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Re: [PSUBS-MAILIST] UW-DGPS
Vance, I've got some background with GPs units, selective availability, and
differential correction. I designed a computer program that would plot data
for our field monitoring teams at our nuclear plant. This information could
be plotted on a map of a ten mile radius of the plant. Just a few point
about GPS:
1) Heavy tree cover can attenuate (shield) the satellite signal. 2) A few
inches of water would totally attenuate the GPS signal from the satellite.
3) The DOD introduces "selective availability" into their satellite signals
which cause error of up to 100 meters. This is decoded by military GPS
units. The selective availability is use by DOD so that foreign enemies
cannot use our GPS units for vectoring missiles at us or our friends. 4)
Differential correction is a way around selective availability. Accuracy can
be obtained down to centimeters. A separate receiver VHF or coast guard
beacon can be used to correct the signal by triangulation. Kind of defeats
the DOD's selective availability, but that's the DOD for you. 5) The DOD has
either 24 or 32 (can't remember, I'm getting old) non-geosyncronous
satellites that show up on a GPS receiver. I've detected a minimum of 4 and
a maximum of 11. Three is the minimum for triangulation.
6) There is a NEMA connector on most GPS units for differential correction
input signals. 7) There is a serial connection on most GPS receivers for
connection to a computer. 8) There are GPS cards on the market that do
everything a handheld GPS unit does, except that it directly interfaces with
the PCI or indirectly the ISA bus in a computer. 9) It would be very easy
for me or anyone else with programming skills to take a GPS unit digitize
the data (it's already digitized and available to the serial port), and send
it via acoustic link to the sub. this is a "no brainer" to some of the guys
on here.
OK, here's the rub:
The GPS unit does all the triangulation and algorithm integrations of the
signals received from the satellites. This unit is on the surface. Placing
it underwater will attenuate the signals from the satellite, making it
non-functional. Say that you kept the unit above water, and "digitized" the
data and sent it to the sub. The sub would know exactly at all times where
the GPS unit was (were the surface craft was), but wouldn't know where the
sub was. The real key here when you think about it is the location on a TM
grid of the antennae. As the antennae moves along the grid, the distance
from each satellite, and the time to receive the signal changes. Without
locating the antenna on the sub, I really don't see how GPS underwater could
work. maybe I'm missing something here. If someone knows how this problem is
bypassed, let me know.
Suds
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