Re: GPS speed indications

Rich Piehl wrote:
Larry Sheldon wrote:
This is a pure question without preconceived notions as to what the answers might be.

What I think I know is that the unit-in-hand locates satellites and does timings to determine where it is in three dimensions.

What I don't know is how often they do that.

Seems like if the interval is very long it would not know about altitude changes, ziggy-zaggity twisty roads and while it would know position accurately it might not know exact distance traveled between fixes and thence would not be able to calculate speed accurately.

I don't exactly how often it updates. But I was watching my unit on two lane roads and freeways where the posted speed limit changes. My unit displays the road speed limit and dead on where the the posted speed limit was my unit showed the speed limit. This occurred numerous times. The only way it could know I was passing that point is by updating every half second or so.

Additionally my unit shows the speed I'm driving. As I adjust my speed the unit adjusts my speed almost immediately.

I'm guessing it updates its data 3-4 times per second

Hmm, that's one thing I was disappointed in with my Garmin - the speed limit display is very inaccurate - e.g. near my house it displays 25 MPH when the posted is actually 35, same thing happens near my work, it displays 40 when it's really 45... yes, I updated the maps as soon as I had the opportunity, it was useless until I did so as there are so many new roads and reconfigured interchanges around here - especially in areas that I'd really rely on it (places I'd have to go for work, that I'm not really familiar with)

Now for the ground speed, I'd guess it updates something like 2x a second; I don't see the numbers smoothly changing as I accelerate like a digital speedo, but it is reasonably fast.


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