In message , Peter Masson
writes

"John Rowland" wrote in message
...
Snip
Something along the lines of a transponder at the start of each platform
which is detected by each door and enables the doors on the appropriate
side, and another one at the end of each platform which disables each door.
Other interlocking means that doors cannot actually open until the train has
stopped, and failsafe precautions could cancel the enabling if the train
hasn't stopped within, say, one minute of passing the transponder. Some
complications for platforms on reversible lines, but I can't help thinking
that something like this has the potential for being more reliable than the
GPS technology (Can GPS identify which line the train is on if adjoining
platforms are different lengths?)
Peter


With difficulty, because GPS has an accuracy of +/- 100m, unless of
course you are using Differential GPS, but that is mainly a maritime
system.
--
Regards,

James Christie

"In the beginning the Universe was created. This has made a lot of people very
angry and been widely regarded as a bad move."
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"James Christie" wrote in message
...



With difficulty, because GPS has an accuracy of +/- 100m, unless of course
you are using Differential GPS, but that is mainly a maritime system.
--

It's been somewhat better than that for a while, since the "random error"
was removed. IME the error is probably nearer 2 metres. Though the powers
that be can reintroduce the random factor, or turn it off entirely, should
they feel the need.

G

On Thu, 23 Sep 2004 18:25:33 UTC, "Gavin Hamilton"
wrote:

: It's been somewhat better than that for a while, since the "random error"
: was removed. IME the error is probably nearer 2 metres. Though the powers
: that be can reintroduce the random factor, or turn it off entirely, should
: they feel the need.

You have to be careful not to confuse the random precison errors with
the unrandom accuray ones. Civilian GPS is designed to be precise to
about +/- 10m, whereas military GPS, which uses different signals, is
precise to +/- 1m. Those errors are random - there is nothing you can
do about them. Selective availability was a deliberate degradation of
accuracy, done by effectively instructing satellites to tell porkies
in their signals, and thereby displace all GPS positions in a
particular area by an ordained amount. That's what doesn't happen
(much) any more, but the precision errors remain.

Ian
--

"Ian Johnston" wrote in message
news:cCUlhtvFIYkV-pn2-KSqR179sj7HU@localhost...

You have to be careful not to confuse the random precison errors with
the unrandom accuray ones. Civilian GPS is designed to be precise to
about +/- 10m, whereas military GPS, which uses different signals, is
precise to +/- 1m. Those errors are random - there is nothing you can
do about them. Selective availability was a deliberate degradation of
accuracy, done by effectively instructing satellites to tell porkies
in their signals, and thereby displace all GPS positions in a
particular area by an ordained amount. That's what doesn't happen
(much) any more, but the precision errors remain.


I have experienced an distinct improvement in accuracy over the last few
years but I am also aware that in certain circumstances GPS is not to be
relied upon. Such events occur, for example, in narrow valleys where the
signals can be "deflected" for want of a better description so GPS wouldn't
work very well in cuttings - or tunnels for that matter.

SA would not be very effiective if all satellites were to displace their
positions by the same amount in the same direction - AFAIK each satellite
had its own displacement which was random - watching a GPS position on a
chart plotter was quite interesting in those days. Now the position doesn't
move and will even plot a position on the correct side of a pontoon.

However when the authorities are playing silly b*ggers with the signal it
tends to be anounced in navigation warnings.....

G

JRS: In article , dated Thu, 23
Sep 2004 18:58:46, seen in news:uk.transport.london, James Christie
posted :

With difficulty, because GPS has an accuracy of +/- 100m, unless of
course you are using Differential GPS, but that is mainly a maritime
system.

Differential GPS with respect to a set on the platform (or station)
seems an obvious move - even deals properly with landslides.

But I believe that a platform is the only extended object which comes
that near to just under a train door; if so, use a pair of solid object
detectors below door level, one at each side of the doorway.

Sonar or radar could be used. ISTM likely that if a suitable
electromagnetic structure were fixed to platform edges (e.g. a
perforated metallic strip) then one might use resonant reflection, and
be platform-specific.

--
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