Re: [gpsd-users] GPS speed and direction reporting

[Yan Seiner]

Yan Seiner wrote:
> I'm playing with gpsd as a way to record the trips I take...  
> Basically I've hacked gpxlogger to monitor the gps, and every mile or 
> so run a webcam program to take a snapshot of the road.
>
> That part works wonderfully.
>
> The webcam program also writes the GPS coordinates, time as reported 
> by gpsd, and the direction and speed.
>
> it's the last two where that I'm having issues.  gpsd seems to report 
> about a 5 minute rolling average of speed and direction.  I'm not sure 
> if this as artifact of gpsd or the gps puck I'm using.
>
> Is there some way to get gpsd to report a, say, 10 second average?  I 
> know that the noise in the gps signal will make this more variable.  
> Right now, gpsd reports a speed of 25MPH a full 6 minutes after I 
> parked the vehicle.....
OK, I figured out what is going on.  Since I'm only interested in 
readings every 60 seconds or so, I put a sleep(1) in the same loop as 
the select(...).  Apparently this causes a FIFO queue to build up, and 
causes the readings to be delayed as the FIFO needs to be drained.

I replaced the sleep(1) with a usleep(10000) and all is well.

Moral of the story, the polling should be continuous or nearly so.

--
Few people are capable of expressing with equanimity opinions which differ from 
the prejudices of their social environment. Most people are even incapable of 
forming such opinions.
   Albert Einstein