Bus Information Signs
 

I have no idea what they're called, the LED signs in some bus stops
that tell you how long the busses are supposed to be, my question is,
how do they work? how do they update?

A few times I've seen that my bus is due next in a couple of minutes
but it just doesn't arrive, several later busses come and then it
vanishes off the sign, abducted by aliens?

Then there are the times I've seen my bus is due in 10 minutes and
decided that instead of standing in the cold I'll walk to the next or
previous stop only for it to come sailing past when I'm mid-way
between stops.

And how many seconds are in a London Transport minute anyway? 90 at
least.

Obviously the system is not perfect (no system is) but if I knew what
made it tick I may be better equipped for getting around.

And why I'm grumbling about busses, why is it so rare for a 38 to get
to the end of it's route? I live near clapton pond and often need to
travel to victoria, when I have the time I'd rather sit on a bus than
go underground but often they terminate at hyde park corner or green
park, same the other way often only making it to hackney central, in
both cases I need to get another bus, pain in the arse, especially as
the end of their route is only 5 minutes away.

grumble grumble
--
Mark.
www.MarkVarleyPhoto.co.uk
www.TwistedPhotography.co.uk
www.TwistedArts.co.uk
www.FacelessLadies.com
www.BeautifulBondage.net

Bus Information Signs
 

On 11 Apr, 17:20, MarkVarley - MVP
wrote:

I have no idea what they're called, the LED signs in some bus stops
that tell you how long the busses are supposed to be, my question is,
how do they work? how do they update?

A few times I've seen that my bus is due next in a couple of minutes
but it just doesn't arrive, several later busses come and then it
vanishes off the sign, abducted by aliens?

Then there are the times I've seen my bus is due in 10 minutes and
decided that instead of standing in the cold I'll walk to the next or
previous stop only for it to come sailing past when I'm mid-way
between stops.

And how many seconds are in a London Transport minute anyway? 90 at
least.

Obviously the system is not perfect (no system is) but if I knew what
made it tick I may be better equipped for getting around.


The system is called Countdown, and as part of the iBus project it is
set to become much more reliable and accurate.

Countdown:
http://www.tfl.gov.uk/corporate/proj...ment/2369.aspx
or via http://tinyurl.com/4swhsj

iBus:
http://www.tfl.gov.uk/corporate/proj...ment/2373.aspx
or via http://tinyurl.com/yu6wwe

Bus Information Signs
 

In message , MarkVarley -
MVP writes

I have no idea what they're called, the LED signs in some bus stops
that tell you how long the busses are supposed to be, my question is,
how do they work? how do they update?

The current system, called Countdown, is fairly primitive. When a bus
passes a fixed point (with a large bar-code) earlier along the route,
the system simply adds the standard time it takes to get from that fixed
point to the bus stop at which you are waiting. If the bus subsequently
gets delayed, it knows nothing about it. Equally, if the bus is diverted
or is too far from the curbside to trigger the reader, or if it is
overtaking another bus (or any large vehicle) as it passes the bar-code,
the system know nothing about it.

The Countdown system is currently being replaced by a rather better,
GPS-based system.
--
Paul Terry

Bus Information Signs
 

On Apr 11, 5:48*pm, Mizter T wrote:
On 11 Apr, 17:20, MarkVarley - MVP
wrote:





I have no idea what they're called, the LED signs in some bus stops
that tell you how long the busses are supposed to be, my question is,
how do they work? how do they update?

A few times I've seen that my bus is due next in a couple of minutes
but it just doesn't arrive, several later busses come and then it
vanishes off the sign, abducted by aliens?

Then there are the times I've seen my bus is due in 10 minutes and
decided that instead of standing in the cold I'll walk to the next or
previous stop only for it to come sailing past when I'm mid-way
between stops.

And how many seconds are in a London Transport minute anyway? 90 at
least.

Obviously the system is not perfect (no system is) but if I knew what
made it tick I may be better equipped for getting around.

The system is called Countdown, and as part of the iBus project it is
set to become much more reliable and accurate.

Countdown:http://www.tfl.gov.uk/corporate/proj...nologyandequip...
or via http://tinyurl.com/4swhsj

iBus:http://www.tfl.gov.uk/corporate/proj...nologyandequip....
or via http://tinyurl.com/yu6wwe-

I wonder if it means more reliable in terms of whether it breaks down
or reliable in terms of the content, which must be down to where the
locations of the bus-detectors rather than the system that passes
information to the punters.

I suspect that the information is very accurate, but that it doesn't
necessarily relate to the location of the bus stop.

That is, some stops will display information about buses that have
already gone past (and therefore seem to disappear) and others
disappear the information off long before the bus has turned up.

I also recall a problem with a bus whose starting point was just round
the corner, and it turned up unannounced because the system didn't
seem to be aware of it at all (a nice surprise in that case).

Bus Information Signs
 

On Fri, 11 Apr 2008, Mizter T wrote:

On 11 Apr, 17:20, MarkVarley - MVP
wrote:

I have no idea what they're called, the LED signs in some bus stops
that tell you how long the busses are supposed to be, my question is,
how do they work? how do they update?

The system is called Countdown, and as part of the iBus project it is
set to become much more reliable and accurate.

Which is a roundabout way of saying that the present system is not very
reliable or accurate :P.

As another poster pointed out, the present system is rather Heath-Robinson
and failure-prone. It was, though, the best that could be done with the
technology of the time it was implemented. Frustrating as it is, it's
better than nothing.

tom

--
Ed editor textorum probatissimus est -- Cicero, De officiis IV.7

Bus Information Signs
 

On Fri, 11 Apr 2008, MIG wrote:

On Apr 11, 5:48*pm, Mizter T wrote:
On 11 Apr, 17:20, MarkVarley - MVP
wrote:

I have no idea what they're called, the LED signs in some bus stops
that tell you how long the busses are supposed to be, my question is,
how do they work? how do they update?

The system is called Countdown, and as part of the iBus project it is
set to become much more reliable and accurate.

I wonder if it means more reliable in terms of whether it breaks down
or reliable in terms of the content, which must be down to where the
locations of the bus-detectors rather than the system that passes
information to the punters.

The new system won't use bus detectors, it'll have a GPS unit on each bus
which radios its position back to base (by GPRS?). That gives it higher
resolution, and makes it impossible for a bus to fall off the system, as
they can now.

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? Maybe just having the receiver on the top of the bus will be
enough. Newer GPS chipsets are also getting better at operating in urban
environments - for instance, i hear that the SirfStar III is much better
than older kit:

http://wiki.openstreetmap.org/index.php/SiRF_III

I suspect that the information is very accurate, but that it doesn't
necessarily relate to the location of the bus stop.

That is, some stops will display information about buses that have
already gone past (and therefore seem to disappear) and others
disappear the information off long before the bus has turned up.

Neither of those should happen. The main failure mode will probably be the
display (or its communication link) breaking down altogether.

Another plus of iBus is that it will let the bus operators and TfL track
all buses all the time, which will give them a much, much better ability
to (a) regulate the service and (b) monitor performance. Operators will
find it much harder to get away with providing a substandard service,
particularly at night, when i understand tfL's inspectors don't operate.

tom

--
Ed editor textorum probatissimus est -- Cicero, De officiis IV.7

Bus Information Signs
 

On Apr 11, 7:04*pm, Tom Anderson wrote:
On Fri, 11 Apr 2008, MIG wrote:
On Apr 11, 5:48*pm, Mizter T wrote:
On 11 Apr, 17:20, MarkVarley - MVP
wrote:

I have no idea what they're called, the LED signs in some bus stops
that tell you how long the busses are supposed to be, my question is,
how do they work? how do they update?

The system is called Countdown, and as part of the iBus project it is
set to become much more reliable and accurate.

I wonder if it means more reliable in terms of whether it breaks down
or reliable in terms of the content, which must be down to where the
locations of the bus-detectors rather than the system that passes
information to the punters.

The new system won't use bus detectors, it'll have a GPS unit on each bus
which radios its position back to base (by GPRS?). That gives it higher
resolution, and makes it impossible for a bus to fall off the system, as
they can now.

Ah right. That should definitely be an improvement, as long as the
right direction has been programmed in ...

Does the use of GPS have implications for Oyster and flat fares?

Bus Information Signs
 

On Apr 11, 6:52*pm, Tom Anderson wrote:
On Fri, 11 Apr 2008, Mizter T wrote:
On 11 Apr, 17:20, MarkVarley - MVP
wrote:

I have no idea what they're called, the LED signs in some bus stops
that tell you how long the busses are supposed to be, my question is,
how do they work? how do they update?

The system is called Countdown, and as part of the iBus project it is
set to become much more reliable and accurate.

Which is a roundabout way of saying that the present system is not very
reliable or accurate :P.

As another poster pointed out, the present system is rather Heath-Robinson
and failure-prone. It was, though, the best that could be done with the
technology of the time it was implemented. Frustrating as it is, it's
better than nothing.

Was it a development of the BESI system used in the 1970s?

Bus Information Signs
 

On 11 Apr, 18:52, Tom Anderson wrote:

On Fri, 11 Apr 2008, Mizter T wrote:

On 11 Apr, 17:20, MarkVarley - MVP
wrote:

I have no idea what they're called, the LED signs in some bus stops
that tell you how long the busses are supposed to be, my question is,
how do they work? how do they update?

The system is called Countdown, and as part of the iBus project it is
set to become much more reliable and accurate.

Which is a roundabout way of saying that the present system is not very
reliable or accurate :P.

As another poster pointed out, the present system is rather Heath-Robinson
and failure-prone. It was, though, the best that could be done with the
technology of the time it was implemented. Frustrating as it is, it's
better than nothing.


Heath Robinson-esque is quite the right sounding description! I
couldn't quite remember when Countdown began, but after a bit of
googling I found this post by the great Mr Arquati, formerly (and I
still think occasionally) of this parish...

http://groups.google.co.uk/group/uk....6435d488fdeb6a

....in which it is stated that the Countdown system went live in 1996
(the above thread incidentally is from 2005 and concerns the Countdown/
iBus overhaul), when GPS technologies were pricey and the government
hardly splashed LT with cash.

I don't know the plotted history of the system but I wouldn't be at
all surprised to learn that it was only installed on a few routes at
first.

Anyway, 'tis a grand thing that it will be made better. It's most
frustrating to have to guess whether the display is telling the truth
or not.

Bus Information Signs
 

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.

Is there a way to tell if a bus is using iBus data or Countdown. Have
any iBus-based displays even been deployed yet?

U

--
http://londonconnections.blogspot.com/
A blog about transport projects in London

Bus Information Signs
 

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. 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.

Is there a way to tell if a bus is using iBus data or Countdown. Have
any iBus-based displays even been deployed yet?

I-Bus equipped vehicles have a blue and yellow sticker in the
windscreen. There is also a console in the cab - typically just above
and to the right of the driver's head - and this shows the headway gap
and schedule "divergence" on a display to the driver.

I am not aware of a way of knowing if a bus is Countdown equipped as the
bus mounted kit is on the rear axle AIUI. This is one reason for the
unreliability of the system - hard to maintain and takes hours to fix
and means the bus is off the road. Not what an operator under a
performance based contract wants.

I am not aware that any Countdown displays have been integrated into the
I-Bus system yet - I think this part of the project is running late.
Certainly we have some local Countdown displays that are on I-Bus
equipped routes and I have not noticed any substantial change or
improvement to the data. There will be a further 2000 stop displays as
part of I-Bus but this phase is only just out to tender. I don't know if
it incorporates the integration of existing displays as part of the
scope or if it is simply the displays themselves and installation.

--
Paul C


Admits to working for London Underground!

Bus Information Signs
 

On Fri, 11 Apr 2008 22:36:08 +0100, Paul Corfield
wrote this gibberish:

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. 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.

I was on a 38 the other day that gave it's location correctly at the
stops but always announced that it was going the other way!
messing with the tourists again :-D
--
Mark.
www.MarkVarleyPhoto.co.uk
www.TwistedPhotography.co.uk
www.TwistedArts.co.uk
www.FacelessLadies.com
www.BeautifulBondage.net

Bus Information Signs
 

MarkVarley - MVP wrote:
I have no idea what they're called, the LED signs in some bus stops
that tell you how long the busses are supposed to be, my question is,
how do they work? how do they update?

A few times I've seen that my bus is due next in a couple of minutes
but it just doesn't arrive, several later busses come and then it
vanishes off the sign, abducted by aliens?

If you read through the archives of this newsgroup you will find similar
statements and much gnashing of teeth. The classic is the bus coming in
the other direction showing up on the display or, during the night, the
bus travelling at much greater than expected speeds and thus the 10
minute allowance for a juicy kebab turns into a rapid dash, without any
greasy and dubious sustenance.

All in all, Countdown is a disappointing experience and while iBus
promises much greater accuracy, it will no doubt lull the average
traveller into a false sense of security much as the present system
does. It would be good if the displays could correct for inaccuracy,
and calculate averages depending on the day and time, well within the
capabilities of even the most modest microcontrollers, but they won't.
Instead, vague promises of an impending bus will remain long after it
has left or will appear well in advance of the actual reality.

ESB

Bus Information Signs
 

In message , at 17:20:26 on
Fri, 11 Apr 2008, MarkVarley - MVP
remarked:
A few times I've seen that my bus is due next in a couple of minutes
but it just doesn't arrive, several later busses come and then it
vanishes off the sign, abducted by aliens?

Signs like that normally have two modes of operation (but they fail to
explain this).

If the arrival time says "10.35" that means the timetable says there
should be a bus at 10.35. Nothing more.

If the arrival times says "3 minutes" that means they've tracked an
actual bus and it's 3 minutes away.

The former will just scroll off irrespective of the passage of buses.
--
Roland Perry

Bus Information Signs
 

On 12 Apr, 02:15, Ernst S Blofeld
wrote:

MarkVarley - MVP wrote:
I have no idea what they're called, the LED signs in some bus stops
that tell you how long the busses are supposed to be, my question is,
how do they work? how do they update?

A few times I've seen that my bus is due next in a couple of minutes
but it just doesn't arrive, several later busses come and then it
vanishes off the sign, abducted by aliens?

If you read through the archives of this newsgroup you will find similar
statements and much gnashing of teeth. The classic is the bus coming in
the other direction showing up on the display or, during the night, the
bus travelling at much greater than expected speeds and thus the 10
minute allowance for a juicy kebab turns into a rapid dash, without any
greasy and dubious sustenance.

All in all, Countdown is a disappointing experience and while iBus
promises much greater accuracy, it will no doubt lull the average
traveller into a false sense of security much as the present system
does. It would be good if the displays could correct for inaccuracy,
and calculate averages depending on the day and time, well within the
capabilities of even the most modest microcontrollers, but they won't.
Instead, vague promises of an impending bus will remain long after it
has left or will appear well in advance of the actual reality.

ESB

Yeah, you're right, you might as well just give up, there's no point
in trying to make anything better whatsoever. In fact they might as
well give up on running buses, anyone who needs to get anywhere should
just get a car. I can't see any problem with that idea.

Bus Information Signs
 

On 12 Apr, 10:36, Roland Perry wrote:

In message , at 17:20:26 on
Fri, 11 Apr 2008, MarkVarley - MVP
remarked:

A few times I've seen that my bus is due next in a couple of minutes
but it just doesn't arrive, several later busses come and then it
vanishes off the sign, abducted by aliens?

Signs like that normally have two modes of operation (but they fail to
explain this).

If the arrival time says "10.35" that means the timetable says there
should be a bus at 10.35. Nothing more.

If the arrival times says "3 minutes" that means they've tracked an
actual bus and it's 3 minutes away.

The former will just scroll off irrespective of the passage of buses.


I'm almost certain that the London Countdown system doesn't have any
capability to work in the first way you describe - i.e. by simply
listing buses according when they should arrive according to the
timetable.

Bus Information Signs
 

On Fri, 11 Apr 2008, MIG wrote:

Does the use of GPS have implications for Oyster and flat fares?

I don't think so. We had the technical capacity to have non-flat fares
before GPS, and indeed did so. The flares were flattened as an act of
policy, not practical exigency. I suppose with GPS they could introduce
some truly fiendish true-distance-based fare system, but i can't see why
they would.

tom

--
The MAtrix had evarything in it: guns, a juimping off teh walls, flying
guns, a bullet tiem, evil computar machenes, numbers that flew, flying
gun bullets in slowar motian, juimping into a gun, dead police men,
computar hackeing, Kevin Mitnick, oven trailers, a old womans kitchen,
stairs, mature women in clotheing, head spark plugs, mechaanical squids,
Japaneseses assasins, tiem traval, volcanos, a monstar, slow time at
fastar speed, magic, wizzards, some dirty place, Kung Few, fighting,
a lot of mess explodsians EVARYWHERE, and just about anything else yuo
can names!

Bus Information Signs
 

On Fri, 11 Apr 2008, MIG wrote:

On Apr 11, 6:52*pm, Tom Anderson wrote:
On Fri, 11 Apr 2008, Mizter T wrote:
On 11 Apr, 17:20, MarkVarley - MVP
wrote:

I have no idea what they're called, the LED signs in some bus stops
that tell you how long the busses are supposed to be, my question is,
how do they work? how do they update?

The system is called Countdown, and as part of the iBus project it is
set to become much more reliable and accurate.

Which is a roundabout way of saying that the present system is not very
reliable or accurate :P.

As another poster pointed out, the present system is rather Heath-Robinson
and failure-prone. It was, though, the best that could be done with the
technology of the time it was implemented. Frustrating as it is, it's
better than nothing.

Was it a development of the BESI system used in the 1970s?

Pass!

tom

--
The MAtrix had evarything in it: guns, a juimping off teh walls, flying
guns, a bullet tiem, evil computar machenes, numbers that flew, flying
gun bullets in slowar motian, juimping into a gun, dead police men,
computar hackeing, Kevin Mitnick, oven trailers, a old womans kitchen,
stairs, mature women in clotheing, head spark plugs, mechaanical squids,
Japaneseses assasins, tiem traval, volcanos, a monstar, slow time at
fastar speed, magic, wizzards, some dirty place, Kung Few, fighting,
a lot of mess explodsians EVARYWHERE, and just about anything else yuo
can names!

Bus Information Signs
 

On Fri, 11 Apr 2008, 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.

Sounds plausible. Also, you don't need an accurate fix all the time: even
if you can only get one every fifty metres, it might do; you could fill in
with dead reckoning in between.

tom

--
The MAtrix had evarything in it: guns, a juimping off teh walls, flying
guns, a bullet tiem, evil computar machenes, numbers that flew, flying
gun bullets in slowar motian, juimping into a gun, dead police men,
computar hackeing, Kevin Mitnick, oven trailers, a old womans kitchen,
stairs, mature women in clotheing, head spark plugs, mechaanical squids,
Japaneseses assasins, tiem traval, volcanos, a monstar, slow time at
fastar speed, magic, wizzards, some dirty place, Kung Few, fighting,
a lot of mess explodsians EVARYWHERE, and just about anything else yuo
can names!

Bus Information Signs
 

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

--
The MAtrix had evarything in it: guns, a juimping off teh walls, flying
guns, a bullet tiem, evil computar machenes, numbers that flew, flying
gun bullets in slowar motian, juimping into a gun, dead police men,
computar hackeing, Kevin Mitnick, oven trailers, a old womans kitchen,
stairs, mature women in clotheing, head spark plugs, mechaanical squids,
Japaneseses assasins, tiem traval, volcanos, a monstar, slow time at
fastar speed, magic, wizzards, some dirty place, Kung Few, fighting,
a lot of mess explodsians EVARYWHERE, and just about anything else yuo
can names!

Bus Information Signs
 

"Roland Perry" wrote in message
...

If the arrival time says "10.35" that means the timetable says there
should be a bus at 10.35. Nothing more.

We have the sma problems in railway world... PIS pulls off the PTT and
assumes everything is ok. When a train is CAPD and the system is not told is
assumes its just not left yet, and either add's 2min's a time to its ETA.
Once it passes its due time at any point along the route (if it ever dose)
it will vanish without a trace.

If its delayed on leaving its origion then it will just rackup time by
2min's at every refresh, or if there is no recording of it it will vanish
anyway and show the next booked service, and then flup between the late one,
and the booked one.

If the arrival times says "3 minutes" that means they've tracked an actual
bus and it's 3 minutes away.

This is the same as 'LULTime' where in it might be 3 min's away, but not
moving - so it will never arrive in 3 min's


There is the added fun of turn-a-rounds at either end of the route, PIS (and
others) have no idea or concept of it, and can't compute the 'next working'
This can cause all sorts of fun when things go a bit broke.

What we should be doing with I-Bus is getting the backend to process what's
actually going on in real time based on live GPS data, LTIS, SWS, and real
time congestion data. They should have the ability to calculate the arrival
of the service at any given point based on all the information above. It
should also be aware of things like driver changes, and what happens when a
bus is pulled half way on its route.

In railway world all we need to do is process the TDS information a bit
better, work in turn-rounds, and have some hook into the SSI (Assuming we
don't role out a GPS system across the NR network...


My 2p worth.

Bus Information Signs
 

On 12 Apr, 13:53, Tom Anderson wrote:

On Fri, 11 Apr 2008, MIG wrote:

Does the use of GPS have implications for Oyster and flat fares?

I don't think so. We had the technical capacity to have non-flat fares
before GPS, and indeed did so. The flares were flattened as an act of
policy, not practical exigency. I suppose with GPS they could introduce
some truly fiendish true-distance-based fare system, but i can't see why
they would.


That's not really right - it's a more complicated picture than you
paint and I think it's fair to say that the bus fares were flattened
as both an act of policy *and8 as a matter of practical exigency.
Making a smartcard system work on buses with a variable (i.e.non-flat)
fare structure introduces significant complications, in that either
passengers have some kind of transaction where they with specify the
length of their journey (whether with a driver or a machine) or
alternatively passengers need to touch-out at the end of their journey
before they alight from the bus. Or, I suppose, all bus stops are
fitted with Oyster readers ala Tramlink and passengers have to touch-
in and out on them.

I started a thread back in January on this very topic, and it contains
a very interesting, comprehensive and informative contribution from
Paul Corfield who was there in the early days of Oyster's development
and outlines some of the thinking that went on with regards to how to
make the smartcard system work on buses in London. He also shared his
experiences of how smartcards systems work on the transport systems of
Hong Kong and Singapore.

The thread is he
http://groups.google.co.uk/group/uk....a7d0d50c9d3b1/

For those who don't want to plough through all that, the most relevant
paragraph of what Paul C had to say with regards to the specific issue
of flat or variable fares on London's buses is this one:

"My guess is that the reality of making zonal fares work on buses
forced TfL into considering flat fares (although simplification had
been working its way through LT Buses for several years). This
consideration then meshed nicely with the aspirations of the Mayor to
drive up bus usage. The rest, they say, is history."

You're right in saying that GPS tracking (as part of the iBus project)
doesn't make any real difference to this whole issue - variable fares
are technically feasible without any such extra tracking technology.

I think that flat fares in London are here to stay, thankfully. I
don't think any future Mayor would dare change it, given the
tremendous advantage of quick boarding and thus reduction in dwell
time that it offers.

Bus Information Signs
 

In message
, MIG
writes
On Apr 11, 6:52*pm, Tom Anderson wrote:
On Fri, 11 Apr 2008, Mizter T wrote:
On 11 Apr, 17:20, MarkVarley - MVP
wrote:

I have no idea what they're called, the LED signs in some bus stops
that tell you how long the busses are supposed to be, my question is,
how do they work? how do they update?

The system is called Countdown, and as part of the iBus project it is
set to become much more reliable and accurate.

Which is a roundabout way of saying that the present system is not very
reliable or accurate :P.

As another poster pointed out, the present system is rather Heath-Robinson
and failure-prone. It was, though, the best that could be done with the
technology of the time it was implemented. Frustrating as it is, it's
better than nothing.

Was it a development of the BESI system used in the 1970s?

There was something else between BESI and Countdown and I can't for the
life of me remember what it was and can't be bothered to Google.
Someone will tell me any time now.......
--
Ian Jelf, MITG
Birmingham, UK

Registered Blue Badge Tourist Guide for London and the Heart of England
http://www.bluebadge.demon.co.uk

Bus Information Signs
 

On Apr 12, 11:55*am, Mizter T wrote:
On 12 Apr, 10:36, Roland Perry wrote:





In message , at 17:20:26 on
Fri, 11 Apr 2008, MarkVarley - MVP
remarked:

A few times I've seen that my bus is due next in a couple of minutes
but it just doesn't arrive, several later busses come and then it
vanishes off the sign, abducted by aliens?

Signs like that normally have two modes of operation (but they fail to
explain this).

If the arrival time says "10.35" that means the timetable says there
should be a bus at 10.35. Nothing more.

If the arrival times says "3 minutes" that means they've tracked an
actual bus and it's 3 minutes away.

The former will just scroll off irrespective of the passage of buses.

I'm almost certain that the London Countdown system doesn't have any
capability to work in the first way you describe - i.e. by simply
listing buses according when they should arrive according to the
timetable.-

I am sure that the disappearance is due to the locations of detectors
and faster than expected arrivals. Eg if the nearest detector is in a
location deemed to be 10 minutes away, and the bus does it in 7
minutes, it will show the bus as still expected for 3 minutes after
it's gone. (Although could the detectors be that far apart?)

Bus Information Signs
 

On Apr 12, 4:46*pm, Ian Jelf wrote:
In message
, MIG
writes





On Apr 11, 6:52*pm, Tom Anderson wrote:
On Fri, 11 Apr 2008, Mizter T wrote:
On 11 Apr, 17:20, MarkVarley - MVP
wrote:

I have no idea what they're called, the LED signs in some bus stops
that tell you how long the busses are supposed to be, my question is,
how do they work? how do they update?

The system is called Countdown, and as part of the iBus project it is
set to become much more reliable and accurate.

Which is a roundabout way of saying that the present system is not very
reliable or accurate :P.

As another poster pointed out, the present system is rather Heath-Robinson
and failure-prone. It was, though, the best that could be done with the
technology of the time it was implemented. Frustrating as it is, it's
better than nothing.

Was it a development of the BESI system used in the 1970s?

There was something else between BESI and Countdown and I can't for the
life of me remember what it was and can't be bothered to Google.
Someone will tell me any time now.......

I can't find much about it at all, but I've seen pictures of "radio-
controlled" buses in the 1970s passing some kind of scanners in the
street. (It stood for "Bus Electronic Scanning Indicator"?)

It would have been used for managing buses, but I suppose that once
you've got a system for locating and identifying buses, the next stage
is to link the information to PIS.

Would the system in between have been perfecting the management part,
or would it be to do with the PIS part?

Bus Information Signs
 

On Fri, 11 Apr 2008 11:22:49 -0700 (PDT), MIG
wrote:

Does the use of GPS have implications for Oyster and flat fares?

Not under the current Mayoral regime as I don't see Ken removing flat
fares.

However I think GPS or something similar is used in Singapore to
determine bus position relative to bus stops and this triggers the card
readers to become active and able to read and write to cards. This is
because Singapore has entry and exit validation on bus. Works very well
to be honest but that's as much a cultural phenomenon as a technological
one. TfL are proposing a new form of bus ticket machine that will act
as a technology hub linked into new radio and I-Bus. There is some
mammoth document on the TfL website that lists all of the projects in
the next Business Plan and this is in it. It could form the backbone of
allowing exit validation and distance based charging.

If you were feeling particularly nasty you could infer that the lack of
mention of fares policy in Boris's manifesto and the proposed
reintroduction of conductors could presage a return to some form of
distance based fare in London. On the face of it Mr Paddick's "I hour
and as many changes as you like" ticket must presumably be flat within
whatever area it applies in. From what little I've read about it that
policy was off the back of shortening routes that run in to Zone 1 and
forcing people to change buses - a recipe for failure if ever I read one
given the long established travel patterns on core routes into Zone 1.

--
Paul C


Admits to working for London Underground!

Bus Information Signs
 

On Sat, 12 Apr 2008 07:14:04 -0700 (PDT), Mizter T
wrote:

I think that flat fares in London are here to stay, thankfully. I
don't think any future Mayor would dare change it, given the
tremendous advantage of quick boarding and thus reduction in dwell
time that it offers.

Given how simplistic the policies are from the other mayoral candidates
I fear your thoughts may be misplaced. As they understand so little
about how transport works in the capital and are making easy promises
I'd guess they believe they have "easy" options with regards to fares
when they face a financial squeeze and decide to increase them. Child
fares will be the first to return and then we'll have different fares
for express routes, different fares for conductor buses because they're
"better" but cost more to run and then longer routes are more expensive
to run so we need to charge more .....
--
Paul C


Admits to working for London Underground!

Bus Information Signs
 

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!

Bus Information Signs
 

On Sat, 12 Apr 2008 16:46:02 +0100, Ian Jelf
wrote:

In message
, MIG
writes

Was it a development of the BESI system used in the 1970s?

There was something else between BESI and Countdown and I can't for the
life of me remember what it was and can't be bothered to Google.
Someone will tell me any time now.......

PIBS? Passenger Information at Bus Stops.
--
Paul C


Admits to working for London Underground!

Bus Information Signs
 

On 12 Apr, 19:06, Paul Corfield wrote:
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.

Might be that the programmed positions of the stops was off:
http://www.tfl.gov.uk/assets/downloa...tions_v00e.pdf

(long paper on TfL trying to figure out exactly where there bus stops
are)

U

--
http://londonconnections.blogspot.com/
A blog about transport projects in London

Bus Information Signs
 

Mizter T wrote:
Yeah, you're right, you might as well just give up, there's no point
in trying to make anything better whatsoever. In fact they might as
well give up on running buses, anyone who needs to get anywhere should
just get a car. I can't see any problem with that idea.

I did not for one moment suggest that things should not be made better,
rather I suggested they probably would not. I would love to see highly
reliable displays but, for whatever reason, it just hasn't been managed
yet. I appreciate having good, timely information so that when delays
occur I can take alternative routes but the Countdown system does not
provide this. When, for example, you let a few buses pass because a
preferential one is coming in "2 mins" - it is damn annoying when this
doesn't transpire even 10 minutes later and that bus has disappeared
entirely off the display like a ghostly dream. The difficulty then is
after a few big let-downs, the information ceases to be trusted.

I also suggested that the displays should become a little more
intelligent, using statistics to improve the accuracy of the
information, flagging delays etc. This would not necessarily need
real-time route position information but could be estimated just by
sensing the buses as they pass the stop. Such a system would
self-calibrate and have an number of other advantages. They would also
display the current time of day obtained via the 60KHz time signal.

ESB

Bus Information Signs
 

On Sat, 12 Apr 2008, Mr Thant wrote:

On 12 Apr, 19:06, Paul Corfield wrote:

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.

Might be that the programmed positions of the stops was off:
http://www.tfl.gov.uk/assets/downloa...tions_v00e.pdf

(long paper on TfL trying to figure out exactly where there bus stops
are)

Eight pages!

Summary:

- GPS doesn't work well enough in cities (31.53 metre 95% error)

- plotting on a map based on the textual description they already have can
work well, but depends on the description being adequate, which not all
are (11.48 metre 95% error)

- going round bus-stops with a map and marking their location on it can
work very well (they did this with electronic maps on laptops, rather than
a paper map), but needs the plotter to be computer-literate do so (6.45
metre 95% error, 28.22 if computer illiterate!)

- stops and routes (which there is already a database of, based on OS road
data) can be mutually validated by trying to place stops on routes, and
looking for various cases indicative of error - this bit is quite clever

- they're working on using GPS logs from the buses to further validate and
refine the coordinates

tom

--
London has a suburb for every emotion. -- Cliff Laine

Bus Information Signs
 

In article , Tom
Anderson writes
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.

I don't know if it'll help, but here's how I explain it to my kids.

You're lost because it's foggy or pitch black. You have a map of the
area but can't see any landmarks. You've lost your watch. However, you
know that the local church clocks are accurate, they strike exactly one
second between beats, and each has a different set of chimes so you can
tell which one you're hearing.

You hear a clock chime and strike the hour. 20 seconds later (using the
first set of strikes to time things) you hear another clock. Since sound
travels at 300m/s you know that you're 6km closer to the first clock
than the second one. Some careful thought allows you to draw a curve on
your map which is all the places 6km closer to the first church than the
latter - you are somewhere on that curve.

Meanwhile, 12 seconds after the second clock you heard a third one. You
are therefore 3.6km closer to the second than the third and 9.6km closer
to the first than the third. These let you draw two more lines that
you're also on and, hopefully, all three lines cross at exactly one
place, which is where you are.

GPS uses radio (which moves much faster) rather than sound, and the
transmitters keep moving; however, the signal coming from them says
exactly where they are, so that isn't a problem. Nonetheless the
principle is the same.

--
Clive D.W. Feather | Home:
Tel: +44 20 8495 6138 (work) | Web: http://www.davros.org
Fax: +44 870 051 9937 | Work:
Please reply to the Reply-To address, which is:

How GPS works (was Bus Information Signs)
 

Clive D. W. Feather wrote:
In article , Tom
Anderson writes

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.

I don't know if it'll help, but here's how I explain it to my kids.

You're lost because it's foggy or pitch black. You have a map of the
area but can't see any landmarks. You've lost your watch. However, you
know that the local church clocks are accurate, they strike exactly
one second between beats, and each has a different set of chimes so
you can tell which one you're hearing.

You hear a clock chime and strike the hour. 20 seconds later (using
the first set of strikes to time things) you hear another clock.
Since sound travels at 300m/s you know that you're 6km closer to the
first clock than the second one. Some careful thought allows you to
draw a curve on your map which is all the places 6km closer to the
first church than the latter - you are somewhere on that curve.

Meanwhile, 12 seconds after the second clock you heard a third one.
You are therefore 3.6km closer to the second than the third and 9.6km
closer to the first than the third. These let you draw two more lines
that you're also on and, hopefully, all three lines cross at exactly
one place, which is where you are.

GPS uses radio (which moves much faster) rather than sound, and the
transmitters keep moving; however, the signal coming from them says
exactly where they are, so that isn't a problem. Nonetheless the
principle is the same.

How do the receivers cope with dozens of satellites all broadcasting on the
same frequencies? Time splicing?

How GPS works (was Bus Information Signs)
 

In article , John Rowland
writes
How do the receivers cope with dozens of satellites

31

all broadcasting on the
same frequencies? Time splicing?

As I understand it, the signals are not on a single frequency but rather
jump around a set of adjacent frequencies ("spread spectrum"). Each
satellite uses a different pattern of jumps repeating every millisecond,
so you can tell which satellite you're picking up by recognising the
pattern, and the different patterns mean that two satellites are never
(or hardly ever) on the same frequency at the same time.

--
Clive D.W. Feather | Home:
Tel: +44 20 8495 6138 (work) | Web: http://www.davros.org
Fax: +44 870 051 9937 | Work:
Please reply to the Reply-To address, which is:

How GPS works (was Bus Information Signs)
 

On 20 Apr, 01:42, "Clive D. W. Feather" cl...@on-the-
train.demon.co.uk wrote:
As I understand it, the signals are not on a single frequency but rather
jump around a set of adjacent frequencies ("spread spectrum"). Each
satellite uses a different pattern of jumps repeating every millisecond,
so you can tell which satellite you're picking up by recognising the
pattern

I think how it works is the receiver has to jump between frequencies
on the same pattern as the satellite it wants to listen to. The
receiver therefore needs to know which pattern each satellite is
using, and it also needs a separate tuner for each satellite. Because
most have far fewer tuners than there are satellites (usually 12, vs
30ish) they need to know in advance which satellites are overhead.

So this means a receiver needs to already know current approximate
location and the current time, and the orbit and frequency pattern
information about the satellites. This is why it takes ages for a
brand new or freshly reset GPS unit to get any sort of lock (up to an
hour).

U

--
http://londonconnections.blogspot.com/
A blog about transport projects in London

How GPS works (was Bus Information Signs)
 

So this means a receiver needs to already know current approximate
location and the current time, and the orbit and frequency pattern
information about the satellites. This is why it takes ages for a
brand new or freshly reset GPS unit to get any sort of lock (up to an
hour).

And also, presumably, when you turn on the GPS a long way from its location
it was last used?

How GPS works (was Bus Information Signs)
 

In uk.transport.london message ,
Sun, 20 Apr 2008 01:42:16, Clive D. W. Feather clive@on-the-
train.demon.co.uk posted:
In article , John Rowland
writes
How do the receivers cope with dozens of satellites

31

all broadcasting on the
same frequencies? Time splicing?

As I understand it, the signals are not on a single frequency but
rather jump around a set of adjacent frequencies ("spread spectrum").
Each satellite uses a different pattern of jumps repeating every
millisecond, so you can tell which satellite you're picking up by
recognising the pattern, and the different patterns mean that two
satellites are never (or hardly ever) on the same frequency at the same
time.

ftp://tycho.usno.navy.mil/pub/gps/gpssy.txt, "GPS SIGNAL
CHARACTERISTICS". Let us hope that their technology is better than
their spelling.

--
(c) John Stockton, nr London, UK. Turnpike v6.05.
Web URL:http://www.merlyn.demon.co.uk/ - w. FAQish topics, links, acronyms
PAS EXE etc : URL:http://www.merlyn.demon.co.uk/programs/ - see 00index.htm
Dates - miscdate.htm moredate.htm js-dates.htm pas-time.htm critdate.htm etc.

How GPS works (was Bus Information Signs)
 

In article
, Mr
Thant writes
I think how it works is the receiver has to jump between frequencies
on the same pattern as the satellite it wants to listen to.

True, I believe.

The
receiver therefore needs to know which pattern each satellite is
using,

But this will be pre-programmed in some way.

and it also needs a separate tuner for each satellite. Because
most have far fewer tuners than there are satellites (usually 12, vs
30ish) they need to know in advance which satellites are overhead.

So this means a receiver needs to already know current approximate
location and the current time, and the orbit and frequency pattern
information about the satellites. This is why it takes ages for a
brand new or freshly reset GPS unit to get any sort of lock (up to an
hour).

That's not my experience, nor do I see why it needs to be like that.

You listen in on one of the frequencies (call it A) and look for a
regular "blip" (the frequency hopping sequence repeats every
millisecond). If 8 satellites are visible you should get 8 such blips at
various points in the cycle. You choose one of them and pick another
frequency (B) and start listening in an
ABBBBB...BBABBBBB...BBABBBBB...BBABBBB pattern until you get two blips
per millisecond. Hopefully only one or two satellites are using a
pattern which matches that and you can quickly find which. If the
ABBBBB... pattern gives you more than two blips, you've been unlucky and
found two satellites hitting A at exactly the same moment, but again you
pick one of the B hits and go for a third frequency.

For that matter, once you've locked in on an A signal at regular
intervals, you can simply try each of the 31 sequences and see which
ones work.

[Since there are less frequencies than there are steps in the sequence,
it's a bit more complicated than this. But that's the principle.]

I believe most of the delay in syncing up with satellites is reading all
the data from the signal, which is transmitted at a relatively slow
rate.

--
Clive D.W. Feather | Home:
Tel: +44 20 8495 6138 (work) | Web: http://www.davros.org
Fax: +44 870 051 9937 | Work:
Please reply to the Reply-To address, which is:

How GPS works (was Bus Information Signs)
 

On Mon, 28 Apr 2008, Clive D. W. Feather wrote:

In article ,
Mr Thant writes

I think how it works is the receiver has to jump between frequencies
on the same pattern as the satellite it wants to listen to.

True, I believe.

I believe it's actually CDMA rather than frequency-hopping. Although the
two are probably equivalent in some deep way.

The receiver therefore needs to know which pattern each satellite is
using,

But this will be pre-programmed in some way.

Yes, this isn't complicated. There's a pseudo-random number generator
algorithm, for which each satellite has its own seed, and the receiver
knows the algorithm and the seeds. The generator and seed are used to
produce a 1023-bit code which is used to modulate the carrier in the CDMA
scheme.

and it also needs a separate tuner for each satellite. Because
most have far fewer tuners than there are satellites (usually 12, vs
30ish) they need to know in advance which satellites are overhead.

So this means a receiver needs to already know current approximate
location and the current time, and the orbit and frequency pattern
information about the satellites. This is why it takes ages for a
brand new or freshly reset GPS unit to get any sort of lock (up to an
hour).

That's not my experience, nor do I see why it needs to be like that.

You listen in on one of the frequencies (call it A) and look for a
regular "blip" (the frequency hopping sequence repeats every
millisecond). If 8 satellites are visible you should get 8 such blips at
various points in the cycle. You choose one of them and pick another
frequency (B) and start listening in an
ABBBBB...BBABBBBB...BBABBBBB...BBABBBB pattern until you get two blips
per millisecond. Hopefully only one or two satellites are using a
pattern which matches that and you can quickly find which. If the
ABBBBB... pattern gives you more than two blips, you've been unlucky and
found two satellites hitting A at exactly the same moment, but again you
pick one of the B hits and go for a third frequency.

For that matter, once you've locked in on an A signal at regular
intervals, you can simply try each of the 31 sequences and see which
ones work.

[Since there are less frequencies than there are steps in the sequence,
it's a bit more complicated than this. But that's the principle.]

That would make perfect sense if the satellites used frequency hopping.

Rather, there's one frequency, producing a stream of bits which are the
sum of the signals from all the satellites. CDMA lets you filter that
stream and recover a single satellite's signal. Basically, the CDMA
decoder takes the raw, summed signal, plus one of the 1023-bit codes, and
gives you back the satellite signal that was modulated with that code. The
code input and satellite signal need to be in sync, so this takes some
time to get right: the modulation is at 1.023 MHz, so it takes 1 ms for a
repetition of the code, and there are 1023 possible offsets of the code
and signal inputs, which will thus take 1.023 seconds to work through.
Once you've got one, you remember it, and try another code, until you've
got enough satellites. Even running through all 30 of the satellite codes
will only take 30 seconds to do this.

I believe most of the delay in syncing up with satellites is reading all
the data from the signal, which is transmitted at a relatively slow
rate.

That's certainly what wikipedia says: the 'navigation message' goes at 50
bits per second, and carries 1500-bit frames, each of which takes 30
seconds to transmit. 600 bits of that are a segment of the 'almanac',
which is a collection of information you need to work out position; the
almanac is 15 000 bits long, so it takes 25 frames to transmit the whole
thing. That's 12.5 minutes. Once you have a copy of the almanac, you can
use the transmitted segments to keep it up to date, but you never have to
do the 12.5 minute wait again. Unless you stop getting updates for some
period of time, eg because the receiver's switched off. The almanac is
valid for 180 days, though, so if it's switched on again inside this time,
it can go straight into action.

Although the 12.5 minute figure is based on receiving all the segments one
after another. Since each satellite is transmitting segments
independently, if they were arranged cleverly, you should be able to
receive several at once, which would reduce the time taken. I don't know
if they are arranged cleverly, or if the satellites transmit in sync with
each other.

tom

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