Re: Re: Solar Bursts May Threaten GPS

On Thu, 05 Apr 2007 12:50:34 -0400, Neon John <no@xxxxxxxxx> wrote:

BTW, as I understand it, the practice of parking new satellites as
spares is past. After the newly launched satellite is proved
functional it is put into service. Many times these replace
end-of-life units that are spared out but not turned off. This is
fuzzy memory and I'm not motivated to look it up so if I didn't get it
quite right, sue me.


Many times they are launched as spares. When put into orbit they are
ops checked and then parked as a spare if they are not needed as a
replacement. It takes 24 to make system fully operational world wide.
Current GPS broadcast to signals, one for civilian usage and the other
for military. The civlilian one had a induced error signal on it until
2000 that limited repeat accuracy to about 200 to 300 feet. They also
disabled the error for a while during the gulf ware when the military
was pressed into using civilian GPS units in the field due to lack of
military ones. The switching off of the error on the signal allowed
the current wave of map based GPS's in the civilian market to take
hold and florish because they would not have been viable with error
signal present There is also the WAAS network that augments GPS which
is basically a network of 25 stations in US that monitor GPS units in
orbit and determine erros in the signals and locations using this
network and then correction data is uploaded to a few geo syncronous
satillites that then broadcast thes signal to GPS units on earth that
are WAAS enabled which improves their accuracy further. WAAS enable
unit will usually have a accuracy diplay in feet to show relative
accuracy on demand and I have seen it as low as 10 to 15ft with
civlian units when WAAS is in use. It takes 2 locks to get a approx
position, 3 to get a pretty good fix and 4 to get good altitude info
too. The more locks you get the greater the accuracy. Three data
streams are on the signal, one is ID, the next is satellites current
positsion and the last is a time bases health signal, The thoery that
GPS works on is that all of them ared sycn'd clock wise within less
than 1 billionth of a second and there is a time based modulation
marker on the signal and the GPS receivers compare these signals and
the time variance between there reception between them togehter with
the known positions of satellites and calculate your position. (the
theory is simple but the application was a pain when it was first
tested in lated 70's and early 80's under the project name of NavStar
for DOD) . There is a LOT of math involved and the minaturation of
recievers togehter with faster low power CPU's have allowed them to
become cheap and plentifull I spent a little time around it in
earlier 80's because the test aircraft was based with a test wing I
was working for so I have some first hand knowledge of early problems
with system. Even today I am amazed still how when our test gear took
up over half of a C141 cargo bay during these early tests and now you
can wear on on your wrist if you want too. Also when Neon John
mentioned the clock change it was done to enhance accuracy because
thought they are sync'd with the ground even the smallest amout of
timing errors between satillites can effect over all accuracy because
without at extremely accurate and stable time referance between them
GPS is not very accurate and it is its weakest link. If you can
distrupt the clock syncs, GPS will become unusable even if the units
in orbit are fully operational otherwise.
-----------------
TheSnoMan.com
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