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A balise // is an electronic beacon or transponder placed between the rails of a railway as part of an Automatic Train Protection (ATP) system. The French word "balise" is used to distinguish these beacons from other kinds of beacon.
Balises constitute an integral part of the European Train Control System, where they serve as 'beacons' giving the exact location of a train. The ETCS signalling system is gradually being introduced on railways throughout the European Union.
A balise which complies with the European Train Control System specification is called a Eurobalise.
A balise typically needs no power source. In response to radio frequency energy broadcast by a Balise Transmission Module mounted under a passing train, the balise either transmits information to the train ('Uplink') or receives information from the train ('Downlink,' although this function is rarely used). The transmission rate is sufficient for a complete 'telegram' to be received by a train passing at any speed up to 500 km/h.
A balise may be either a 'Fixed Data Balise,' or 'Fixed Balise' for short, transmitting the same data to every train, or a 'Transparent Data Balise' which transmits variable data, also called a 'Switchable' or 'Controllable Balise'. (Note that the word 'fixed' refers to the information transmitted by the balise, not to its physical location. All balises are immobile).
A fixed balise is programmed to transmit the same data to every train. Information transmitted by a fixed balise typically includes: the location of the balise; the geometry of the line, such as curves and gradients; and any speed restrictions. The programming is performed using a wireless programming device. Thus a fixed balise can notify a train of its exact location, and the distance to the next signal, and can warn of any speed restrictions.
A controllable balise is connected to a Lineside Electronics Unit (LEU), which transmits dynamic data to the train, such as signal indications. Balises forming part of an ETCS Level 1  signalling system employ this capability. The LEU integrates with the conventional (national) signal system either by connecting to the lineside railway signal or to the signalling control tower.
Balises must be deployed in pairs so that the train can distinguish the direction of travel 1→2 from direction 2→1, unless they are linked to a previous balise group in which case they can contain only one (1) balise. Extra balises (up to 8 per group) can be installed if the volume of data is too great.
Balises operate with equipment on the train to provide a system that enhances the safety of train operation: at the approaches to stations with multiple platforms fixed balises may be deployed, as a more accurate supplement to GPS, to enable safe operation of automatic selective door opening.
The balise is typically mounted on or between sleepers or ties in the centre line of the track. Balises within a group are typically spaced 3 metres apart.
A train travelling in the normal direction will meet the controlled balise and then the fixed balise.
A train travelling at maximum speed of 500 km/h will transmit and receive a minimum of three copies of the telegram while passing over each balise.
The train's 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 the train to come to a safe standstill if required.
The data in the balise can include the distance to the next balise. This is used to check for missing balises which could otherwise lead to a potential wrong-side failure.
Balises have been used in Germany to transmit tilting instructions for curves to tilting trains.
At the start and end of ATP equipped territory, a pair of fixed balises are often used to inform the onboard ATP equipment to start or stop supervision of the train movements.
Each pair of balises usually consists of a switchable balise and a fixed balise. A balise transmits a 'telegram' of either 1023 bits (93*11) or 341 bits (31*11) in the channel encoding with 11 bit per symbol. The user data block is cut into 10-bit user symbols before the scrambling and shaping operation - the effective payload of signalling information is 830 bit (83*10) for the long telegram and 210 bit (21*10) for the short telegram. The final telegram consists of
- shaped data (913 bit or 231 bit) containing the payload (830 or 210 bit)
- control bits (Cb, 3 bit)
- scrambling bits (Sb, 12 bit)
- extra shaping bits (Esb, 10 bit)
- checksum (CheckBits, 85 bit)
The telegram is broadcast in a cyclic manner as the train passes over the balise. To avoid transmission errors the payload is scrambled (avoiding burst errors), substituted with a symbol code of different Hamming distance, and a checksum is added for validity checks. Since the checksum is computed after the symbol substitution the telegram contains extra shaping bits to allow the resulting checksum bits to be filled up in a way that only valid symbols of the chosen channel code are in the telegram where each symbol has 11 bits.
The payload data consists of a header followed by multiple packets defined in the ERTMS protocols. Typical packets are Packet 5 - Linking, Packet 12 - Movement Authority, Packet 21 - Gradient Profile, Packet 27 - International Static Speed Profile, Packet 255 - End of information. Many applications include optional packets like Packet 3 - National Values, Packet 41 - Level Transition Order, and Packet 136 - Infill Location Reference. If the telegram maximum of 830 bits is reached then more packets can be sent in the following balises of the same balise group - with up to 8 balises in a balise group the maximum ERTMS message per balise group can encompass 8 * 830 = 6640 bits (note that every telegram must contain a header and the trailer packet 255). A fixed balise transmits a stable message which typically can include the linking information, gradient profile, and speed profile. It may also contain track information such as route suitability data for different train types and axle load restrictions.
Almost all packet types contain a parameter flagging whether its information is relevant for the "nominal" or "reverse" direction (or both). If a train sees balise 1 before balise 2 then it passes over the group in the nominal direction. Consequently some packets may be dropped by the application software of the receiver if they are not designated for the relevant direction. The ERTMS header block of 50 bits contains the ETCS version, the current number and total count of balises within a balise group (up to 8 balises), a flag whether it is a copy (up to 4 copies) that increases chances for the receiver to see the telegram of the balise in a group, a serial number flagging whether the message has changed lately, a 10-bit country identifier along with the 14-bit balise group identifier allowing for a unique ID of every balise group. The linking information informs about the distance to the next balise group (one linking packet per direction) and the required train reaction if the next balise group is missed (e.g. train stop). The movement authority packet defines a maximum speed that may be used for a given maximum distance and maximum time - setting the maximum speed to zero on a variable balise will force the train to stop. The gradient profile may have a variable length based on the contained pairs of section length (scalar and number in the metric system) and section gradient (uphill/downhill flag and a number in %). Similarly the international static speed profile is given in a variable count of section parts with each part denoting the section length (number in meters - the scale is only given once at the start of the packet for all sections), the maximum speed (number * 5 km/h - allowed numbers are 0-120 i.e. some spare values are left over) and a flag if the speed restriction applies to the front or rear end of the train (possibly allowing for a delay). The trailer packet only contains its packet id with no parameters where 255 equals the state of all bits set in the 8-bit packet id field (11111111).
Balises are made by several different companies; while they may vary in the details, they are manufactured to meet the same standards. The principal manufacturers of Eurobalises belong to a group of seven firms (Alstom, Ansaldo STS, Bombardier, Invensys, Siemens, Sigma-Digitek, Thales) within the UNIFE federation of railway suppliers. This group cooperated in developing the specifications for Eurobalises. Specifications for Eurobalises are governed by the European Railway Agency.
Eurobalises are used in
- ETCS – the European-wide train protection system
- Chinese Train Control System versions CTCS-2 and CTCS-3, used on high speed rail lines in China
- EuroSignum – a variant of the earlier Swiss Integra-Signum train protection system
- EuroZub – a variant of the earlier Swiss ZUB 121 train protection system
- SCMT – an Italian train protection system
- TBL1+ – a train protection system used in Belgium
- GNT – the system to control tilting trains in Germany
- ZBS – a new rapid transit control system for the S-Bahn Berlin
Balises other than Eurobalises are used in
- KVB - a train protection system used in France
- ASFA - a train protection system used in Spain
- ACSES - a train protection system used by Amtrak on the Northeast Corridor in the USA
- EBICAB - a train protection system used in Norway, Sweden and other countries, including parts of Spain
- Automatic Warning System
- Cab signalling
- Positive Train Control
- Train Protection & Warning System
- 'EC sets out ERTMS deployment deadlines,' http://www.railwaygazette.com/news/single-view/view//ec-sets-out-ertms-deployment-deadlines.html
- ERTMS Levels http://www.ertms.com/media/2428/fact-3.pdf
- Connor, Piers (March 2013). "Southern's Class 377/6". Modern Railways: 40–44.
- Warren Kaiser, Stein Nielson (2008-03-14). "The Core of ATP - Data Engineering". IRSE Technical Meeting "All About ATP" Sydney.
- Specification for Eurobalises http://www.era.europa.eu/Document-Register/Documents/SUBSET-036%20v241.pdf
- Test Specification for Eurobalises http://www.era.europa.eu/Document-Register/Documents/SUBSET-085%20v222.pdf
- SUBSET-036 Specification for Eurobalises (PDF; 1,12 MB)
- (German language) Eurobalise S21 by Siemens Transportation Systems (PDF; 1,33 MB)
- (English language) Eurobalise S21 by Siemens Transportation Systems (PDF; 1,33 MB)
- Eurobalise by Alstom
- Beijing Railway Signal Factory