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

Sun May 4 01:22:59 CDT 2008

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.

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").

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.

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?

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.

-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
_______________________________________________
DPRGlist mailing list
DPRGlist at dprg.org
http://list.dprg.org/mailman/listinfo/dprglist