[LINK] limits of technology

Gordon Keith gordonkeith at acslink.net.au
Tue Dec 12 12:00:43 AEDT 2006

On Tue, 12 Dec 2006 10:56, Stewart Fist wrote:
> I note that the Link Subject line has drifted somewhat.   My original
> question was mainly in reference to the whether the scientists were capable
> of determining the positional orbits of the satellites that were being used
> for measuring GPS and sea levels.
> I appreciate that these are two quite different processes, but the both
> depend on knowing where one or more satellites are in relationship to the
> surface of the Earth (or should that be 'Centre of the Earth') before any
> accuracy can be determined.

Wikipedia has a good discussion of GPS accuracy 

It is also worth noting that you don't need to know where the satellites _are_ 
in the relationship to the earth, but where they _were_ when the measurement 
was taken.

The chief difference is that where they _are_ relies on observation of the 
satellites position before the event and prediction of its orbit and 
behaviour. Where they _were_ is calculated by observation of the satellites 
position before and after and knowledge of its behaviour. So some of the 
errors are observable and so removable in data processing.

For high accuracy applications such as measuring sea levels, data about the 
observed orbits of the satellites is one of the data streams that feeds into 
the calculations. This data is not available in real time.


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