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    GPS selective availiability 'switched off'
    From: JC Sutherland
    Date: 2000 May 04, 18:15 EDT

     Hi all;
    Selective Availiability was so arranged that the precision of the a
    of fixes taken over a period of time could not be improved by
    With SA discontinued this is no longer the case and much smaller circles
    of probable position can be obtained by averaging techniques even
    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
    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
    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
    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
    may not be long before the first GPS assisted collision occurs! After
    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.

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