ATS-Sx is a collective term for various related warning and protection systems. In its simplest form, ATS-S warns the driver about an upcoming red signal, ATS-SN immediately stops the train when passing a red signal, and ATS-SF/SK/SS/ST/SW perform additional speed checks on approach to red signals or curves.

A track-side beacon called "S long" is placed about 600 meters in front of any signal. The beacon transmits a frequency of 129.3 KHz or 130 KHz when the signal is red, or at a Nop (no-operation) frequency of 103 KHz when the signal is not red. When a train passes this beacon, it picks up the beacon frequency to determine the signal state. If the signal is red, an alarm starts to sound that must be acknowledged within 5 seconds, otherwise an emergency brake application is performed. After acknowledging the alarm, a chime starts to sound to remind the driver of the upcoming red signal. The chime must only be canceled once the driver has visual confirmation that the signal is not red any longer. In this basic form, ATS-S only prevents passing a red signal if the driver is incapacitated when passing the beacon.

In addition to the S long beacon, an immediate stop beacon is placed about 20 meters in front of any signal. This beacon transmits at a frequency of 123 KHz if the signal is red, or at the Nop frequency of 103 KHz if the signal is not red. When passing this beacon, the train applies the emergency brakes immediately if the signal is red. The safety system must then be reset manually in order to continue driving.

In addition to the S long beacon and the immediate stop beacon, these systems perform speed checks on approach to signals, curves and pointwork. The concept is to place two beacons that transmit at a frequency of 108.5 KHz in close proximity to each other. When the train passes the first beacon, a timer of 0.5 seconds starts to count down in the train. If the second beacon is reached before this timer runs out, the train travels too fast and an emergency brake application is performed. If the timer runs out before the second beacon is reached, the train travels slow enough. The exact distance between the beacons determines the speed that is checked against.

The difference between ATS-SF/SK/SS/ST and SW is mainly the length of the timer. ATS-SF uses 0.55 seconds, ATS-SK uses 0.45 seconds, and the others use 0.5 seconds.

This is an overview on the frequencies used by ATS-Sx.

Beacons with particular combinations of active (red signal) and idle (non-red signal) frequency are given designations for easy identification. Beacons that are always active are not given an idle frequency in the table below.

Please note that 129.3 KHz and 130 KHz are considered equivalent.

All information I have on ATS-Sx was obtained from this very comprehensive Japanese source.