Eurobalise
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. 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 wayside power source. Coupling between the balise and the vehicle-mounted antenna is by magnetic induction, similar to a transformer - although operating at frequencies typical of radio, it is not a radio system. In response to "telepowering" by means of a 27 MHz, CW-modulated magnetic field transmitted by a mounted under a passing train, the balise transmits information to the train by means of a 4.234 MHz FSK-modulated magnetic field. The original provisions for Eurobalises to receive information from the train have 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. The BTM may be integrated into the on-board antenna or a separate electronic module, according to the preference of the system vendor.Image:Balises in Finland.jpg|thumb|Balises on the Orivesi–Jyväskylä railway in Muurame, Finland
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 or reverse direction. 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 fixed-data, transmitting the same data to every train, or transparent data, switchable, or controllable, transmitting variable data.
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, 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 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. 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.Safety Considerations
ETCS and other applications using Eurobalise technology typically include protection against failure and theft of balises. Balises identify themselves to the trainborne equipment as being "linked" or "unlinked". Linked balises may send data to the trainborne equipment containing the identity of and distance to the following balise group or groups. By this means, the trainborne equipment is able to determine where to expect a balise and thus to identify if a balise has failed. Alternatively, in ETCS application levels 2 and 3, the "link list" may be transmitted to the trainborne equipment from the Radio Block Centre. The reaction that the trainborne equipment shall apply in case of a missing or failed balise is itself defined by data transmitted with the link list and could typically be to make an enforced brake application or to display a message to the train driver.Balises also provide a set of fixed position references to the ETCS onboard equipment, allowing the latter to correct its odometry system for cumulative errors accrued during travel from one balise group to the next and maintain accurate tracking of its position relative to the end point of the current movement authority. These errors typically result from slippage and sliding of the train wheels on damp rails, or from controlled-creep traction.
Balise designs usually include multiple redundant electronic circuitry, allowing them to continue providing normal service for many years despite partial failure.
Although not a radio system, the Eurobalise transmission can under very specific and unusual circumstances be affected by strong radio sources operating at the same frequency, causing the ETCS onboard equipment to apply a safety reaction.
Modulation
The downlink uses an amplitude modulation on the 27.095 MHz frequency. This frequency is used to power the passive balises.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 three copies of a telegram to a train passing at 500 km/h.
The Euroloop frequency was moved to a centre of 13.54750 MHz.
In a practical setup the BTM requires 65 watts 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 or 341 bits 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 for the long telegram and 210 bit for the short telegram. The final telegram consists of- shaped data containing the payload
- control bits
- scrambling bits
- extra shaping bits
- checksum
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
Almost all packet types contain a parameter flagging whether its information is relevant for the "nominal" or "reverse" direction. 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, a flag whether it is a copy 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 and the required train reaction if the next balise group is missed. 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 will force the train to stop. The gradient profile may have a variable length based on the contained pairs of section length and section gradient. Similarly the international static speed profile is given in a variable count of section parts with each part denoting the section length, the maximum speed and a flag if the speed restriction applies to the front or rear end of the train. 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.