OT : How does NAV system work underground?

Verne · 29th November 2001, 03:22

Hi Everyone.

I'm not too sure if this topic has been discussed before, but its been bugging me ever since my father purchased a (E38) 728i this year with a Sat-Nav system.

How does the Nav system work underground? Like if you're travelling in a tunnel for example? From what i know, the GPS relies on uplinks to three satellites so as to triangulate one's position on the ground. Or at least, thats how those handheld ones work. Is it any different from the ones vehicles are using? If they aren't, how is it possible that the system is still able to track your position when in a tunnel.

Unless the satellites don't use line-of-sight (which errmm, sounds weird to me, but i'm not a tech junkie), i can't really think of a way that the Sat Nav is able to update the car's position in a tunnel. Can the system switch over to another mode whilst underground? For example, using the speedo/odometer as well as the steering to calculate how far and which way you're heading underground. Or does it just assume that you won't veer off the tunnel underground? But that doesn't explain how it tracks u coming out thru' different exits of the tunnel?

Well, hope some of you guys can enlighten me cause its reallie puzzling everytime i drive that car thru a tunnel and still see the sat-nav system tracking. Its probably some technical thing i'm unaware of but i'll be grateful to learn how the system works anyway.

Regards,
Verne

tfung · 29th November 2001, 03:57

good question.. I always wondered about that as well.. can anyone shed some light on this? My theory is that even though you don't use your nav system all the time, it constantly updates your position through the gsp. When you go underground or in a garage, it takes the last known position and calculates where you are, by the distance you drove and also what steering inputs you make.... And it updates your location according to that information...

Verne · 29th November 2001, 04:12

That theory was bouncing around my brain quite a bit. And if it really is fact, then i'm pretty amazed at how the programmers of the on-board computer at BMW take into account so many factors. Like, when contact with the satellites were lost and steering input to calculate direction, which i think isn't a very simple task.

Regards,
Verne

rvacha · 29th November 2001, 04:12

Hi Verne-
I cannot speak for the exact method(s) used by the BMW system, but all navigation systems basically work the same way. They augment GPS with "dead reckoning" techniques. The two most common are accelerometers and wheel rotation counters. Accelerometers are sensors that detect acceleration and deceleration of the vehicle. They can tell if the car is accelerating or braking, turning right or left, and at what rate these things are happening. They are handy sensors but are subject to so-called drift errors. Basically what happens is this: if you are cruising at a steady speed, then your acceleration should be exactly zero. The sensor will have small errors that cause the nav system to believe that the car is slowly increasing or decreasing in speed. After some length of time, it will eventually falsely deduce that the car is either stopped or maxed out (at 155MPH or so....). G-tech users have no doubt seen inaccuracies caused by these drift errors. As a sanity check, the system usually consults with wheel rotation sensors, which give very accurate speed readings but are weak at sensing acceleration. The wheel rotation sensors are the same sensors utilized by the ABS/DSC system and are therefore "free" (no additional system cost). With these two tools, the nav system can guess with a fair amount of accuracy where it is. The last trick is software which is written to "snap" to a road. As the errors build up, the system would begin to show you driving in absurd locations such as lakes, ponds, and desert sand dunes. The software recognizes the silliness of the situation and periodically relocates your vehicle to the closest road ("snap"!) that makes sense based on your bearing. Dead reckoning is an important part of nav systems not only because of tunnels as you mention, but also because GPS receivers can take a long time to "lock on" after the satellite signals have been lost. The signals can be lost while the car is parked in a garage, the battery is replaced, etc

The system is never perfect, however. My beast routinely loses contact with GPS after parked in my garage for a while. Since I live "off the digitized map area", the system can drift way off target as there are no known roads to "snap" to. Once I finally get to an area that contains mapped roads, the system will eventually snap to the correct road and get its bearings

Verne · 29th November 2001, 04:23

Thanx for the reply!

So if i'm reading you correctly. When the car is underground, its most probable that the system has lost contact with the satellites and is using the 'dead reckoning' programming. It has to be pretty accurate then in order to know which exit you're exiting from out of the tunnel, especially if they're multiple exits. Kudos to the programmers in these systems then.

Well, as for the "snap" feature, i'm not too sure about that. Because there was once i was forced to make a detour on a temporary road (which obviously wasn't included in the map) and the GPS just showed me driving over grass. Does that mean that the snap feature only works when one has lost contact with the satellites? And if contact is maintained, then the system will try to pinpoint your exact position whether you're on or off-road?

I guess people with X5s that go frequently off road would be able to tell us.

Regards,
Verne

rvacha · 29th November 2001, 04:53

The snap feature does not happen instantaneously. If it did, it would incorrectly snap you to the wrong road evertime you drove on a road that was not in the database. It will wait a while before deciding that something is wrong. I assume (but do not know for sure) that the snap algorithm varies from vehicle to vehicle - SUVs are probably less likely to snap to road quickly than say, an M5. I also assume that the snap feature is rather intelligent. If it figures that the dead recking accumulated errors can only amount to 1/4 mile error, it probably would not attempt to snap you to a road that is, say, one mile away from your currently assumed location

tonyo · 29th November 2001, 06:01

There is also a gyro that may help the dead-reckoning algorithm.

Need4Spd · 29th November 2001, 08:05

I think the NAV is tied into speedo, accelerometer, and/or other sensors so it keeps track of changes since last link and knows roughly where you are. This is one of the advantages of a built-in vs. handheld Nav (like a Magellan).

Karl G · 29th November 2001, 09:51

Need4Spd, you're right - the built-in nav takes advantage of the wheel speed sensors and a built-in gyroscope to predict the motion of the car when contact with a satellite cannot be established, as well as augmenting its sensitivity during normal operational conditions. Handheld and "on-dash mounted" GPS systems sometimes use accelerometers to perform the same function - but they're usually a bit more inaccurate.

blue max · 29th November 2001, 10:03

That explains why my NAV was totally confused after a 45 minute ferry crossing. It calculated my distance from home as 400 + miles when in reality I was only 60 miles away. But the vehicle location was accurate. After about 1/2 hour of driving, the system self-corrected.

Cheers.