DPRG mailing list archives: [DPRG] GPS and ground planes

Sun May 4 01:52:25 CDT 2008

Kenneth Maxon wrote:
> 
> Hi Jeff,
> 
> I had many problems with a SMIR(III) based GPS receiver.  Adding a ground
> plane helped some of the problems but not all.  Additionally, making sure
> that I used a very short extremely low bonding resistance connection
> improved the signal returns.

Short connection from where? From ground plane to the chassis? Or 
between the ground plane and the receiver case? I also have a SIRF III 
(the EM-406A from Sparkfun) that I could ground the case. I'm not happy 
with the performance of that one yet. Maybe I just haven't played with 
it enough yet.

> GPS has a primary wavelength of just under 7.5", so a ground plane that is a
> full wavelength or larger will work well.  (bigger is better w/diminishing
> returns.)   If you have the room to make a round plate with a 15" diameter
> you will not get much better than that.   I haven't had time to go out and
> solve this one for a circularly polarized wave, I just did it
> experimentally.   BTW, In my solution I ended up using a much smaller piece
> than that (square ~7").

I might try a square or rectangular plate. At the moment I am just 
carrying the GPS. So I have more options in what I use. The 15" circle 
may work on the robot when I get that far.

> If you care to see, a few months back (late fall / early winter) there were
> a number of messages back and forth on this list for reference with some
> great input from several group members to help track down some of the
> sources of error.

I knew there was some discussion, but I wasn't sure what to search for 
or what the topic was. I'll check it out. I remember some discussion 
about ice.

> My system still suffers from excessive wandering in the GPS signal but that
> wandering is now almost always contained to ~50ft.  On a rare occasion it
> will still wander up to ~100ft away from actual position, however this is
> rare compared to the previous errors of 280ft that would happen several
> times over the course of a days worth of captures...
> 
> As a side note, I also have a Garmin CSX60 and it has identical performance
> issues.   I took the Garmin on my last trip to Savannah with me and inside
> my hotel room running for 2hrs capturing data it never wandered more than
> ~30ft (15ft radius) without any ground plane... Since I am only 250miles
> south of you here in the upper mid-west, I wonder if there is something
> geographically related to it?

I did notice that that my few short tests did have a whole bunch of 
wandering when I first started my path recording. Either I did not wait 
long enough to get a good fix or the internal software required that I 
move before it decided on its calibration. Possibly it was assuming too 
much gain because it had not seen me move yet. I'll play with that again 
  and see which is causing it.

> Question: On the SMIRF(II) chipset that you have been using, have you ever
> been able to get a lock on the WAAS signal?  On the SMIRF (III) I have not
> been able to receive it and do not know if it is my location or the way I am
> interfacing to the chipset.

A year or so ago when I played with this I saw a flag which inidated 
DGPS, which I assume was WAAS. I haven't looked at the data this time 
around on either eceiver. I'll check on that.

> -Kenneth
> 
> 
> 
> -----Original Message-----
> From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org]On
> Behalf Of Jeff Sampson
> Sent: Sunday, May 04, 2008 12:11 AM
> To: dprglist at dprg.org
> Subject: [DPRG] GPS and ground planes
> 
> 
> Hi,
> 
> I played with my GPS. It is a Rikaline USB GPS module for a laptop
> computer and has the Sirf II receiver. I noticed the resulting path that
> I recorded didn't look like I expected. It was close, but looked sloppy.
> So I stuck it to a piece of steel sheet, about 8"x9" and ran it again.
> (This is one of those that has a magnet to stick to the top of your
> car.) It worked much better the second time.
> 
> Does a GPS receiver prefer a ground plane? Or did I just have a better
> fix the second time? If a ground plane is preferred, does anybody know
> the recommended size of the plane?
> 
> --
> Jeff Sampson
> http://tcrobots.org/members/jsamp.htm