"Ian Johnston" wrote in message
news:cCUlhtvFIYkV-pn2-KSqR179sj7HU@localhost...

You have to be careful not to confuse the random precison errors with
the unrandom accuray ones. Civilian GPS is designed to be precise to
about +/- 10m, whereas military GPS, which uses different signals, is
precise to +/- 1m. Those errors are random - there is nothing you can
do about them. Selective availability was a deliberate degradation of
accuracy, done by effectively instructing satellites to tell porkies
in their signals, and thereby displace all GPS positions in a
particular area by an ordained amount. That's what doesn't happen
(much) any more, but the precision errors remain.


I have experienced an distinct improvement in accuracy over the last few
years but I am also aware that in certain circumstances GPS is not to be
relied upon. Such events occur, for example, in narrow valleys where the
signals can be "deflected" for want of a better description so GPS wouldn't
work very well in cuttings - or tunnels for that matter.

SA would not be very effiective if all satellites were to displace their
positions by the same amount in the same direction - AFAIK each satellite
had its own displacement which was random - watching a GPS position on a
chart plotter was quite interesting in those days. Now the position doesn't
move and will even plot a position on the correct side of a pontoon.

However when the authorities are playing silly b*ggers with the signal it
tends to be anounced in navigation warnings.....

G