[OSM-newbies] Consistently off tracks on second or third tracking

[Harald Kirsch]

Am 20.01.2008 23:03 schrieb Franc Carter:
> Out of curiosity, how do you know it's off,

If I ride three times the same street and no two recorded tracks agree
with each other, then obviously at least two of them are off. In my
original post I was not suggesting that one of them is 'the truth'.

HaraldK

> cheers
> 
> On Jan 21, 2008 7:48 AM, Nick Black <nickblack1 at gmail.com> wrote:
> 
>> On Jan 20, 2008 6:36 PM, Lauri Hahne <lauri.hahne at gmail.com> wrote:
>>> On 20/01/2008, Lars Aronsson <lars at aronsson.se> wrote:
>>>> Harald Kirsch wrote:
>>>>
>>>>> Now, if the GPS would produce a Gaussian error, I would expect a
>>>>> track to zigzag left and right from an older track, but
>>>>> obviously this is not the case.
>>>> The error is random (Gaussian or not), but not in every sample.
>>>> Satellites drift slightly out off course, and are corrected into
>>>> the right orbit once in a while.  You can get exactly the correct
>>>> position relative to the satellite, but the satellite can be 7
>>>> metres out of its planned orbit.  So your position might also be 7
>>>> metres off.  If you could know how much off each satellite is at
>>>> each moment, you might be able to calculate your correct position
>>>> with much higher accuracy.  There are several schemes for this,
>>>> some are military and secret, one is called "WAAS", another is
>>>> called EGNOS.  You can learn more by googling these terms.
>>>>
>>> Actually all GPS satellites broadcast correction parametres for their
>>> orbits. So that isn't a problem.
>> Yes it is - errors occur because of misjudged orbital paths.  The
>> signal broadcast by Navastar contains an estimate of the orbit, not
>> the exact orbit.
>>
>>> WAAS and EGNOS (together known as SBAS) broadcasts information about
>>> ionospheric errors which amount to most error there is. This
>>> information is collected by ground stations who measure their position
>>> and compare that to their known location.
>>>
>>> A quick googling found the following page
>>> http://www.kowoma.de/en/gps/errors.htm It isn't a very accurate paper
>>> but it gives you the basics.
>>>
>>>
>>> --
>>> Lauri Hahne
>>>
>>>
>>> _______________________________________________
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>>>
>>
>>
>> --
>> Nick Black
>> --------------------------------
>> http://www.blacksworld.net
>>
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>>
> 
> 
> 
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Harald Kirsch | pifpafpuf bei gmx punkt de