[OSM-newbies] Detect WAAS/EGNOS mode

[Lauri Hahne]

SBAS satellites generally transmit correct parameters for satellite clock,
ephemeris and ionospheric errors. Of these the ionospheric errors are the
biggest error source in regular GPS system. Ionospheric corrections are
transmitted as a map, not as single variables. The receiver locates it in
this map/grid and uses the local correction values.

The correction grid is generated from information provided by the
landstations. This method is similar to constructing a weather map and thus
the corrections you are using are always "local". If you're out of the
broadcast range, then your receiver won't do any corrections at all. Thus
all your assumptions that you're getting bad correction if you're too far
off is completely bs. The data transmitted by SBAS signals also makes it
possible to calculate accurate approximations for positioning error. Eg
EGNOS promises that using their signal, you can get about 1 metre accuracy
across Europe.

2008/4/25 Andy Robinson (blackadder) <blackadderajr at googlemail.com>:

> ael wrote:
> >Sent: 25 April 2008 12:53 PM
> >To: newbies at openstreetmap.org
> >Subject: Re: [OSM-newbies] Detect WAAS/EGNOS mode
> >
> >I am a bit rusty on WAAS, but surely a major point has been missed so
> >far? The corrections are made with one (or a very few) ground stations
> >sampling the ionsphere disturbances on the path between those points and
> >the satellites. If your unit is "seeing" a different part of the
> >ionsphere, then the corrections maybe be wrong for your path. So it
> >depends on how uniform the ionsphere is at a particular time. And how
> >close you are to a ground station.
> >
> >I may be wrong about this...
>
> As far as I know you are absolutely right. So for instance you might in
> theory pick up signals from one of the WASS satellites on the west side of
> Europe, however they would of little use because the correction basis is
> all
> calculated for the base stations scattered around North America.
>
> So for someone in Korea the same may apply if there are no base stations
> and
> designed service covering that country.
>
> Cheers
>
> Andy
>
> >
> >ael
> >
> >
> >
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>
>
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-- 
Lauri Hahne
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