[Fun] Have you ever wondered of the GPS chip accuracy?

Laszlo KREKACS laszlo.krekacs.list at gmail.com
Wed Aug 5 00:30:51 CEST 2009


When I was on holiday, I ended up to write a gps app,
and I wondered how to measure speed exactly.

My first attempt was, that I take the gps coordinates
every second, and I calculate how many meters I

Then I discovered, that fso reports speed  too (although
in "knot" units), after a bit of research, I multiplied the
knots with 1.852 to finally have my speed in km/hour.

Then I compared the two results, and it was interesting
enough, that I thought would be fun to share with you too.

Here is the generated svg data:

On the horizontal axe you can see the time (in 10 sec unit,
and every minutes and 10 minutes has a longer line).
On the vertical axe there is the speed in km/h.

There are some horizontal area, which indicates speed
limits (I was biking):
greenish: 0-5km/h
yellowish: 5-10km/h
red/lila: 10-15km/h
blueish: 15-20km/h

The black curve is the calculated speed based on the
reported gps coordinates.

The green curve is the speed what the gps chip recorded.

The duration of the whole trip was 15 min, as you can read
from the graphicon.

I think the pictures talks for himself, but everybody can see,
that the calculated speed is really "hairy" compared to
the reported speed by the gps chip.

If anybody interested, I can share the data (from where
the graph was generated)...

I got two questions regarding the gps chip, maybe someone
can answer me:
- is there some settings, where I can fine-tune the gps
chip? (I think what is the method to calculate the average speed?
Is it adjustable?)

- Is there a way to ask for gps coordinates faster then one second?
(for running 0.1 sec would be awesome). And why the time is
only reported in sec unit? (not fraction of the second)
1249424982.764385 vs. 1249424982
Is it a limitation of the gps chip?

Best regards,

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