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

Wed Jan 2 07:35:50 CST 2008

I'll have to disagree with Ed; it's hard to know anything with almost- 
certainty given the info from your two emails (unless maybe Ed has  
more info about your setup than I do?).  Your description of the  
coordinates getting worse in a matter of a minute only to return  
inside of 5 minutes... that sounds like it could *also* be a  
satellite going out of view and your receiver is searching for and  
switching to a new one (which is something you can verify from the  
NMEA data stream; I don't have scripts that check for that yet).  So  
there is more than one possible reason to explain your observations.

Do you know what chipset your GPS device uses?  Modern device usually  
have more advanced firmware that will reduce the errors that occur  
during things like satellite-switch-over.

Multipath is very hard to measure and very hard to fix (unless you're  
an antenna and DSP specialist).  Measure what you can, fix what you  
can and re-assess if you need to worry about multipath at that point.

!!Dean

On Jan 1, 2008, at 23:59 , Kenneth Maxon wrote:

>
> Dean,
>
> I will give the site a thorough read and get back to you, thank you  
> for posting.
>
> To fill in some detail as I have run a number of expirements  
> already and have verified a few data sets against google earth...    
> I agree completely on the need for more satellites in view.    
> ProtoBot throws away all data until at least 6 satellites are in  
> view.  The data always tracks, meaning that the data points  
> reported do not hop around, but rather follow transitionary paths.   
> Often the reported point will stay with-in a few feet boundary of  
> its last reported position for 15 or 20 minutes at a time, however,  
> afterwards it may up and track away 100 or more feet in a matter of  
> a minute or less.  Often the reported position will return too  
> within ~20ft of where it should be (google earth reporting) inside  
> of ~5 minutes.
>
> The screen snap shot (sorry for the poor picture) was taken after  
> ~20minutes of data capture showing a 102 ft by 88 ft wandering path  
> when the unit was stationary.   I have captured much longer paths  
> that look similar, however my camera has significant difficulty  
> capturing this display w/wo flash other other settings...
>
> http://nikita.argia.net/kmaxon/sat_wand.jpg
>
> I am most definitely not an expert in the realm of GPS and those  
> that I do know who are, work in another division that precludes  
> them from talking with to me.  Sigh...  So close and yet so far...
>
> -Kenneth
>
> -----Original Message-----
> From: Dean Hall [mailto:dwhall256 at gmail.com]
> Sent: Tuesday, January 01, 2008 11:35 PM
> To: dprglist at dprg.org
> Cc: Kenneth Maxon
> Subject: Re: [DPRG] Further Quandries on GPS...
>
> 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...
>> _______________________________________________
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>

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