edwin_m
Veteran Member
Certain GPS errors can result in a totally fictitious postion being reported, particularly when line of sight is blocked. Mentioned above that there are tunnels near to this crossing.With regard to GPS signals, surely the answer is that it shows when the position signal last refreshed - either as a timer or, say, a red outline if it's been more than a period of time.
Thus in this scenario, the signaller would know almost exactly where it was or say that it's not safe to cross presently until the train is located exactly.
From a layman's perspective, similar questions are asked about aviation; why either some fairly mainstream piece of helpful technology isn't commonly employed, or why some antiquated technology is still heavily relied on.
I think the challenge would be phasing GPS into other working practices and standards and training up vast numbers of staff so that it doesn't all go wrong when somebody gets too reliant on what a dot on the map is telling them.
In this era of 'The Internet of Things' GPS in rail vehicles feels entirely possible, so there must be a compelling reason for why it isn't done.
But if there is no alternative system or process, they have nothing to rely on except the dot on the map!
Traditionally GPS wasn't accurate enough on a moving vehicle to tell which of several adjacent tracks it was on, which is obviously a major restriction for use in signalling. Modern systems should be more accurate, particularly if using differential techniques as mentioned above, but as far as I'm aware they aren't used for vital location purposes on railways anywhere in the world. This is somewhat surprising as it's used to navigate various missiles, but the military version has various enhancements that aren't available to other users, and missiles generally operate at an altitude where line of sight to satellites is always available.