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 Hi all;
Selective Availiability was so arranged that the precision of the a
group
of fixes taken over a period of time could not be improved by
statistical
methods.
With SA discontinued this is no longer the case and much smaller circles
of probable position can be obtained by averaging techniques even
without
P-code reception.
Yesterday I set up my GPS at home and took fixes approximately every 10
mins throughout the afternoon, over a period of about 10 hrs. taking a
total of 50 readings.
This data gave a Probable Error of +/- 14.7 meters with a maximum
diffference of 65 metres from the mean position. However this mean
position was in error by 40 metres. But of course this  could be  an
error in  the map I was using.
Later last night when the atmosphere was less turbulent I took 50 more
readings as rapidly as I could, (about 6 secs between each).
This second set gave a Probable Error of +/-1.5 metres and the mean fix
was closer to the map position.
I have drawn some interesting conclusions from this data which I would
like to pass on for discussion.
POINT 1
The more fixes you average the more precise you will think you will know
your position. Also the more rapidly you take fixes the smaller will be
the resultant scatter in the data. BUT THERE IS NO GUARANTEE THAT THE
SMALLER GROUP WILL BE ANY MORE ACCURATE! In fact, because of the physics
of propagation the smaller group could  just as likely to be centered
around a deflected position.
POINT 2
The propagation of radio waves through the atmosphere is dependant on
the
ionization of the outer layers and this ionization is affected by Solar
activity. We will need to be more consious of Solar flare predictions
before we belive in this apparantly improved accuracy.
POINT 3
What is the point of knowing your position in Lat and Long more
precisely
than the quality of the Chart or Map you are using. Sandbanks, mudflats
and estuary bars as well as navigation buoys are known to move about.
POINT 4
A lot of almanacs and  pilot books are giving lists of Waypoints. Some
of
these are inevitably in strong tide areas and consequently many boats
could be aiming for the same waypoint at the same time. Now that the
autopilot position fixing could be accurate to less than a boat length
it
may not be long before the first GPS assisted collision occurs! After
all
if all GPS sets are receiving the same data at the same time they will
all have the same position error!
The moral is "Choose carefully your own waypoints" .
Hope this is food for thought.
Regards
CLIVE

   

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