I regularly drive through the Madrid inner ring road tunnel (approx. 12km long, many entrances and exits, also alternative route choices, i.e., a sat nav woould be useful to strangers). On previous journeys the signal has been lost as expected. Yesterday, although there were no signal bars showing, it kept on track throughout the full length, my speed was shown, and the spoken messages were spot-on for indicating the correct lane when there is a choice of route and the exit was indicated.
How? Freak weather conditions? Have they added an additional signal (radios and mobiles work in the tunnel)? Some of the older sat navs with first generation chips had the option of dead reckoning based on connection to the odometer and motion sensors to take account of turns - doubt this applies with the bog-standard Toyota integrated TomTom
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For starters it will guess your location based on the speed and direction you were heading before the signal was lost. This is the most likely thing at work here. I suppose the integrated TomTom might have a gyroscope and possibly a link to the speed info but I doubt the latter.
The GPS signal is very weak. It would not penetrate a tunnel. In 'urban canyon' scenarios then GPS chipsets for years have processed reflected signals to increase accuracy but I again doubt this is at work in a tunnel.
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Its standard TomTom guestimate, based on mapped road and entry speed.
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Most sat navs use other inputs to guess, especially built-in ones which can use vehicle speed, reverse signal, accelerometer and steering angle to estimate where they are after GPS loss. Smartphones are probably also better for similar reasons than dedicated standalone units.
Last edited by: sooty tailpipes on Sat 22 Sep 12 at 17:05
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>> Most sat navs use other inputs to guess, especially built-in ones which can use vehicle
>> speed, reverse signal, accelerometer and steering angle to estimate where they are after GPS loss.
Only built in ones use these inputs.
>> Smartphones are probably also better for similar reasons than dedicated standalone units.
Mostly smartphones dont use the accelerometer for GPS mapping, only the GPS signal, because most are ported programmes from other devices.
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And the built in device in this thread is a TomTom unit. Not likely to have any other input but none of us know for sure.
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TomTom units dont, even the ones made for individual car makes.
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I didn't think they would - too difficult to integrate. The TT 900 when it came out had a gyroscope to help a little but most TT units didn't. And still don't.
My first sat nav (an HP iPAQ running Navman and then TT 2 with a Navman BT GPS unit) used to take an age to get a fix. Speeded up in TT2 by telling the software to tell the GPS unit where it was. Fix instant after that. But when you came out of a tunnel and had a turn soon after it didn't have a clue.
Things have moved on. Software and the ability of a GPS receiver to work out where it is quickly. And most modern GPS units now also use the Russian GLONASS. Some even the upcoming Galileo.
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My faithful old TomTom 700 ( looks like a CRT telly ) loses it's signal in tunnels and is refreshingly honest about it. It just sort shrugs it's little electronic shoulders, downs tools and petulantly refuses to even guess where it is.
The built in system in my new car though at least pretends to know where it's going in tunnels. I went under the Dartford crossing the other night and it confidently kept showing my location. It's either jolly clever or maybe it's just a good bull****** !
Last edited by: VxFan on Sun 23 Sep 12 at 00:18
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Our Up! came with 'maps & more' as standard, which is basically a Navigon unit that mounts on the dashboard. It definitely takes a data feed from the CANBUS because the trip computer stuff can be displayed on it, along with various things like revs and water temperature. Whether that also links to the sat nav bit I dont know - the car has only done 200 miles and we've only used the nav to test which way it sent us to get to Guildford. Actually offered three options, one of which was the right one IMO (and didn't involve the Hindhead tunnel...) :-)
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>> Most sat navs use other inputs to guess, especially built-in ones which can use vehicle
>> speed, reverse signal, accelerometer and steering angle to estimate where they are after GPS loss.
Only built in ones use these inputs. I said that, but so do many aftermarket ones, not TOM TOM or Halfords, I'm talking Pioneer, Becker etc...
>> Smartphones are probably also better for similar reasons than dedicated standalone units.
>Mostly smartphones dont use the accelerometer for GPS mapping, only the GPS >signal, because most are ported programmes from other devices.
But iOS/Android location APIs don't just use GPS, they use assisted GPS and cell-tower and wifi.
Last edited by: sooty tailpipes on Sat 22 Sep 12 at 19:22
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With all respect, can't you folk use a map?
It seems like if the GPS/TomTom/SatNav goes south, then the world will end!
Yes, I use my on-phone navigator if I'm in an area I don't know, but to get close-by, I normally have a damned-good idea where I'm heading to, area-wise.
Or am I just being old-fashioned? I can tell the time, and tie shoelaces as well, arts which seem to be lost on the younger generation.
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You make a fair point Ian. Indeed I can use a map. However, if I'm going somewhere I've not been before and sat-nav is available, then why not use it? It's just easier.
Also, I don't just use it to find my way. Often I know exactly where I want to go but the system in my car is "smart" and can "see" traffic jams ahead etc and suggests alternative routes. Very handy. Maps don't generally do that. Plus, it also continuously re-calculates how long the journey is likely to take given the current traffic conditions. That too can be very useful if working to a schedule of appointments.
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Humph, I'm all for the technology, and happy to have it, but you have raised the important point - you CAN use a map. and if push comes to shove can use one.
How many of the modern generation can't?
And as a result, they are stuffed when the satnav plays up, or they happen to be on foot somewhere?
It isn't me being a luddite, it is more my concern that everything is too easy these days, so therefore the basic skills are lost.
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>> It isn't me being a luddite, it is more my concern that everything is too
>> easy these days, so therefore the basic skills are lost.
But there are loads of life skills we have forgotten. How do you knap a flint arrow head? How many know how to hunt, trap, skin, gut and cook food on the paw or hoof? Who knows which fun-guys are killers or edible, or the berries?
Last edited by: Zero on Sat 22 Sep 12 at 20:29
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>> Humph, I'm all for the technology, and happy to have it, but you have raised
>> the important point - you CAN use a map. and if push comes to shove
>> can use one.
>> How many of the modern generation can't?
>> And as a result, they are stuffed when the satnav plays up, or they happen
>> to be on foot somewhere?
>>
>> It isn't me being a luddite, it is more my concern that everything is too
>> easy these days, so therefore the basic skills are lost.
>>
>>
I would think the same as those without a map and when sat nav hadn't been invented back in the day, muddle through, ask people, bit of guess work etc.
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>> With all respect, can't you folk use a map?
>>
It's so easy to get lost in a tunnel.
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>> It's so easy to get lost in a tunnel.
>>
Too true, I have been in tunnels with road junctions complete with traffic lights, but they did have direction signs. :-)
Last edited by: Old Navy on Mon 24 Sep 12 at 19:44
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Not driven in Boston in the last few years !
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>> Not driven in Boston in the last few years !
>>
Or Monte Carlo.
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>> Not driven in Boston in the last few years !
>>
Oh, that verdammte freeway system under the downtown area. How there aren't more mass pile-ups is nothing short of a miracle. Whoever designed it was taking something a little stronger than desirable...
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I drove through when it was under construction and only partly open in 2008. After a long flight just what one wanted. I thought I was sorted with a pre-programmed route in my TT.
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>> With all respect, can't you folk use a map?
Not in a tunnel if the interior light isn't working.
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Not everyone drives Vauxhalls Dave...
:-)
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That was almost as funny as my comment - NOT
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>> But iOS/Android location APIs don't just use GPS, they use assisted GPS and cell-tower and
>> wifi.
As I said, most Sat Nav applications are simply ported and don't use those API's, because the ported programme does not have them, they are simply no-where near accurate enough for car sat nav, and even if they did, how does cell tower and wifi permeate the tunnel?
Honestly, its a simple guestimate, it works well, so there is no need to complicate it.
And most Becker, Blaupunkt, Pioneer & Clarion after market units now merely rely on GPS and have no other inputs.
Last edited by: Zero on Sat 22 Sep 12 at 20:08
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Had to use a compass today on our walk. Albeit in my iPhone but at least I knew we were heading the wrong way....
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My mate swears he can berth his cruiser solely on GPS when the "war games" are being played off the West Coast of Scotland. He reckons the signal is turned up a notch for the military.
I am amazed however at the number of commuters who slavishly stick the satnav to the windscreen when they are on a familiar journey.
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>> My mate swears he can berth his cruiser solely on GPS when the "war games"
>> are being played off the West Coast of Scotland. He reckons the signal is turned
>> up a notch for the military.
I wouldnt rely on that, the Navy have "turned off GPS"* in that area from time to time to screw up the Russians. (and practise war games in an EMP simulation)
* its really really really easy to swamp GPS signals.
Last edited by: Zero on Sun 23 Sep 12 at 11:33
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>>
>> As I said, most Sat Nav applications are simply ported and don't use those API's,
They don't need to. They use the provided location API and the device's implementation handles how it gets a location, using the various mechanisms available to it, transparently to the application. They do not get down and dirty with the GPS chipset directly, in fact they could not do so on an unrooted device.
Road tunnels tend to have GSM access points (and often WiFi for maintenance types) inside them.
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>Road tunnels tend to have GSM access points (and often WiFi for maintenance types) inside them.
The GSM in the tunnel is one cell, the wifi is one access point, all telling you you are in the tunnel. Useful.
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>>Mostly smartphones dont use the accelerometer for GPS mapping, only the GPS signal, because most are ported programmes from other devices.
If that's the case, then a better map application is required. Most have an onboard accelerometer these days and not using it in the application is criminal.
Where they really score over GPS units is the additional info available via WiFi and cellular comms. They can get a good idea of where they are via cell location and surrounding WiFi points. Also they can download the current GPS almanac over broadband quickly rather than waiting for it to trickle in from the first satellite signal they get. With that combination of a closeish fix and an up to date almanac, they then know exactly which satellites they should be able to get a signal from and can get a GPS fix in seconds from a cold start.
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The additional info does not amount to a pile of beans. Cellular triangulation is far less accurate than GPS, so hardly worth bothering with, Wifi ditto at 70mph. GPS is more than accurate enough, there is no need to implement it with accelerometers, there really isnt any need to add expense and reliance on hardware when the GPS works well enough.
instant data will (and is) come to them of course, but only for "around me" augmented reality facilities (and advertising of course)
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Another consideration is processing overhead. You don't want to be towing your battery.
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As a follow-up to the OP, I did the return trip via Madrid today, and note the GPS was working all through the tunnel. I disconnected it from the dashboard and used it as a hand-held and it was still working (plenty of time as the tunnel had sprung a leak and 5 lanes were having to filter into 2 slowly!); so it appears they've managed to incorporate a navigation signal in addition to the existing radio and mobile telephone signals.
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>> The additional info does not amount to a pile of beans.
As I said, it does for that time to first fix. On a cold start, giving the chipset a vague idea of where it is and an up-to-date almanac to hand gets that down to single-digit numbers of seconds. This is critical for smartphones.
In car units cheat a bit by having the luxury of being able to splash a few milliamps on keeping the chipset warm and updated when it's not in use, courtesy of that monster ampere-houred lead-acid confection under the bonnet. As most cars are subject to daily use, this means that a fix can be had pretty much instantly as it still has an updated almanac and knows where it is. This is also why, when you've left the thing in an underground carpark for a day or two, you're back to waiting a minute or so for that fix as it's been incommunicado for the duration. A data connection and initial cellular fix would speed this up no end.
I'll agree that once that fix is aquired the additional data available makes no odds either way.
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Sat navs in tunnels - rtj70
