Tutorial TETRAPOL

Tutorial: TETRAPOL

What it is. TETRAPOL is a digital trunked radio system used by some public-safety and other organisations (for example in France and a few other countries). It has its own air interface and is not TETRA. OnAir reads the network information that the control channel broadcasts in the clear.

Read this first. Only network information is decoded. Voice, short data, identities and encrypted traffic are not decoded, and nothing is transmitted; this is a decision of the project. In many countries it is illegal to intercept or disclose private radio communications. Check the law where you live. See Legal.

What you need

• A radio that tunes 68 to 520 MHz (the mode table range). The mode asks for 250 ksps and works from 64 ksps, so an RTL-SDR is enough. The channel width in the table is 200 kHz; the signal itself is 10 or 12.5 kHz wide.
• A VHF or UHF antenna.
• Frequency: the mode starts on 392 MHz. The database lists it as the TETRAPOL downlink app default (France and others). TETRAPOL control channels are on frequencies set by the network; use the spectrum to look for a continuous narrow carrier.

Before you start

The signal is narrow and the sample rate low, so there is no need for a fast radio. A low sample rate also keeps CPU use small.

Step by step

1. In the mode list open Two-way radio and choose Network info, UHF.
2. Pick your radio under Source and start the receiver.
3. Tune a suspected control-channel frequency.
4. Look at the cell table (check the panel for its tab). Wait for the status line: the receiver first hunts for the scrambling ("scramble hunt, GMSK 8 kbit/s").
5. The ? button explains what the mode reads.

What you should see

Once a D_SYSTEM_INFO broadcast is recognised, a row appears with the country, system, cell, location area, channel and scramble value. Until then the status shows the hunt.

Reading the screen

Columns: Country, System, Cell, LA (location area), Ch, Scramble, SNR, Last. There is a Log of recognised frames.

Options and settings

Clear forgets the cells. There are no other settings; check the panel.

Troubleshooting

• Hunt never finishes. The frequency is not a control channel, or the signal is weak. Try neighbouring channels or improve the antenna.
• The signal is TETRA, not TETRAPOL. A 25 kHz wide pi/4-DQPSK carrier is TETRA: use Tutorial-TETRA.
• Frequency drift. Retune slightly; narrow signals need a stable radio.
• Encrypted calls. They only show as encrypted traffic; nothing is decoded.

Tips

Look for the control channel by its steady, continuous GMSK carrier.

Legal and privacy notes

Only the network information broadcast to all terminals is shown. Voice and data are not decoded.

Under the hood

GMSK at 8 kbit/s on 10 or 12.5 kHz channels. The standard is the TETRAPOL Forum specification PAS 0001-2.

Related

Mode-TETRAPOL, Tutorial-TETRA, Modes, Main-window, Supported-radios, Troubleshooting, FAQ, Legal, Voice-codecs.