Hi, Just having a conversation which has turned into why GPS, DAB radios and
other stuff doesn't work at all well on the more modern rolling stock i.e.
not picking up a signal. I'm guessing it's got something to do with the
window materials, but just wondered if anyone knew here?

Example train (http://static.flickr.com/2/1408980_241dea4228_o.jpg)

Thanks

On Fri, 8 Sep 2006 16:34:15 +0100, "elyob"
wrote:

Hi, Just having a conversation which has turned into why GPS, DAB radios and
other stuff doesn't work at all well on the more modern rolling stock i.e.
not picking up a signal.

That's a good question. On Virgin's Pendolinos you can get a DAB
signal if you stand by an external door, but that's about it. And it's
*very* irritating.

--
James Farrar
. @gmail.com

elyob wrote:

Hi, Just having a conversation which has turned into why GPS, DAB radios and
other stuff doesn't work at all well on the more modern rolling stock i.e.
not picking up a signal. I'm guessing it's got something to do with the
window materials, but just wondered if anyone knew here?

Example train (http://static.flickr.com/2/1408980_241dea4228_o.jpg)

Thanks

Well GPS probably doesn't work because the trains moving all the time
and all the clutter between you and the low down weak satellite signal
is killing the reception. As for DAB , dunno, perhaps the windows are
designed to block mobiles and block DAB too? Does FM radio work?

B2003

Boltar wrote:

elyob wrote:

Hi, Just having a conversation which has turned into why GPS, DAB radios and
other stuff doesn't work at all well on the more modern rolling stock i.e.
not picking up a signal. I'm guessing it's got something to do with the
window materials, but just wondered if anyone knew here?

Example train (http://static.flickr.com/2/1408980_241dea4228_o.jpg)

Thanks

Well GPS probably doesn't work because the trains moving all the time
and all the clutter between you and the low down weak satellite signal
is killing the reception. As for DAB , dunno, perhaps the windows are
designed to block mobiles and block DAB too? Does FM radio work?

Just realised that GPS isn't geostationary so the satellites arn't low
down.
Even so , perhaps it can't get a lock on enough of the satellites from
a
train window.

B2003

"Boltar" wrote in message
oups.com...

Boltar wrote:

elyob wrote:

Hi, Just having a conversation which has turned into why GPS, DAB
radios and
other stuff doesn't work at all well on the more modern rolling stock
i.e.
not picking up a signal. I'm guessing it's got something to do with the
window materials, but just wondered if anyone knew here?

Example train (http://static.flickr.com/2/1408980_241dea4228_o.jpg)

Thanks

Well GPS probably doesn't work because the trains moving all the time
and all the clutter between you and the low down weak satellite signal
is killing the reception. As for DAB , dunno, perhaps the windows are
designed to block mobiles and block DAB too? Does FM radio work?

Just realised that GPS isn't geostationary so the satellites arn't low
down.
Even so , perhaps it can't get a lock on enough of the satellites from
a
train window.

B2003

Dunno what happens in the UK but in Sydney some trains have a coating on the
windows to reduce the amount of sunlight (we have plenty here :-) And that
also effects mobile phone and GPS reception.

cheers
Peter
Sydney

So consider the situation, you are wishing to receive GPS which is a
very weak and very high frequency signal at about 1.2 to 1.5GHz with
corresponding wavelengths at 23 to 20cm whilst sitting in what is
electrically a leaky faraday cage with an indoor antenna.

There are seveal reasons why modern stock presents more problems than
the already challenging environment (any train) you are sitting in:

The majority of modern stock types are made from extruded aluminium
planks, welded together and thus present about a 6mm (internal and
external sections combined) thickness for a signal to pass through. At
RF frequencies, the skin effect whereby RF currents penetrate less and
less depth of any given metal goes up with frequency. Essentially, the
weak signal is hitting the train, and being concentrated in its
outermost fraction of a millimentre.

The only opportunities the signal has to get inside the train are areas
which offer poorer screening performance such as windows and in
gangways. Windows commonly contain metallic films such as gold to tint
them, and once again as frequency rises the tint film's RF screening
performance improves.

I suspect that sucessful GPS reception with an indoor antenna on board
a modern train is going to be severely compromised by the limited field
of view of the sattelite constellations through a window or other RF
leaky part of the train as well as the general constraints of the
changing, surrounding tolpology. One thing is for sure, that signal is
not getting in through the metallic parts of the train.


Turning to DAB radio, this is a digital transmission in the region of
200MHz with wavelengths of 1.5 metres. With powerful transmitters
spread across the country as well as more local relays, its field
strength at any given place is going to be much higher than ever GPS
will be and herin lies a snag.

When DAB standards were set, terrestial broadcasters could predict the
likely field strengths for which a receiver would have to operate,
taking into account an economical transmitter network and to ensure
consumer uptake, a relatively low tech and cheap receiver with
(compared to professional products,) mediocre selectivity and
sensitivity. In essence despite the very much higher field strengths
experienced by a DAB receiver when compared to a GPS receiver, the
receiver doesn't capitalise on it because it is built to a price (as is
the transmitter network).

The screening performance of the train structure and window tint films
are poorer at DAB frequencies than GPS frequencies, but the DAB
receiver requires a higher signal strength at its antenna to "make it
go".

A further consideration: People are analogue. Analogue radio works well
in marginal reception conditions because the signal gets noiser and
noiser as it gets weaker, but our brains have the innate capability to
pick out intelligible speech and music in noisy audio frequency
conditions. It may not be pleasant to listen to, but we can make sense
of it.

With a digital broadcast signal, the receiver mutes the audio output
until its own internal algorithms tell the receiver that it isn't
producing unintelligble mush. So it's all or nothing with DAB, a bit
like GSM as well (although that has a much lower acceptable error
threshold than DAB). Believe me, if you take the mute line off a DAB
receiver and get it to receive marginal signals (with high error
rates), it sounds like a cross between a machine gun, a stuck CD and
Sir Patrick Moore at warp factor 9 - not pleasant and certainly not
intelligible.

I hope that answers why RF devices don't work well in trains. Anything
with an internal antenna in a leaky faraday cage isn't going to work
well, it is suprising just what performance you can tease out of a GPS
receiver in such situations though.

Ah, one more point; older trains had certain structural features which
were much more leaky to RF. Mk 1 stock had a very large proportion of
untinted windows and wooden floor panels for example.

Richard