ATS-Ps adds a speed pattern on top of ATS-Sx in order to achieve functionality that resembles ATS-P. The difference between ATS-Ps and ATS-P is that ATS-Ps is an analog system that only works by detecting beacon frequencies, while ATS-P uses digital transmission. ATS-Ps is always installed on top of the existing ATS-Sx infrastructure, while ATS-P is usually the only safety system on the route.

ATS-Ps is capable of employing two independent signal patterns with downslope compensation, as well as four independent speed patterns to upcoming route speed limits and two independent immediate speed restrictions.

The existing infrastructure for ATS-Sx is used, although adjustments may be necessary. The S long beacon is placed around 600 meters in front of the signal, and the immediate stop beacon around 20 meters in front of the signal. In addition, the Ps1 beacon is installed exactly 655 meters in front of the signal, and the Ps2 beacons exactly 390 meters in front of the signal. The Ps3 beacons can be optionally installed at exactly 100 meters in front of the signal.

The Ps1 beacon transmits at 80 KHz if the signal is red, and at the Ps Nop (no-operation) frequency of 73 KHz otherwise. When the train picks up the 80 KHz frequency, it generates a brake pattern for the signal under the assumption that the signal is exactly 655 meters way. The brake pattern comes with a release speed of 65 km/h, meaning the allowed speed will not drop below this value. If the 73 KHz frequency is picked up, this only activates ATS-Ps, but does not generate a brake pattern.

The S long beacon is placed around 600 meters in front of the signal and transmits at 129.3 KHz (or 130 KHz) if the signal is red, and at the idle frequency of 103 KHz otherwise. If the train detects the 103 KHz frequency, the pattern to the signal is cleared. The same idle frequency is also used by the Ps2 beacons, the Ps3 beacons and the SN immediate stop beacon, meaning any of those beacons can clear the signal pattern.

The Ps2 beacons are actually Sx speed check beacons, usually checking for a speed 5 km/h greater than the speed limit of the yellow aspect. The speed check beacons are engaged to the signal, meaning they only transmit at their active frequency of 108.5 KHz if the signal is red, and at 103 KHz otherwise. If the train picks up the Ps2 beacons and the signal is red, the signal pattern is refreshed under the assumption that the signal is exactly 390 meters away. The release speed is now 10 km/h.

In case of a station that has multiple platforms, the signals at each platform can be at different locations. In this case, the Ps1 and Ps2 beacons are often placed to point to the closest signal. If the train passes a branch that leads to a signal farther away than the closest signal, the Ps3 beacons can be placed to refresh the signal pattern under the assumption that the signal is exactly 100 meters away. Depending on the circumstances, the Ps2 beacons could also be placed again which refresh the signal pattern under the assumption that the signal is exactly 390 meters away.

The Ps3 beacons consist of a 108.5 KHz beacon followed by an 80 KHz beacon, spaced roughly 2 meters apart. If this sequence is picked up by the train, the signal pattern is refreshed under the assumption that the signal is exactly 100 meters away. The release speed is 10 km/h.

The SN immediate stop beacon is placed around 20 meters in front of the signal and transmits at 123 KHz if the signal is red, and at the idle frequency of 103 KHz otherwise. If the signal is red, ATS-Sx engages the emergency brakes, while ATS-Ps creates a 15 km/h brake pattern for the next 80 meters.

Given that ATS-Ps requires the Ps1 beacon to be 655 meters in front of the signal, the situation can arise that two signals are spaced closer than that distance, for example a home signal followed by a departure signal. For this case, ATS-Ps supports two independent signal patterns, called pattern A and pattern B. Pattern A supports the Ps1, Ps2 and Ps3 beacons, while pattern B only supports the Ps1 and Ps2 beacons. For this reason, pattern B is usually used for the home signal, and pattern A for the departure signal.

For pattern B, all Ps1, Ps2 and SN beacons are prefixed by a 95 KHz beacon that is always active regardless of the signal state. This way, ATS-Ps can distinguish between pattern A and pattern B. The 95 KHz beacon is usually placed about 2 meters before the beacon it prefixes.

Care must be taken with the S long beacon. When the signal is not red, the S long beacon usually transmits at the idle frequency of 103 KHz. Without prefixing with a 95 KHz beacon, this would clear pattern A instead of pattern B. If there is no pattern A at this point, there is no problem except that the beacon does not clear pattern B. However, if pattern A is already set up, or if clearing pattern B is desired, the S long beacon must be prefixed with a 95 KHz beacon. This beacon should be placed about 3 meters before the S long beacon, or at any other distance that does not interfere with the Ps downslope compensation (see below).

Given that the first Ps1 beacon must be exactly 655 meters in front of the signal, the speed a train can travel at is limited to under 140 km/h. This assumes a deceleration of around 4 km/h/s. On downslopes, this deceleration cannot be safety guaranteed. Therefore, ATS-Ps supports downslope compensation.

Downslope compensation works by telling ATS-Ps about the downslope and then using different distances for the Ps1 and Ps2 beacons than usual. In order to set up the downslope, the S long beacon is prefixed by a 108.5 KHz beacon (pattern A) or 95 KHz beacon (pattern B).

The distance between these beacons determines the downslope. After the downslope is set up, any Ps1 or Ps2 beacon that refreshes the pattern assumes a different distance to the signal depending on the downslope.

Usually, the Ps1 beacon is placed first, then the prefixed S long beacon is placed between the Ps1 and Ps2 beacons to set up the downslope, then the Ps2 beacon follows. It is also possible to first set up the downslope and then install the Ps1 beacon.

An an example, for a downslope of -20/1000, the Ps1 beacon is placed 970 m in front of the signal, the prefixed S long beacon 600~700 meters in front of the signal, the Ps2 beacons 560 meters in front of the signal, and the SN beacon 20 meters in front of the signal. The distance between the 108.5/95 KHz downslope prefix beacon and the 129.3 KHz S long beacon must be 3.3 - 4.8 meters, ideally 4.0 meters.

ATS-Ps supports four independent patterns to upcoming speed limits. In order to install speed limits, a 90 KHz beacon is followed by a 95 KHz beacon (divergence), a 90 KHz beacon (temporary), a 85 KHz beacon (curve), or an 80 KHz beacon (slope). The beacons must be placed 555 meters before the point of danger.

The distance between the beacons determines the speed limit.

All speed limits are added 10 km/h as tolerance. The divergence speed limit (95 KHz) is released automatically 50 meters after the point of danger. The other speed limits must be released using beacons by spacing them 2 meters apart.

ATS-Ps supports two independent patterns for immediate speed restrictions. One is the irekae speed restriction, used for the entrance or exit of a siding. The other is the yuudou speed restriction, used for call-on operations (when a red home signal may be passed in order to merge with the preceding train).

In order to set up an irekae speed restriction, a 108.5 KHz beacon is followed by a 90 KHz (or 95 KHz) beacon. The spacing of the beacons determines the speed limit.

The speed limit is added 5 km/h as tolerance. The irekae speed limit is released with a 103 KHz or a 123 KHz beacon.

In order to set up or to clear a yuudou speed restriction, a 108.5 KHz beacon is followed by an 85 KHz beacon. The spacing of the beacons determines the behavior.

The speed limit is added 10 km/h as tolerance. The beacons are usually installed just before the SN immediate stop beacon and are only engaged to the 108.5 KHz + 85 KHz frequencies if the home signal is red and the yuudou signal is clear. The yuudou speed limit can be released by placing the beacons roughly 4 meters apart.

Here are all other functions not listed above.

There is a special Nop (no-operation) frequency for ATS-Ps of 73 KHz. Contrary to the 103 KHz frequency, this one does not clear the signal pattern, but does indicate that ATS-Ps is available.

ATS-Ps assigns a special meaning to Sx speed check beacons, namely to refresh or clear the signal in form of the Ps2 beacons. Therefore, routes using existing Sx speed limits might interfere with the Ps2 function. For this and other reasons, it is possible to ignore the Ps functionality of beacons by prefixing them with a 85 KHz beacon. This also works with S long or SN beacons, or any other that transmit frequencies that have a meaning in ATS-Ps.

In order to temporarily disable all ATS-Ps functionality, for example during construction work, the combination 90 KHz + 108.5 KHz is used. The spacing of the beacons determines whether to activate or deactivate ATS-Ps.

The following is an overview of all frequencies that have a meaning in ATS-Ps.

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-Ps was obtained from this and this very comprehensive Japanese source.