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GPS Dead Reckoning using G Sensor - Any takers?

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By sama, Senior Member on 18th July 2008, 11:29 AM
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We've all seen it. We're stuck in traffic and the GPS signal starts going crazy! Or we're in a tunnel, or in an urban canyon.

It is possible to navigate using purely a g-sensor. The army does this.

The G Sensor information can be combined with the GPS signal, to clean the GPS signal, and make it more accurate.

Here's a thread about dead reckoning. Now we just need a brave soul to attempt it!
 
 
19th July 2008, 11:46 AM |#2  
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I can help with this. I'm a Java programmer and can help getting the maths required to do it.
19th July 2008, 01:58 PM |#3  
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I've noticed something strange with the Tomtom7 software.

I have never used TT7 before or any other TT product, have always used Route66 on my Nokia 6110.

When I was driving in an underground tunnel, there was no signal, but the car icon on TT kept on going? I think it's just taking last average speed and continuing on until it gets a signal lock again.

I thought it was cool.
19th July 2008, 05:06 PM |#4  
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Quote:
Originally Posted by MrDerrickC

When I was driving in an underground tunnel, there was no signal, but the car icon on TT kept on going? I think it's just taking last average speed and continuing on until it gets a signal lock again.

Yes, that's right. This behavior helps in tunnels, but it's quite annoying, when stopping the car underneath a huge bridge because of a red traffic light.
The GPS signal is lost and the navigation just drives along the route assuming you are still going about the same speed and gives nice advices while you're waiting on the traffic light. Then, when you start driving again the GPS reception comes back and the navigation realizes: "Uhh, I have some mismatch - need to recalc!". This route recalculation can lead to missing navigation advices - so everything has it's pros and cons.
19th July 2008, 08:35 PM |#5  
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this is where dead reckoning would be perfect. the g sensor would say "I'm not moving" and clean the gps signal to tomtom

the way it would be done by software is that an app would read the gps signal, and the g sensor signal, then provide a new serial port for tomtom to use. to that serial port, a cleaned gps signal would be given.
21st July 2008, 08:08 AM |#6  
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Quote:
Originally Posted by sama

this is where dead reckoning would be perfect. the g sensor would say "I'm not moving" and clean the gps signal to tomtom

the way it would be done by software is that an app would read the gps signal, and the g sensor signal, then provide a new serial port for tomtom to use. to that serial port, a cleaned gps signal would be given.

Of course, the sensor wouldn't say "I'm moving/not moving", but "my situation is steady/changes so and so" (just nit-picking!)

Anyway, yes, this would be a great idea, but I really doubt we'll be able to see something useful if the gps software companies do not choose to build it in...
21st July 2008, 08:41 PM |#7  
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We don't need to wait for GPS vendors to do this, it can be done with some s/w.

I would do this myself, but I'm a Java programmer and I'll have to learn .net thinking and syntax to do it, and I honestly have no time! The only tricky bit I see is configuring a kalman filter, but not for someone that's familiar with the maths.





If you look at the picture above (taken from here, which is a detailed discussion of dead reckoning), you can see that the GPS signal stops after Kalman Filter #1, the dead reckoning block shown in blue would be the software filter.


I'm happy to help with the design, and will try my best to get the maths done as psudeo code.

so... one more time ... any takers?
21st July 2008, 08:55 PM |#8  
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ok maybe im missing something but this idea has issues with 2 things IMO

1) the G sensor will not be able to differentiate between travelling at a constant speed and not moving at all, because in both situations the G-force is zero [it can only detect acceleration/deceleration (hence accelerometer nomenclature) or phones orientation]

2) the phone may get all mixed up depending on how you mount the phone in the car (landscape, portrate, flat on dash), and if you move the phone in the car with your hand (eg you receive a call or just need to move it around in the car). lol

the way i can see to avoid problem 1 is if the G-sensor assumes that if you decelerate in a tunnel you are about to stop, if you accelerate you are moving, but the accuracy of this is affected by the hardware and problem 2, Overall i find it hard to see this working too well. *wonders how the army use it*

Defo worth a try tho is someone feels like putting the effort in
21st July 2008, 09:07 PM |#9  
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agreed regarding point 2. If you pick up the phone, you're not going to be able to use it, but then, if you're on the phone you won't be able to use the navigation either. Once the phone is stationary again, the filter would work.

Also, it's possible to detect erratic movement and use primarily the gps signal when that happens.

regarding point 1, this is where the combining magic happens. The GPS signal in a constant vector is an indication that all is good with the gps. when it starts to get erratic, we consult to the g-sensor's vector. In essence, both gps and innertia signals would be producing a stream of navigation data, and the filter would make the best of the data.

ps. left/right g is also measurable.
21st July 2008, 09:11 PM |#10  
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think of the g sensor data like a joystick top down view, recorded over time as a stream. that stream is the g-sensor's interpretation of the current path.

the gps signal, is also recorded as a stream.

overlay them, and if they agree, we're good. If one misbehaves, consult the other.
21st July 2008, 09:40 PM |#11  
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You guys forgot another problem. When you're moving into a tunnel or something the device will detect the declining of the angle of the car you're driving. So it will detect that you're actually accelrating even if you don't.
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