On Fri, 11 Apr 2008, Paul Corfield wrote:

On Fri, 11 Apr 2008 13:54:58 -0700 (PDT), Mr Thant
wrote:

On 11 Apr, 19:04, Tom Anderson wrote:
Provided that the bus can get good GPS and GPRS signals. GPS is
notoriously inaccurate in built-up areas, as buildings block lines of
sight to the satellites, and introduce reflections which confuse the
receiver (like ghosting on the telly). I wonder what they're doing to deal
with this?

I've read a TfL paper that I can't find right now that found it to be
surprisingly accurate, something like dead on 98% of the time.
Presumably to do with being aerial mounting a large antenna on the bus
roof where it has a good view of the sky, rather than the tiny
internal one in consumer gear that gets blocked by the car roof.

My general experience of I-Bus has been good even though my route runs
out of the first garage equipped with it so we've had all the bugs and
changes to contend with. There is still the odd bus with non functioning
displays or visual but no audible (or vice versa) announcements. Only
twice has something really silly happened - the first was leaving Wood
Green and I-Bus believing we were still heading there. Even when we
passed ourselves going the other way (IYSWIM) the system did not correct
itself.

ISTR reading of exactly this failure mode in GPS-driven information
systems on trains, on this very group. Somewhere on SWT? Unbelievable that
the software doesn't catch it.

The other was the system seeming to be completely dead and then suddenly
springing into life after leaving a stop. I have no idea how GPS works
but if it uses any form of cellular pattern to locate a vehicle I did
wonder if we had crossed from one cell to another in the second example.

It doesn't. Unlikely as this may sound, it works by picking up
synchronised radio signals from members of a family of 31 satellites
orbiting 20 000 km above the earth, measuring the time differences between
them with an accuracy of a few nanoseconds, which tells you the
differences in distances to the satellites with an accuracy of a few
metres (out of twenty million - not bad!), then doing geometric
calculations to work out where that means the receiver must be. It's the
kind of thing that if it didn't exist, you'd think it was an absurd idea.
A bit like the London Underground!

Working out your position depends on being able to pick up the signals
from the satellites, and buildings and other features of cities can
interfere with this. I suspect the jumpiness is caused by this: if the bus
has lost the signal for a moment, it won't fire off its announcement until
it's picked it up again and worked out where it is.

tom

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