 Car Stops & doors
On Thu, 17 Jul 2008 20:33:56 +0100, Chris Read wrote:
> With SDO and related GPS gizmos on the 377s, I was wondering, would the
> doors have actually released on the carriages not in the platform? Or
> are the systems not quite that clever?
GPS isn't accurate enough to rely on to tell if a train has stopped 4
cars short - that distance is still with in your typical error circle.
With differential correction it might be accurate enough much of the
time, but not ALL the time.
GPS will be used to look up the station in a database and tell the train
how long the platform is - as the typical error will still place the
train in general vicinity of the station, so it's a pretty reliable bet
that you ARE at that particular station. It's not going to be able to
tell if the driver stopped short - not with out beacons on the track the
train reads. (oi, I'm stopped on the 8 car beacon but I'm 12 cars long)
I gather people think GPS is more accurate than it really is as all
those domestic sat-navs 'cheat' by following roads, thus hiding the
errors. (A feature of the Nokia Maps application in my phone that I find
really really annoying :-)
Google maps for mobile shows the uncertainty circle on top of the map,
it's quite instructive to see the error circle change as you move and
satellites come in and out of view (usually shaded by nearby buildings). |