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Nobby10
Occasional Visitor
Joined: Sep 26, 2005
Posts: 18
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 Posted: Wed May 24, 2006 12:02 am Post subject: GPS Mode: WASS / EGNOS or Normal? |
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What GPS Mode should I be using on my i3? WASS / EGNOS or Normal?
Never really been sure and of late seem to be having problems losing satelites. This is especially evident when driving in London. i.e. in between tall biuldings. A pal of mine has a Tom Tom (don't know which one) and it doesn't lose satelites in London - at all!
Bit of a choker when I've bought a Garmin! Any ideas?
P.S. I do have the latest software installed - 3.10 |
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Skippy
Pocket GPS Verifier
Joined: 24/06/2003 00:22:12
Posts: 2946
Location: Escaped to the Antipodies! 36.83°S 174.75°E
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| Posted: Wed May 24, 2006 1:31 am Post subject: Re: GPS Mode: WASS / EGNOS or Normal? |
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| Nobby10 wrote: | | of late seem to be having problems losing satelites. This is especially evident when driving in London. i.e. in between tall biuldings. |
Tall buildings cause problems for all GPS units. Put simply, if you can't see the sky very well then you will have trouble getting a position fix.
The SiRF Star III based GPS units (Nuvi and newer Bluetooth GPS units) are better at holding a fix than the older GPS units.
When a GPS reciever is "cold" (switched off for more than about 6 hours) the GPS must "see" the satellites continuously for a minute or two so it can lock on and use them to calculate your position. This initial aquisition is critical. If something keeps blocking the view then it can't aquire the satellites. Each time the view gets blocked, you have to wait another minute for the GPS to aquire the satellite.
Although the GPS only needs 3 sats for a 2 dimensional fix, it can lock onto and track 12 at once and use the best ones available. For best performance, you should stop in an open area and wait 5 minutes so it can lock on to ALL the visible satellites before you go into a difficult area. Unfortunately, Garmin have "dumbed down" the i3 and removed the satellite status screen so you can't see what is happening here. :x
Switching on WAAS will allocate two channels to tracking the WAAS/EGNOS satellites (which you have got almost no chance of seeing in Central London because they are very low in the sky) so at best it will probably make no difference and at worst you will waste two of the 12 channels that you could be using to track other more useful satellites. Outside of the heavily built-up area the WAAS setting may make a useful improvement to your GPS accuracy if you have a good view of the south eastern sky.
Once the GPS has a lock on a particular satellite, it can reaquire it almost instantly after the signal is blocked (say by a building). If you are driving into central London after being out in the open for a while then you should be OK because you have already aquired a lock on all the available satellites . The worst scenerio is when you go into London and leave your GPS off for 6 hours then try and get a fix from cold.
The really good (ie, expensive £2,000 in-car) GPS systems have "dead reckoning" which uses a speed sensor and compass to compensate for when the GPS loses lock.
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matb
Frequent Visitor
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Joined: Mar 20, 2006
Posts: 258
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| Posted: Wed May 24, 2006 7:07 am Post subject: |
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have a look around the forum as there are some really in depth discussions on this top, including links to web sites about waas and its european sister egnos.
as many of us have tried this route, not because of problems, but to make good, better, purchase one of these aerials £9.99.
once fitted then try either setting and see which best suits you.
it would appear that as described above you are suffering from "cannonying"
http://www.maplin.co.uk/search.aspx?MenuNo=1428&MenuName=GSM+%26amp%3b+GPS+Antenna&worldid=5&FromMenu=y&doy=24m5
if the problem continues then consider changing from f/w 3.1 to either 2.7 or beta 2.82. both and others, available from my sticky at the top of the forum.  |
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Pootle
Regular Visitor
Joined: Oct 25, 2005
Posts: 111
Location: Nottingham
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| Posted: Wed May 24, 2006 8:25 pm Post subject: |
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After reading opinions here and getting educated as a result, I switched my i3 to WAAS/EGNOS.... and all has been super duper, turns are more accurate etc... until today. Going towards Derby from Matlock my i3 lost sat reception twice due to terrain, Hills, Cliffs, no young ones and over hanging trees. Once it sorted itself out but the 2nd time went completely loopy and I turned it off only to turn it back on and it had me doing 363mph across country!
In general usage I would go with WAAS/EGNOS which appears to be more accurate. The next time I head out towards Matlock I'm going to give it ago with GPS to get a comparison.
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matb
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Joined: Mar 20, 2006
Posts: 258
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| Posted: Wed May 24, 2006 8:41 pm Post subject: |
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i am wassed and have been from the start.
yes more accurate but at times longer to lock on sats from a cold start and i also, once only, lost reception under very heavy tree coverage.
i do flick between the two settings but always revert back to waas.
even better with external aerial |
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