DPRG mailing list archives: [DPRG] Further Quandries on GPS...

Tue Jan 1 23:35:14 CST 2008

There are plenty variables about your GPS that I don't know about so  
I could speculate plenty and that's all it would be.  Here is  
information on experiments I have performed:
http://deanandara.com/Argonaut/Sensors/Gps/

Notably, coordinates can be off by as much as 120 meters just after  
module power-up or when in view of only 5 or 6 sats.  The SiRFstarIII  
that is in my GPS module will give coordinates with increasing  
accuracy after taking multiple readings; a.k.a. after time elapses.   
Multipath effects are not going to be something you can measure  
(unless you have fancy equipment I don't know about; which, if you  
did, you probably wouldn't be speculating like this), so start with  
what you can measure.   I suggest letting your receiver sit in one  
place for a minute or five and examine the behavior of the  
coordinates: are they erratic or are they converging?  If they are  
erratic, try again a different location & time.  If they are  
converging, find out more information, like how many sats in view?  
and how long until the coordinates are within 3 meters of actual (I  
use Google Earth for that)?

By running little experiments like this, I know that my robot warm-up  
procedure will include turning on the GPS module and waiting until a  
significant number of sats have been in view for an amount of time  
that I determine.  Only then will I start trusting my GPS coordinate  
values.  I have written a few Python scripts that help me capture,  
convert and view data from my GPS module.  For example, just reading  
NMEA sentences with the receiver at one location for 5 minutes, save  
the data, convert to a KML file, plot it as a path and view it using  
Google Earth.  This helps visualize where your receiver thinks it  
is.  Scripts are here:
http://deanandara.com/Argonaut/Sensors/Gps/Scripts.html

!!Dean

On Jan 1, 2008, at 21:18 , Kenneth Maxon wrote:

>
> Hello DPRG...
>
> Here is one for further conversation in the "Hmmm...?" realm...
>
> It was up to a fair 8-deg today and although there is a significant  
> snow pack on the ground the sun was bright and the sky was clear  
> for a few hours so ProtoBot ventured back out into the back yard  
> for more GPS expiremental data gathering.
>
> This time I made sure that ProtoBot was a full 12 ft away from the  
> house and this time, the house does not have a 1/2"~3/4" glaze of  
> ice hanging on it.   There is however a 3/4" layer of solid ice  
> under the snow back that ProtoBot is sitting on.   I pulled a  
> bonehead move and forgot to take my hand held Garmin + adapter &  
> extension cord out to verify the readings, however, surprisingly  
> ProtoBot still recorded erronous readings, like it was doing inside  
> and in the previous outdoor trial up along side the house.
>
> I have taken time to swap out the GPS module / antenna combination  
> on ProtoBot with the spare unit that I'll be using on an upcoming  
> project.  And given that ProtoBot's last round of testing performed  
> in a similar (although less drastic from an error standpoint)  
> manner than did the comercial Garmin unit, I still have a quandry  
> here.   Yes, I am still wondering *IF* this could be coming from  
> multipath off of the ice layer.  I theroise that it is not as the  
> 10" of snow pack sitting on top of the ice should be enough to  
> completely disburse the energy, and all of the sattelites reported  
> by the module as being used in the actual position calculation had  
> a good signal strength reading.
>
> I don't know that anyone on this list will have input, however it  
> is an interesting quandry that I am working through and I thought  
> it might be of interest to the group, as there are others in this  
> group that activly use GPS on their robots.
>
> Of course I'll post futher when I get to the bottom of the  
> issue...  -Kenneth
>
> BTW, Ron, the package arrived, thank you.  You didn't have to  
> return it but since you did I'll put it to good use in the next  
> project.  (IE, you've saved me from some eBay surfing).  -Thanks...
> _______________________________________________
> DPRGlist mailing list
> DPRGlist at dprg.org
> http://list.dprg.org/mailman/listinfo/dprglist

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://list.dprg.org/pipermail/dprglist/attachments/20080101/e6d580d2/attachment-0001.html