Humminbird Side Imaging Forums
Side Imaging Forums => 798c SI => Topic started by: tomhigher on July 20, 2009, 04:12:02 PM
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Hi my name is Tom ;D
i was wondering about the precision of the gps signal because the gps told me an speed of 0,1 - 0,4 kph
but this is not true ..because the 798ci stays on the desk in my garden ;)
and i get only "3D Fix" in the Display ..so what is wrong ?! ..thanks for your help
regards
TOM
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Hi Tom,
the "speed" you get shown as your unit sits on the desk in your garden is "produced" by the satelites traveling through the sky (the sats are not stationary above the earth). So if your unit doesn't move, the sats are in motion and the unit calculates the different distances of the sats and turns out a "speed" even if you're not moving at all.
You can observe the motion of the sats, if you switch the unit into GPS Status Mode and watch it for a while. Some sats show up, some disappear, but it takes some time to see this.
To get a decent speed fix, the unit has to travel for a minute or two to calculate the "right" speed.
So don't worry, my GARMIN does the same stunt ;).
Regards from Germany
Harry
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Hi Harry,
thanks for the answer..shows your system "enhanced fix" or "3D fix" in the display..and what does it means (quality of the signal ?!)
regards
TOM
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Actually the speed shown when a GPS unit is stationary is due to the inaccuracies of the GPS system that we are allowed to use (the U.S. Government keeps the really accurate stuff for their own use!). This is further affected by the accuracy of the GPS Receiver that you are using plus the quality of what you are referring to as the “Fix Type”. When the GPS Receiver decides that your position has moved, due to these inaccuracies, it will show the calculated speed that you would have had to move from where you were before to where you are now. To see the a speed while sitting still is not that unordinary, especially if the GPS Receiver does not have a clear view of the GPS satellites.
We use the terms “2D Fix”, “3D Fix” and “Enhanced Fix” to show progressively better satellite configurations. Each gives a more accurate positioning than the one before with the Enhanced meaning that the GPS Receiver is tracking and using at least one of the WAAS satellites (for U.S. areas). So you should see lower speeds with a 3D or Enhanced Fix than you would with a 2D Fix.