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Not quite of the significance you might think...
The GPS constellation consists of 2 totally separate systems, Precise
Positioning System (PPS) and Standard Positioning System (SPS). PPS is
available to the US Military only, and is rumored to have real time,
sub-centimeter accuracy in all 3 axis' (Yes, GPS does do 3D and is starting
to make pressure based altimeters obsolete). SPS, the system available to
the rest of the world, is said to have accuracy "within several meters" and
was added to the system late in the design phase, the intention being to
replace a quickly degrading LORAN-C system and to help justify/defray the
cost of the constellation in an era of decreasing defense spending. One
well known trouble with SPS (at least among fishermen) is that its
repeatability (ability to locate the same spot, such as some under sea topo
feature) is poor at best when compared to LORAN-C, having something to do
with postioning being triangulated from different combinations of constantly
moving satelite, rather than the fixed land based stations in LORAN-C. Add
to this Selective Availability (SA), which was intended as a facility that
could be switched on in a time of military emergency to further degrade the
accuracy of SPS, theory being the degradation of the SPS signal to the point
where it was useless, strategically speaking.
As the LORAN-C network continued to crumble and the Coast Guard had several
budget scares that could have resulted in the plug being pulled prematurely
on the LORAN-C network, the Coast Guard, with significant support of the
private sector, began design and constuction of a network of land based
differential beacon stations to cover our coastal and inland waterways.
These differential beacons, now mostly completed, receive SPS, analyze it
and transmit a correction factor. Specially equipped receivers, referred to
as Diffential Global Positioning System (DGPS) receivers, receive the
correction factor as well as the positioning data from the constellation and
are able to resolve it in real time, displaying significantly more accurate,
but more importantly, more reliably repeatable positioning data. This is
not a global strategic issue, as it just covers the continental US at this
time.
Now, back to the subject at hand, SA. SA was never intended to be left on
all of the time, rather it was to be switched on in times of crisis. Its
first big showing was during the Persian Gulf conflict. Apparently, a
significant number of our troops were not equipped with PPS capable
receivers, and after several incidents of troops getting lost in the desert,
civilian SPS receivers started showing up in the theater, but guess what,
with SA switched on, SPS was virtually useless. At some point, the decision
was made to turn off SA and widely distribute the much cheaper and readily
available SPS units. All of that being said, its really no big deal that
they are canning SA, is it?
So, we
really don't have anything more than we started out with and we still
don't have the military only PPS.
This archive was generated by hypermail 2b29 : Thu Jun 01 2000 - 22:37:10 PDT