Satellite Navigation on the Underground

[MarkS]

The firm working on the [Jubilee] line has admitted it needs at least 10 weekend shutdowns next year to introduce a satellite navigation signalling system.

What?  Satellite Navigation?  How does that work then?

http://www.thisislondon.co.uk/standard/article-23774039-another-six-months-of-closures-on-jubilee-line-to-finish-botched-upgrade.do

[Mac]

Thats realy weird, because they have spent the last three years installing a totally different system. it similar but not GPS.
anyway, the way the GPS system works.

The driver eneter his destination in to  tube tube, Just kidding.

In a nutshell

There is a main central computer, that has a detailed 3d plan of the entire Line that the train is running on.
Down to the mm!!
This includes all the curves, gradients uphill and downhill, line speed.
It also holds the specification of the trains are also loaded, Acceleration, breaking efficiency, weight, etc etc.

The train takes its gps position and the relays this information back to the computer.
So the computer now knows where each and every train is.

Using this model it can keep all the trains a safe minimum distance apart for any given speeds.
After calculating the parameters for accelerating, breaking, coasting ect, this information is then sent to each train.
and the trains control themselves.

There are also position detectors that are installed along the lines,normally at stations, which give an exact position back to the computer,
should the gps system fail.

They are using this system in canada,

But not on the Jubilee.

The Jubilee is using the same system as the docklands

Which is phase loop detection,
a twin cable is laid in the track bed (4 foot)
and every 7 meters the cable is crossed over itself,
7 meters later its crossed back.
detectors in the train read the data overlaid in this cable and count the phase changes (every 7 meters).
The difference is the computer is on the train.
and the trains know the positions of every other train.

Again there are position detectors at each station to reset the position of the train.

The cool thing, is if one train is stationary, then the train behind can creep up on it.
Technically until its touching the train in front.
But they programme larger margins into the distances.

Mac.

[Rick]

should the gps system fail

...or, indeed, fail ever to work at all, what with there being a fair bit of rock in the way of the GPS satellite signals whenever the trains are in a tunnel.

I guess this is a case of acronym collision, or possibly a typo, because it makes absolutely no sense at all to have trains running underground relying on GPS for positioning if GPS refers to the satellite-based Global Positioning System.

[mickw]

They're actually calling it a Satellite Navigation System, perhaps satellite as in "remote".

Looks like the master plan is for automated trains on the Jubilee Line 

[Rick]

They're actually calling it a Satellite Navigation System, perhaps satellite as in "remote".

Thought up by some technically-ignorant manager or marketing droid, presumably... 

Looks like the master plan is for automated trains on the Jubilee Line 

Well, at least the drivers won't go on strike, then... 

[Mac]

They're actually calling it a Satellite Navigation System, perhaps satellite as in "remote".

Nope. the system actually does rely on the GPS system, (Canadian one that is)
Your are correct in the fact that GPS dosent work underground, thats why the Jubilee system is totally different and dosen't use the GPS system.

If the GPS system goes down (loss of signal) tunnel ect, the system then relies on the position detectors, to locate the position of the train.

Because the rail system in canada has something like 2-3 trains an hour the safety margin is quite large,
with say position detectors every 5-10 miles, so they know where the trains are to the nearest 5 miles.

On the underground they have currently 28 trains an hour, which they are trying to get up to 35 trains an hour.

If it was using the GPS system and there was a failure then it would grind to a halt. that's why they are using the phase loop system.


Quick school on railway signalling so its easier to understand.

london underground, BR and probably 70% of the modern world us a system called fixed block signalling.
where each signal is controlled by the tracks.

one track is a fixed block, i.e. it cant change its length or position.
8 represents a signal.
¦ is a break in the track which isolates it from the next one.
* is the overlap of the signal. which is where the train will stop should it pass a signal at danger (SPAD)



                  ******                ******                                  ******
train             8                         8                                           8
-----¦--------¦----------¦-------¦-------¦--------¦------¦-------¦-------¦-------¦
 A        B             C           D         E             F        G         H          I

Tracks are numbered A-I
the limit of the control of the signal is up to and including the overlap of the signal ahead.
for the first signal to show a green  tracks C, D, and E have to be clear.
for the second signal tracks E, F, G, H, & I have to be clear.

If a train is on E track, the first two signals are at danger.
On BR the lengths of these tracks vary from 1/2 mile up to 4-5 miles each!
the distance between the signals also vary.

London underground these track circuits vary between 50-300 m in length and the distance between the signals are probably no more then 500m -800m.
THESE DISTANCES ARE FIXED!!!!
these distances are calculated using the line speed, and breaking efficiency of the trains, ect.

Moving block systems
These systems rely on the fact that there are no fixed track circuits or signals (sort of).
The position of the train is determined by various methods, GPS, phase loop, position detectors, ect.
The speed of the train, track, gradient ect, ect is calculated to that a safe distance between it and the next train is kept.
The train is then controlled via computer to maintain this distance.
If the train in front slows down, then the train behind has to slow down, but the distance between them can now decrease
as the stopping distance has changed.

ie,



Train 1                                                                                 Train 2
---------------------------------------------------------------------------

Train2 80 mph, Train 1 80mph, safe distance 2 miles

Train two now slows to 30mph,


Train 1                      Train 2
---------------------------------------------------------------------------

Train 1 will also slow to 30mph, but the safe distance has now reduced to say 1/2 mile.
so train 1 will catch up to train2 and maintain that safe distance.
so you can actually get more trains in each area. safely.

Look at London bridge and any LU station, all the trains a re following each other just waiting for the train ahead to clear the limit of controll.
If the systems were moving block, then the trains could creep up on each other whilst the first one was unloading.

PS
It my Job.
I was involved in the surveys for the Jubilee and the northern lines. (Yup they are getting the same system as well)
as well as installing the test track at Highgate. (the one that mayor Livingston rode all those years ago)

If you want an in-depth chat on railway signalling systems, i can bore you to tears on them.

Well, at least the drivers won't go on strike, then

wrong, the trains still need a driver. although he will be there in event of a system failure, he also there for public reassurance.
its a requirement

[mickw]

the system actually does rely on the GPS system, (Canadian one that is)

The Canadians have GPS that works through rock ? 

[Mac]

nope, 95% of their rail system is overground!!
the rest is tunnels, ect.

Thats why the use the position detectors (axel counters) at things like tunnels so that they know where the train is.

They count them in, they count them out.

http://findarticles.com/p/articles/mi_m0BQQ/is_6_42/ai_88099062/
http://www.transportation.siemens.com/en/data/pdf/ts_ra/produkte/ds_azs350u_en.pdf

[mickw]

So "GPS" could actually be used on parts of Metropolitan and District lines.
All seems pretty daft to have 2 safety systems that are not a "reduntant" systems.
It's like saying if your brakes don't work, screaming helps 

Axle counting seems to make sense as does fixed block.

Was GPS just a bad description of the system ?

[Rick]

Nope. the system actually does rely on the GPS system, (Canadian one that is)

Errr... I must've missed the junction. When did we start talking about the Canadian railway system?

wrong, the trains still need a driver. although he will be there in event of a system failure, he also there for public reassurance. its a requirement

...but the trains would in fact run quite happily without one, so it's a bit like the "man walking with a red flag" requirement...

[Mac]

So "GPS" could actually be used on parts of Metropolitan and District lines.

in therory it could, but you then have the problem of the tunnels again.

the main problem is the frequency of the train service 35 an hour as opposed to 2 or 3.

the met and district are using a different system again.
but its still moving block.

All seems pretty daft to have 2 safety systems that are not a "reduntant" systems

they are a redundant system.

if the gps system fails the system will still work with the axel counters.

also each axel counter is two separate counters, so if one fails the other will work.
There are circuits to check each system works.

British rail, even have detection systems for bulb failures.

Was GPS just a bad description of the system ?

yup, pretty much so.
To say the technology is based on GPS is probably a "WOW" factor.

Railway signalling is designed to fail safe.
the computers have their own safety checking,
(A completely duplicate computer running on exactly the same inputs)
Both of the have to agree for the system to  work. If either one gives a different output the system shuts down.
This is normally blowing an internal board fuse, which cant be changed!!!

If this fails then you have a serious problem.
and the entire systems shuts down for obvious reasons,

The trains (with drivers) then revert to the old rule system, of point to point working.

Errr... I must've missed the junction. When did we start talking about the Canadian railway system?

nope the fact is that they said the Jubilee is working with the GPS system, but is not, the canadian is. (Huge open spaces, 95% above ground, ect)

but the trains would in fact run quite happily without one, so it's a bit like the "man walking with a red flag" requirement..

true, but if the system failed in a tunnel with no driver, your F*****d, its pretty hot in those tunnels.
No one to tell you if its safe or not to leave the train.

Question, what would you do today if you saw the driver of a Victoria line train get out of his cab, close the doors, and watch the train leave the platform?
Probably worry. but the fact is the Victoria trains have been driver less since they were installed. but every time you see on there is a driver sitting there.
LUL require every train to have a driver to assist passengers in the case of emergency's.

So even the Jubilee trains will still have drivers, The DLR still have drivers, except they open and close the doors most of the time.

Mac.