When you open the serial port of the gps device with hyperterminal for example, it shows updates about every second and it seems that the current position is not always sent within that second. Then between 1 second or even >=2 seconds, there is time for position error. We could compute distance error with following formula:
- d_error is distance error
- speed is given in km/h
- refresh_rate is time between each position update in second
for example with the given information from tomcat:
Originally Posted by tomcat
tomcat: did you really mean mph or km/h?
We can verify the formula with a refresh rate from 1: d_error=50/3.6*1~=14m
or if 50 was really given from tomcat in mph: d_error=80.5/3.6*1~=22m
if we double the refresh rate, we double the error. Results seem to correspond to what tomcat wrote.
From my earlier posted information, I said that with a speed of 240km/h I was estimating the error of about 300m-400m, but thinking about it again, I would re-estimate it down to 100m-200m.
d_error=240/3.6*1~=67m and I think that the position refresh rate of my gps device could be within 2 seconds, then the error could be about 140m, which would correlate my new estimation.
Then supposing that my therory is right (some gps guru could tell me if I did a mistake...), how could a navigation system shows correct current location when the current position does not come fast enough from gps device? Then neither SD nor MapPoint could do something. Apart of estimating the position depending on d_error when the navigation is based on a calculated route...
Why is refresh rate so slow? is it because of satellite signal? is it because of gps device? For satellite signal, we could wait for Galileo, maybe it will be better than gps in that case, or in the case of gps device, why not building faster devices???