Tutorial LBJ

Tutorial: LBJ (Chinese railway train approach warning)

What it is. LBJ is the train approach warning of the Chinese railways. Every locomotive sends short messages on a UHF channel at about 821.2375 MHz: an FM carrier with 1200 bit/s FSK data. The messages are POCSAG numeric pages. They carry the train number, the speed in km/h and the kilometre post, and sometimes more. OnAir decodes them itself and lists the trains it hears.

Read this first. Receive only. LBJ is a safety broadcast meant to warn crews and track workers. Check the rules where you live about receiving it. See Legal. If you receive train data, do not use it in any way that could disturb railway operation.

What you need

• A radio that tunes 821.2375 MHz. The mode asks for 960 ksps and works from 256 ksps, so an RTL-SDR is enough. The tuning range of the mode is 821 to 821.5 MHz and the channel width is 35 kHz.
• A UHF antenna. You need to be near the line, since the signal is weak. The frequency database lists 821.2375 MHz as the LBJ train approach warning (China).

Before you start

Locomotives are off frequency by different amounts (up to about 1.5 kHz was seen), so the decoder follows the carrier. Bursts are a few seconds apart for each train and weak; keep gain moderate and do not overload.

Step by step

1. In the mode list open Cellular & rail and choose Train warning, 821 MHz.
2. Pick the radio under Source and start the receiver. The panel says "tune to about 821.2375 MHz and start the receiver" until you do.
3. Tune 821.2375 MHz.
4. Wait for a train. The status line reads "listening" with the level and offset, and counts messages, sync words, blank messages and failed checks.

What you should see

One row per train in the table, updated with each message. A basic message has three numeric code words, 15 characters: five digits of train number, three of speed, five of kilometre post in tenths of a km. For example a message with train 19533, 67 km/h and post 382 means km 38.2.

Reading the screen

Columns: Train, Speed km/h, Km post, Direction (Down or Up), Locomotive, Route, Level, Seen. A blank value had damaged digits. A row in grey had errors that were corrected: check it against the next message of the same train. The ? button explains what LBJ carries.

Options and settings

Clear empties the table. The mode has no other settings; check the panel.

Troubleshooting

• Nothing heard. You must be near a railway that uses LBJ. Check the frequency and antenna.
• Rows with blanks. Weak signal; improve the antenna.
• Direction looks wrong. The mapping of Down and Up is assumed from public descriptions, not proven.
• Locomotive or route empty. Only the long messages carry them. They were not present in the test recordings, so the layout follows public descriptions.
• No GBK route text on Windows. The route is shown as ASCII only there.

Tips

A speed and post that agree between two messages (for example 0.2 km in 11 s at 67 km/h) show the decode is right.

Legal and privacy notes

Receive only. Locomotive numbers identify real trains; do not publish train movements in a way that could be misused.

Under the hood

2-FSK on FM, 1200 bit/s, POCSAG sync word 0x7CD215D8 and BCH(31,21) code words with up to two bit corrections. Address (RIC) 1234000 with function 1 or 3 carries the basic message. Format facts come from public descriptions of the system (see the project's credits); the decoder was written from the POCSAG standard and recordings.

Related

Mode-LBJ, Tutorial-POCSAG, Tutorial-GSM-R, Modes, Main-window, Supported-radios, Troubleshooting, FAQ, Legal.