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TT GO500 New user's first real test - Could have done better

 
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ciderbloke
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Joined: Jul 17, 2005
Posts: 9

PostPosted: Tue Jul 19, 2005 2:33 pm    Post subject: TT GO500 New user's first real test - Could have done better Reply with quote

Purchased my TT GO 500 last week mainly to avoid public transport in London since the events of 7th July. I figured since the company is paying my mileage I may as well travel in relative safety.

My knowledge of the roads in London is very poor but having used the TT to get me to a few destinations last week with no hitches in the Herts / Beds / Bucks area I set off confidently for a meeting in the Aldgate area of London this morning.

Yet again the TT performed brilliantly, the voice prompts combined with the 3D layout of the road taking me faultlessly from J2 of the M1 through the rush hour traffic. Till I got to within about half a mile of my destination....

TT had a strong GPS signal but suddenly decided I was on a road that was running parallel to the one I was actually on, and recalculated my route, asking me to take a non existent left turn. I carried straight on and TT recalculated the route but said I would go through the congestion zone, which I was outside of on my original route.

I refused the route and it suggested a non toll alternative but within a couple of turns again decided I was on a different road to that I was actually on, asking me to take the 3rd exit of a non existent roundabout in front of me and once again recalculating the route as I drove on over the imaginary turn.

TT recalulated once again and once more wanted to take me into the congestion zone. Frustrated and anxious to reach my destination I accepted and from there TT made the final few turns OK.

Same thing happened when i left. Strong GPS signal yet TT seemed to be out of synch in the Aldgate area with my actual position on the map and recalculated the route 3 times after I ignored it's instructions to take non existent turns but once into North London it again performed brilliantly.

Well, mission accomplished ( I guess ). TT took me to Aldgate and home so is fit for purpose, and i'm still generally happy with it. BUT, it's fair to say it left me in the lurch for a couple of hectic miles just when I needed it most and that £8 congestion charge hasn't impressed me.

Cheers
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ridcully
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Joined: Jun 25, 2005
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PostPosted: Tue Jul 19, 2005 6:06 pm    Post subject: Reply with quote

I drive arround the same area in central London as you and yes mine does sometimes get confused, changing its mind on what road you are on. I put this down to reflections from tall buildings. It is especially worse on large smooth glass buildings. Also you dont realise how many trees and bridges there are in London untill you get a GPS.

Ian
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ned1
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Joined: Jun 15, 2005
Posts: 161
Location: Herts

PostPosted: Tue Jul 19, 2005 7:18 pm    Post subject: Reply with quote

Ciderbloke,
Ian has a point because I was on a slip road off my route by about 20ft and my ttg recalculated the route before I was on the rounabout over the top of the road I just came off.
Ned
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sgould
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Joined: Oct 05, 2004
Posts: 1320

PostPosted: Tue Jul 19, 2005 8:28 pm    Post subject: Reply with quote

If you need a better accuracy of position and faster locking, you probably need the external aerial.
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ciderbloke
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Joined: Jul 17, 2005
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PostPosted: Tue Jul 19, 2005 11:08 pm    Post subject: Reply with quote

Thanks for the feedback. I'm not sure that I need an external antenna as TT performs faultlessly elsewhere.

Was interested by Ian's comments about driving in the same area and his theory that glass buildings etc add to the problem. I had just turned into Old Street past a lot of metal and glass buildings when TT threw a wobbly, but at all times the signal was strong.

Does anyone know how these type of structures could affect the signal? I guess the TT is merely a receiver and if the GPS signals are being reflected off a lot of glass and metal the TT's mapping could get confused as to your location on the ground? Any views on this please?

Sorry for newbie questions, still much to learn !!!

Dave.
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smallfreak
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Joined: May 31, 2005
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PostPosted: Wed Jul 20, 2005 8:03 am    Post subject: Reply with quote

ciderbloke wrote:
Does anyone know how these type of structures could affect the signal? I guess the TT is merely a receiver and if the GPS signals are being reflected off a lot of glass and metal the TT's mapping could get confused as to your location on the ground? Any views on this please?

Refrection is a bad problem as it both introduces interference and it does lengthen the signal path. GPS works by measuring the signal runtime from the satellites to the receiver. If for instance the sattelite is left of you but blocked by a tall building and in addition there is another tall building right of you, then the signal still gets to the antenna by reflection. This signal path is a bit longer and the GPS processor calculates a position a little to the right of the true position.

Certainly things are a bit more complicated because there are several satellites that sum up to a certain position, but it still can cause you off by a couple of meters. This could only be avoided, if someone installs local GPS signals on the ground for difference measurements in such regions. The GPS processor on the GO would be able to use such DGPS signals, but I'm not aware that such installments have been made anywhere on earth yet.
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tabatha
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Joined: Jul 14, 2005
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PostPosted: Thu Jul 21, 2005 11:49 am    Post subject: Reply with quote

smallfreak wrote:
ciderbloke wrote:
Does anyone know how these type of structures could affect the signal? I guess the TT is merely a receiver and if the GPS signals are being reflected off a lot of glass and metal the TT's mapping could get confused as to your location on the ground? Any views on this please?

Refrection is a bad problem as it both introduces interference and it does lengthen the signal path. GPS works by measuring the signal runtime from the satellites to the receiver. If for instance the sattelite is left of you but blocked by a tall building and in addition there is another tall building right of you, then the signal still gets to the antenna by reflection. This signal path is a bit longer and the GPS processor calculates a position a little to the right of the true position.

Certainly things are a bit more complicated because there are several satellites that sum up to a certain position, but it still can cause you off by a couple of meters. This could only be avoided, if someone installs local GPS signals on the ground for difference measurements in such regions. The GPS processor on the GO would be able to use such DGPS signals, but I'm not aware that such installments have been made anywhere on earth yet.


How much error would be introduced by an extra few feet of delay caused by a reflection considering that the signal has already travelled around 12,000 miles or more?
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ciderbloke
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PostPosted: Thu Jul 21, 2005 12:20 pm    Post subject: Reply with quote

Fair point.

It seemed to me, as a layman, that the TT knew my position in terms of Longtitude and Latitude well enough, but the map which is ''overlaid'' onto my position was slightly out of synch, and screwed the TT's route planning.

I guess these maps have to be overlaid pretty accurately, especially in an area as tightly laid out as Central London.
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Snig
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Joined: Nov 23, 2004
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Location: Solihull, UK

PostPosted: Thu Jul 21, 2005 1:07 pm    Post subject: Reply with quote

tabatha wrote:
How much error would be introduced by an extra few feet of delay caused by a reflection considering that the signal has already travelled around 12,000 miles or more?


I might be missing something but, since GPS works on the time for the signal to travel from satellite to receiver, wouldn't adding 50 feet (say) to the path put your position out by 50 feet? The 12,000 miles makes no difference.

Regards,
Mark.
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smallfreak
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PostPosted: Thu Jul 21, 2005 1:36 pm    Post subject: Reply with quote

tabatha wrote:
How much error would be introduced by an extra few feet of delay caused by a reflection considering that the signal has already travelled around 12,000 miles or more?


If the signal path is "just a few feet" longer, the processor calculates your position wrong by "just a few feet". The GPS device very accurately measures the true distance between the receiver and the satellite. It's the absolute value of the distance that counts here.

Depending on the position of the satellite in the sky with respect of you, the calculation puts you off track along the line satellite - antenna. If for instance the satellite ist straight above you, prolonging the signal path by 1 meter results in a difference in "height" by that one meter (you seem to be below ground). If the satellite in question is just grazing the western horizon, a reflection that extends the signal path by 30 feet, will calculate your position 30 feet further to the east.

To get your true position, several satellites contribute to the calculation (at least 3) but chances are good that in this case the signal paths of more than one satellite is distorted and the calculated position is quite off your true one. By "weighting" each measurement and averaging the results, the GPS device will still put you roughly in the correct area, but the uncertainty grows considerably.

More or less "intelligent guessing" about where you are heading might cover the problem a long time as the software assumes you have to be "somewhere on the road", but in dense areas it might think you are on a different road if the error is big enough. This leaves room for enhancement with additional constraints like "when driving, it is unlikely that someone jumps between roads, so assume to be on the same road until a junction and a measured turn" or "cars do not jump off bridges to a road underneath" or even "when on a motorway, there is no chance jumping off to a parallel road (there may ba a chance to 'jump' to the other lane if you have a road repair)"
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smallfreak
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PostPosted: Thu Jul 21, 2005 1:48 pm    Post subject: Reply with quote

ciderbloke wrote:
I guess these maps have to be overlaid pretty accurately, especially in an area as tightly laid out as Central London.

Both the map and the measured position must match very precisely which is even more problematic in areas with many alternative roads. The sum of all errors mus not exceed half the distance to the nearest alternative road or the GO will place you there.

Reflection, ionospheric bloating due to the solar radiation, thick clouds,.. all affect signal run time. And the signal itself is so weak.. It's already like magic that this little piece still guesses the location quite good most of the time.
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