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GPS versus Speedo Accuracy

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On some forum somewhere I once asked does GPS accurately measure speed in turns.  I asked the question because I noticed that indicated GPS would drop significantly in a turn.  That is if I went into a turn at an indicated 60mph, it might drop to 55 mid turn and recover to close to 60 after the bike straightened up.  On my GS it was hard to read the analog speedo but the GPS on the Nav 5 was easy to read.

Well I recently set my digital readout to show "speed", the speed that the analog needle was show or real dog on close.  So today I compared the digital readout and Nav 5. I went in and came out at 63.  It read 62 mid turn.   GPS went in at 63 plus or minus one mph.  It dropped to about 55 mid turn.  I say about because I did this several times and it was also plus or minus about 1 mph.

I have my theory but I want to see what you GPS gurus think.

My theory is that it depends...on how quickly the gps is connecting to the satellites and most likely several other variables. If the gps connected every 5 seconds, there's a significant probability that it could miss you making a turn and calculate your speed as if you'd gone straight across the corner. Since it would think you went straight and therefore a shorter distance, you must have gone slower. If it connected to the satellites every 1/10th second, the probability of that happening would go way down...unless you're in an area with significant trees covering the road (another variable).

That's my theory of how it could happen. It's probably a rather simplistic theory, but may still have some credence. I suspect if you had a military spec gps that sampled the satellites at an extremely fast rate, your variation "might" not be as significant.

^ this. The gps speed is calculated based on the elapsed time between the locations it "obtained" from the satellites. So, it's gonna "square off" the corners to some degree.

That said it's still likely to be more accurate than a mechanical speedometer when traveling in a straight line.

To get a three dimensional fix four satellites are required, three for a 2D fix.  It's all geometry.  The data sent by the satellite is processed by your unit.  The unit gets the position of each satellite from the data stream of the that particular satellite.  The distance to the satellite is measured by the time the packet left the satellite to when it was received.  Where things start to mess with the basic solid geometry problem is that the unit may be choosing from four to as many ten satellites in view AND the signals can reflect off of objects making them longer than they should be.  Jim, what you said about the straight line is probably right and why your E911 dispatcher usually gets a more accurate position on cars than something standing still.  Actually I'm not as interested in "why", I'm pretty sure I know the "why".  I'm just wondering how many people notice this change in speed in the turns.

I'm almost always looking through the curve and not down at my GPS.  :-)


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