No Alastair it is the other way round with selective availability (SA). It is
off and will stay off. During the Gulf War the Americans turned off SA and I'm
sure they will leave it turned off during the current conflict. The reason is
that some military personnel and some of the civilians with whom they interact
use the civilian GPS signals - or may do so in an emergency. To degrade the
signal in time of war would be seen to be imposing an unnecessary risk to
American forces and to breach the commitment that Clinton made when he had SA
turned off. During the Gulf war they moved satellites so that they were
concentrated in the area of conflict, but the US Govt guarantees that we will
always have access to four satellites which is what we need to get a 3-D fix.
Not that you should ever pay any attention to the altitude readout on a GPS
receiver.
You can see the PDOP (positional dilution of precision- a measure of the GPS
receiver to satellite geometry) status of the GPS signals, and other info on
the GPS network, on the maps at
http://www.peterson.af.mil/usspace/gps_support/performance_reports.htm. Look at
the' 72-Hour Dilution of Precision Reports' there. The EPE - Estimated Position
Error - that our GPS receivers give us is based on PDOP and various factors
that affect signal quality.
Regards - David
wrote:
> <snip>As for selective availability, the errors purposely introduced by the
> US
> military, my understanding is that these are no longer worldwide but rather
> imposed over a theatre of operations. We can probably assume that there
> would be errors introduced over SW Asia at the moment but I should still be
> able to find the car park after birding here in Oz. Indeed last week while
> birding at the Weddin Mountains, I returned to my campsite almost to the
> metre. The 315 also gives an indication of error (Estimated position error
> EPE) and this is routinely at 4m.
>
> Regards
> Alastair
> ____________________________________________
David McDonald
PO Box 1355, Woden ACT 2606, Australia
Tel: (02) 6231 8904 (h); (02) 6125 5618 (w)
Fax: (02) 6125 0740; Mobile: 0416 231 890
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