(13-05-2019, 06:08 PM)Rob Wrote: When can balises be placed by themselves (i.e. in a balise group containing only one balise)?
https://en.wikipedia.org/w/index.php?tit...=894294214 states that "Eurobalises are typically placed in pairs" and that "Singular balises exist only when linked to a previous balise group or when their function is reduced to provide only the exact position."
A single balise is still referred to as a balise group.
Reasons to have more than one balise in a balise group include:
- Directionality - when a train's onboard unit (OBU) passes over a balise group containing more than one balise it can immediately infer if it is travelling in the normal or reverse direction relative to the balise group and can therefore decide whether the information in the telegram's packets (its data) applies or not.
- Redundancy - telegrams can be duplicated in separate balises reducing the chance a failure results in the train not receiving the intended information.
- Higher data capacity - a single balise can only contain a certain number of bits of data, more balises (up to 8) may be used to transfer all the required data to the train.
However, as far as I can tell, in many situations in a level 2 system a single balise would suffice. Balise linking allows the OBU to be told of balises it can expect to be read beyond its current position. This linking information allows the train to deduce its direction in relation to a single balise based on a history of at least one other balise group (that may also only contain a single balise). So as long as the OBU has passed over two linked singular balises it knows its direction. In level 2 balises normally contain little information - mainly allowing the train to understand and report its position in relation to a fixed location on the track (the balise).
Cases where more than one balise may still be required (ignoring redundancy and data capacity):- Where trains enter the ETCS area (hitting the first balise and needing to know their direction) or wake up
Including exits from sidings, depots and maybe terminus platforms etc
- Maybe balises placed to trip trains in 'staff responsible' mode e.g. at the end of authority at a junction. (presumably this function would still work on a singular balise - but only if the train has the linking information).
So in a level 2 system the majority of balise groups could have only a single balise? What have I missed; why are balises 'typically placed in pairs'?
(Asking for general knowledge rather than directly for the exam - not sure if this is the best place to post this question)
I don't absolutely know but I guess that one significant consideration is establishing directionality at the first available opportunity once position has been lost for some reason, such as being in a degraded mode or when a train reverses and therefore a new cab is opened up. It may well not be as much of a problem now in baseline 3, but I seem to remember it was in baseline 2. Linking from a previous balise group may be sufficient whilst everything is normal, but typically ay offer nothing in a degraded mode for some reason.
As someone who only knows a little about ETCS, some of its basis has always seemed somewhat odd to me. A lot of it seems to stem form the system specifications really only being about the air-gap interface. Certainly, on at least one company's compliant implementation, when a train gets to a divergence rather than the RBC getting any information from the interlocking regarding the lie in which points are controlled and detected, it treats the train's position as being undefined as it is totally ignorant of which way it has gone until the train encounters a Balise Group beyond the facing points and finds out that way. I am rather rusty on this, but wouldn't surprise me that this would need to be a pair in order to establish direction; in a sense I think it is almost a re-birth afresh on a new segment of line.
Hence I think you are probably right that THEORETICALLY quite a few wouldn't 100% need to be more than singles, but by the time one has factored in ensuring a decent degraded mode behaviour in the presence of some faults, some locations which definitely need to be pairs due to any form of start-up consideration, then the potential saving isn't as great as it may have first appeared; in that context one then might then decide just to have pairs everywhere and save all the heart-ache trying to save a few.
The other factor is of course the contract terms; if the supplier has potential to lose a lot of money due to inadequate RAMS performance and one way of mitigating this is to provide more balise which they can argue are needed and therefore get paid for by the client.....
All this is speculation- I don't have experience of this area so just my guesses
