On Sat, 12 Apr 2008 14:07:54 +0100, Tom Anderson
wrote:

On Fri, 11 Apr 2008, Paul Corfield wrote:

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

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.

Thanks for the technical explanation - what was odd about the example I
witnessed was that we'd just crossed the Lea Valley reservoirs where
there are no tall buildings for miles and the bus is fully exposed to
the sky so had every chance to receive its signals over a longish
distance. Still it was the first installation so perhaps it was just
one in a long series of bugs.
--
Paul C


Admits to working for London Underground!