[OSM-newbies] Detect WAAS/EGNOS mode

ael law_ence.dev at ntlworld.com
Fri Apr 25 20:58:41 BST 2008


Lauri Hahne wrote:
> 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.

Thanks for the informed posting. But given that the base stations can
only sample small (integrated) portions of the ionsphere, the correction
grid must still be subject to errors, just like a weather map.
Presumably that 1 metre accuracy must be at some confidence level
reflecting the sampling and typical length and time variations in the
ionsphere.

ael




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