
  • A GUI should appear.
    • You can enable/disable the fading effect with the push button
      • you can adjust slower and faster fading with fading_freq
    • You can enable/disable the interference effect with the push button
      • you can adjust the gain of interference with interference_gain
      • you can adjust the number of occurrences of interference signals with impulse_thres
  • you can adjust noise with noise_gain
• T3: Go to /examples/ALS162_Receiver/
• Run Receiver with: python3 ALS162_Receiver_ExtDetection.py
  • It should be run in the 3rd step.
  • A GUI should appear.
    • for further adjustments see "Signal REception with SDR"
• T4: (compare above) go to /examples/ALS162_Receiver/
• Run the decoder with: python3 DecodeALS162.py
  • It should be run in the 4th step.
  • The terminal T4 should provide received bits continuously, and a time & date message each minute.
  • NOTE: Decoding the very first received minute might fail as some initial bits are lost. Decoding subsequent minutes should work fine.

Signal Reception with SDR

• Set up your SDR with your computer.

• Ensure that the raw ALS162 signal reception at 162.0 kHz is good enough, e.g. using gqrx or another signal analysis tool.

• To start the ALS162 receiver, open the flowchart in /examples/ALS162_Receiver/ALS162_Receiver_ExtDetection.grc with GNURadio Companion.

  • Press run button.
  • First, deactivate the following auxiliary signals in the plot for now (Derivative, +2, +1, 0, -1, -2), and keep the remaining signals (Phase, Symbols, avg. Phase).
  • Wait a few seconds until the Phase signal in the Phase Drift Compensation plot roughly aligns at 0 between the values -1 and 1. If the transient phase of the control loop is still too erratic, reduce the shift step slider slightly (and increase it again later on).
  • reactivate the auxiliary signal Derivative and set the diff_gain slider values so that the amplitudes of Derivative" are roughly at +/-2000 and +/-4000, respectively.
  • Only adjust the sliders for the thresholds, if really needed. You should rather avoid to change them.
  • After sufficient parameter adjustment, the GNURadio Companion debug console should show a debug message each second with either '0' or '1' or '2' (for a new minute). And the plot should indicate these symbols with tags, given symbols is activated.
  • Optionally,
    • you can adjust zoom the signal amplitude of the plot with zoom.
    • you can investigate the results of the threshold values on the derivative of the phase signal by activating +2,+1, 0,-1,-2, respectively.

• Next, open a terminal or Powershell.

  • Change to your cloned repository.
  • Run the DecodeALS162 script with python3 ./python/DecodeALS162.py.
  • The terminal should show the received bits together with a sequence of indices.
  • after approximately 2 minutes, DecodeALS162 should be synchronized with the transmitter. It should provide the current time, date, etc. each minute.
  • NOTE: sometimes a bit can not be decoded correctly, e.g. due to interference or noise or a weak signal. Then the current frame of the minute is corrupted and will attempt to re-synchronize.

REMARKS

• ⚠️ Please note that the ALS162 transmitter in Allouis, France, is sometimes offline due to maintenance work, e.g. every Tuesday morning from 08:00 to 12:00.
• This project has not been tested with other SDR receivers.
• This project has not been tested with a receiver setup using a sound card.
• This project has not been tested with other antennas (e.g. a ferrite antenna).
• A Low Noise Amplifier (LNA) is not needed.
• The simulation of the transmitter channel and receiver does not run at the same speed as ordinary seconds, but rather a little bit faster.
• The SDR project provides the decoded ALS162 signal more or less in real-time, but it is probably not accurate in terms of milliseconds and it is also delayed by approximately 2 seconds.