<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">
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:<div><a href="http://deanandara.com/Argonaut/Sensors/Gps/">http://deanandara.com/Argonaut/Sensors/Gps/</a></div><div><br class="webkit-block-placeholder"></div><div>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)?</div><div><br class="webkit-block-placeholder"></div><div>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:</div><div><a href="http://deanandara.com/Argonaut/Sensors/Gps/Scripts.html">http://deanandara.com/Argonaut/Sensors/Gps/Scripts.html</a></div><div><br class="webkit-block-placeholder"></div><div>!!Dean</div><div><br class="webkit-block-placeholder"></div><div><br><div><div>On Jan 1, 2008, at 21:18 , Kenneth Maxon wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><font id="role_document"> <div> </div> <div><span class="124190303-02012008">Hello DPRG...</span></div> <div> </div> <div><span class="124190303-02012008">Here is one for further conversation in the "Hmmm...?" realm...</span></div> <div><span class="124190303-02012008"></span> </div> <div><span class="124190303-02012008">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.</span></div> <div><span class="124190303-02012008"></span> </div> <div><span class="124190303-02012008">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.</span></div> <div><span class="124190303-02012008"></span> </div> <div><span class="124190303-02012008">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.</span></div> <div><span class="124190303-02012008"></span> </div> <div><span class="124190303-02012008">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.</span></div> <div><span class="124190303-02012008"></span> </div> <div><span class="124190303-02012008">Of course I'll post futher when I get to the bottom of the issue... </span><span class="124190303-02012008">-Kenneth</span></div> <div><span class="124190303-02012008"></span> </div> </font><div><font id="role_document"><span class="124190303-02012008">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...</span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">_______________________________________________</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">DPRGlist mailing list</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><a href="mailto:DPRGlist@dprg.org">DPRGlist@dprg.org</a></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><a href="http://list.dprg.org/mailman/listinfo/dprglist">http://list.dprg.org/mailman/listinfo/dprglist</a></div> </blockquote></div><br></div></body></html>