GPS speed calculation method?

pshute · #1 (**permalink**) 20-11-2007, 12:48 PM

I've been using a free application called bbTracker to create GPS logs of trips. This application records the current speed at each point it records, as well as the coordinates.

The author of this software says that the API returns the speed each time he retrieves the coordinates - he doesn't calculate it himself using the distance travelled since the previous point. He says he has no idea how it calculates the speed it returns.

So, does anyone *know* how the Blackberry 8800's GPS calculates the speed? Does it actually take a couple of readings? If so, how many seconds apart? Does it average over several readings? Any idea how accurate it might be?

GaryCutri · #2 (**permalink**) 20-11-2007, 02:12 PM

"As well as calculating its position, a GPS receiver can also determine its velocity by measuring the Doppler shift: the difference between the expected frequency of the satellite signal and the actual frequency of the incoming signal"

adam89 · #3 (**permalink**) 19-12-2007, 02:15 PM

Now what i want to know is what the hell is altitude measured against?

e.g. Altitude = 110

110 what? Above sea level?

pshute · #4 (**permalink**) 19-12-2007, 04:38 PM

Quote:

Originally Posted by adam89

Now what i want to know is what the hell is altitude measured against?

e.g. Altitude = 110

110 what? Above sea level?

Maybe above the geoid? See Undulation of the geoid - Wikipedia, the free encyclopedia

But it seems to be very inaccurate. I have heard (but not sure I believe it) that a GPS chooses which satellite signals it will use in the position calculation according to which ones will give the greatest positional accuracy horizontally. In doing this it ignores those that might give good vertical accuracy. The result is *very* erratic vertical accuracy. Just a theory.

I'd never trust the heights it returns.

adam89 · #5 (**permalink**) 20-12-2007, 11:46 AM

Quote:

Originally Posted by pshute

Maybe above the geoid? See Undulation of the geoid - Wikipedia, the free encyclopedia

But it seems to be very inaccurate. I have heard (but not sure I believe it) that a GPS chooses which satellite signals it will use in the position calculation according to which ones will give the greatest positional accuracy horizontally. In doing this it ignores those that might give good vertical accuracy. The result is *very* erratic vertical accuracy. Just a theory.

I'd never trust the heights it returns.

Woah, thanks so much for that info!