Tutorial TETRA

Tutorial: TETRA

What it is. TETRA is the digital trunked radio standard used by emergency services, transport and other organisations in many countries. OnAir shows network information only: what a base station broadcasts in the clear to every terminal. You see which networks and cells are around you, not what they say.

Read this first. Voice and messages are not decoded. This is a decision of the project, not a missing feature: OnAir does not decode voice, short data messages or subscriber identities, it does not decrypt anything, and it does not transmit. Encrypted traffic is out of scope; the mode only reports whether the network uses air-interface encryption. Receiving the public broadcast information is legal in most countries, but check the rules where you live. See Legal.

What you need

• A radio that tunes 380 to 470 MHz. The mode asks for 2 Msps and works from 200 ksps, so an RTL-SDR is enough. The mode table covers 136 to 960 MHz; the channel width is 1.8 MHz.
• A UHF antenna.
• Frequencies: the mode starts on 392.5 MHz (emergency services downlink; in Europe the bands are 380 to 385 and 390 to 395 MHz). The frequency database also lists 420.0 MHz (TETRA band 420 to 430 MHz, civil trunking in some countries).

Before you start

TETRA is a continuous signal on the control carrier, so the level is steady. Check the input level in the panel and avoid overload from nearby cellular transmitters; a notch or filter may help.

Step by step

1. In the mode list open the group Two-way radio and choose Network info, 380 - 470 MHz.
2. Pick the radio under Source, start the receiver.
3. Either tune a frequency, or use the span box next to Scan. Its choices are 390 - 395 MHz (emergency services), 410 - 430 MHz, 450 - 470 MHz and Custom (start and end in MHz).
4. Press Scan to step over the span, Stop scan to end. The mode finds carriers by signal over the noise floor.
5. Open Cells and select a carrier.

What you should see

A table of cells and carriers. For a cell you see the network (MCC and MNC), the colour code, the location area, the main carrier with duplex offset, the services offered, the time network and the encryption status. A status line counts blocks searched and signalling blocks decoded.

Reading the screen

• Cells: network code, colour code, location area, offset, last heard.
• Neighbours: neighbour cells from the network broadcast (D-NWRK-BROADCAST), when read.
• Carriers: carriers found in the last block, with level over the floor.
• Log: messages read, such as hyperframe, security class and services.

The ? button explains what the mode reads and what it never does. The panel shows whether the network uses encryption and the security classes; encrypted signalling is only reported as encrypted.

Options and settings

Clear empties the cell list. A saved list can be exported with the save button.

Troubleshooting

• No carriers. Check the span matches your country and use the spectrum to look for 25 kHz wide continuous carriers.
• Carrier found but no network data. Frequency error or weak signal; recentre on the carrier.
• No neighbour list. It arrives only occasionally; wait.
• Encrypted network. You still see the network information; there is nothing more to read.

Tips

Use a span scan first, then stay on the strongest carrier for a minute to collect neighbours.

Legal and privacy notes

Do not try to listen to calls. The project does not decode them, and in many countries it is illegal to intercept private radio communications.

Under the hood

pi/4-DQPSK, 25 kHz channels, four time slots per frame (ETSI EN 300 392-2). OnAir uses the base station broadcast channels (BSCH, BNCH, AACH and MCCH signalling blocks).

Related

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