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Michael Feimster picked up on my original response, but I thought the
information might be interesting to other techwhirlers too:
<<GPS signals are deliberatlly degraded for civilian use. Military GPS
devices can be accurate to within a handful of meters. Civilian GPS devices
are accurate to anywhere from 50 to 150 meters. This was done to prevent
hostile forces from using GPS to guide bombs to their targets.>>
You're referring to "selective availability", in which the satellite signals
were scrambled using a key (whose nature escapes me) that military
transmitters could use to unscramble the results. This did indeed degrade
accuracy, but not as badly as you report, and the results varied based on
how many satellite signals could be received, the quality of the receiver,
and whether the receiver was being used in static or kinematic (moving)
positioning. 100-m accuracy is what you'd get with the US$100 receiver you'd
purchase at Radio Shack, but professional receivers do much better even in
civilian applications.
Military users now have devices such as laser-ring gyroscopes available to
produce inertial navigation systems backed by GPS with accuracies of
centimetres, even while moving. Civilian tech such as the stuff we use in
forestry routinely achieves accuracy down to 1 m (ranging up to 15 m under
really bad conditions) using a technique called "differential correction",
which can be done in real-time (for navigation) or back at the office (for
surveying). Moreover, these results are now sometimes achievable without
differential correction under optimal conditions because "selective
availability" was eliminated last year (May?).
<<In a former life I use to navigate ships in the Coast Guard. We never used
GPS to navigate in a harbor, let alone in the fog, or in the dark.>>
This may have changed. The Canadian Coast Guard now broadcasts differential
correction information via radio beacons to assist navigation. The
information is apparently now available to anyone travelling along the main
navigable waters in Canada, permitting accuracy to well below 5 m. I'm not
sure I'd want to navigate anywhere really tight with that technology, and
particularly not where movable objects such as shoals are concerned, but
it's only a matter of time before this becomes possible. Accuracy has
increased enormously even over the few years I've been involved in editing
research reports on this technology; it's only going to get better as the
math and the electronics improve.
--Geoff Hart, FERIC, Pointe-Claire, Quebec
geoff-h -at- mtl -dot- feric -dot- ca
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