A Eurobalise is a specific type of a balise installed between the rails of a railway. Eurobalises are part of the European train control system (ETCS). The balises are pre-programmed, and contain information that is read by train antennas. One of their many functions is to allow a train to determine its location.
Overview
A balise typically needs no power source. In response to radio frequency energy broadcast by a (BTM) mounted under a passing train, the balise transmits information to the train ('Uplink'). The provisions for Eurobalises to receive information from the train ('Downlink') has been removed from the specification. The transmission rate is sufficient to transmit at least three copies of a 'telegram' to be received by a train passing at any speed up to 500 km/h.
Eurobalises are typically placed in pairs on two sleepers in the center of the track. For ETCS they are typically spaced three metres apart. With the balises being numbered the train will know whether it travels in nominal (1→2) or reverse direction (2→1). Singular balises exist only when linked to a previous balise group or when their function is reduced to provide only the exact position. There may be up to 8 balises in a balise group.
Balises are differentiated as being either a Fixed Data Balise 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).
Fixed Data Balise
A Fixed Data 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 permanent speed restrictions.
Transparent Data Balise
A Transparent Data Balise is connected to a Lineside Electronics Unit (LEU), which transmits dynamic data to the train, such as signal indications and temporary speed restrictions. 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.
Euroloop
A balise transmits telegrams at a specific site. To allow a continuous transmission the telegrams may be sent along leaky feeder cable being up to 1000 metres long. The Euroloop cable is always connected with a balise at its end which serves as the End of Loop Marker (EOLM). The telegram structure is the same as for the balise it is connected to. Originally the Euroloop used the same frequency as the Eurobalises but that was changed for specification 2.0.1 in September 2004. Euroloops had been used in Switzerland which completed the change in July 2010.
Modulation
The downlink uses an amplitude modulation on the 27.095 MHz frequency. This frequency is used to power the passive balises (it is the intermediate channel 11A in CB radio).
The uplink uses frequency-shift keying with 3.951 MHz for a logical '0' and 4.516 MHz for a logical '1'. The data rate of 564.48 kBit/s is enough to transmit 3 copies of a telegram to a train passing at 500 km/h.
The Euroloop frequency was moved to a centre of 13.54750 MHz (exactly half of the Eurobalise power frequency).
In a practical setup the BTM requires 65 Watt to power the Eurobalises and to receive the telegrams with the BTM mounted above top of rail on a bogie.
Encoding
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)