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Wade Joseph

GPS info

5 posts in this topic

With the recent news of people hitting islands at night while navigating with their GPS I thought I would share a little knowledge about GPS with everyone. This may or may not be news to some of you, but I offer it with the hopes that maybe someone else will be spared a tragic accident.

The Military controls the GPS satellites. They control the signals and the strength eminating from theses satellites. They also have an offset programmed for civilain use. This ofset changes frequently and the variance is anywhere from 3 feet to as much as 100 yds. They do this so that a civilian GPS unit cannot be used as a navigation system to hit a target with a missle. Only our military has access to the dead on accuracy of the real deal GPS signals.

So please heed the warning your unit gicves you when you power it up and do not rely on it as your sole source for navigation especially at night on bodies of water you are not intimately familiar with.

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Great advice Wade. Its amazing how much difference there can be in the variance from day to day. If your on a lake you arent VERY familiar with you shouldnt be on plane in the dark. Even if you are familiar with a body of water its not a good idea. You just dont know what boat with lights off, log or bird is there to hit you and knock you out of the boat. (Geese are great at this at dusk on the river.)

Be careful and live to fish the next day.

Oh and of course having too many alcoholic beverages in the ride is never a good idea.

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I never use GPS for high-speed navigation. Even on the lake we have had our cabin on for 24 years, a lake of only about 700 acres I know like I know my own pickup, I don't go on plane and move fast at night in my boat. Like Nlander said, you can't predict the unexpected, and surprises can kill you.

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Yes, the military does have control over how close a person can get to a waypoint. But the average proximity a person has to waypoints has increased dramatically over the years. The military usually allows a variance of fifty feet, which is pretty darn good. Like Steve said, anyone who is planned out when it is dark is just asking for trouble. BC

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Yes the military has a way of messing with the signals but it is turned off right now, has been since 2000. It's called Selective Availability. We use GPS's at work that get accuracy less than a foot, but they cost about 10 grand! Recreation grade GPS's (Garmin, Magellan, etc.) are almost always good to 30 meters (not enough to navigate waterways at night) and usually less than that. This error has nothing to do with the military, it has to do with error between the clocks in the satellites and the receiver, as well as the signal getting bounced off things in the atmosphere. One thing to keep in mind is when your GPS says your Estimated Position Error(EPE) is 20 ft that really means that 50% of the time you are likely to be within 20 ft of where it says you are.

Just wanted to add my .02.

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