Balise

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This article is about an electronic railway beacon, for the programming language, see AIS Balise.

A balise is an electronic beacon or transponder placed between the rails of a railway as part of an Automatic Train Protection system. The French word "balise" is used to distinguish these beacons from other kinds of beacon.

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[edit] Overview

A balise typically needs no power source. In response to radio frequency energy broadcast by an antenna mounted on the train, the balise transmits information to the train, such as: location of the balise; the geometry of the line, such as curves, gradients and speed limits; and the position of any signals. Balises are typically deployed in pairs so that the train can determine its direction of travel A->B from direction B->A.

Balises often operate with equipment on the train to provide a system that enhances the safety of train operation.

[edit] Installation

The balise is typically mounted on or between sleepers in the centre line of the track. The pair of beacons are typically 3 metres apart.

[edit] Encoding

Each pair of balises consist of a switchable balise and a fixed balise. The switchable balise transmits to the train a telegram of about 1000 bytes containing information about gradients and speeds on the line ahead plus the aspect of the signal. The telegrams also contain various checksums and validation codes.

The fixed balise transmits merely an End-of-Message message.

A train travelling in the normal direction will meet the fixed balise and then the switchable balise.

A train travelling at maximum speed of 500km/h will transmit and receive a minimum of 3 copies of the telegram while passing over the balise.

The on-board computer uses the data from the balises to determine the safe speed profile for the line ahead. Enough information is needed to allow for the train to come to a safe standstill if required.

The data in the balise also includes the distance to the next balise. This is used to check for missing balises which would otherwise be a potential wrong-side failure.

At the start and end of ATP equipped territory, a pair of fixed balises turn on and turn off the ATP equipment.

[edit] See also

[edit] References


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