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

Sun Jan 27 21:21:26 CST 2008

David & DPRG,

There is to much rounding error applied to the numbers in the first link in
your e-mail below to tell if these errors are on the order of what I am
seeing or not.  Any chance you could shoot me a set of coordinates defining
the outer boundaries...?

I have finally gotten back to running another set of experiments and I am
still receiving disappointing results.

In the ground plane overkill dept, I added a 18" long by 8" wide by 3/16"
thick solid aluminum plate to the top of the robot's ~4' mast.  I terminated
the ground return lead of the GPS module (SiRF III chipset) to the ground
plane using a <4 milliohm* connection.  *Note: Measured with a 4wire setup.
(4 milliohms is no where near perfect but should suffice for this test).
The GPS module w/integrated antenna I set right down onto the ground plane
separated by only ~1/8" acrylic.  I powered up the entire system and left it
sitting for over an hour and then reset only my processor system, leaving
the power to the GPS system un-interrupted.

I allowed the system to gather and plot data for ~3hrs this afternoon.   As
always I verified a couple of random samples through out the data capture
period against Google Earth to make sure everything was in range with my
processing routines.

During data capture the least number of satellites in view was 12.  My
software tracks how many of them have sufficient signal strength and tracks
a min and max number of satellites used in the position calculation.
During the 3hr time span, the number of active satellites never dropped
below 6 and typically remained at 9 or 10 used in the active calculation.

Interesting observation:  During the time periods of most significant error
there were never less than 9 satellites in use in the active calculation.
During the entire run the HDOP never dropped below 2.4m and there is no
significant correlation between the HDOP or PDOP values and the error form
true position.   I have a plot on the display of my robot but don't have a
way to get that plot back into a PC currently, I'll attempt to forward that.

During the ~3hr run similar errors were observed as those previously
reported.  Today typical values remained in a 47ft by 23ft bounding box,
with several forays of error paths when plotted out over 96ft from the true
actual position**.   **Note: the "true actual position" that I am using
comes from running an averaging function for 8hrs continuous and taking that
value and pumping it into Google Earth for a verification.   It is more than
accurate enough to determine error at the level of which I am seeing them.

Sooo...   This next round has been somewhat disheartening with the addition
of the ground plane and the observations of lack of correlation from
reported PDOP and HDOP to error functions.   Also the lack of correlation of
number of satellites in view to error in signal is also quite disheartening.
(I should qualify that last statement as there is a very strong correlation
there that appears when less than 6 satellites are used.)

Sigh...   Just thought I'd throw out some additional data for anyone
interested, or anyone with further ideas...

I will be back in a much further South Latitude on business here in a few
weeks and will attempt to find a way to get a subset of CCAs through airport
security to test much closer to the equatorially based WAAS satellites that
we can't seem to receive "here in the middle" with either GPS that I have.

-Kenneth

-----Original Message-----
From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org]On
Behalf Of dpa
Sent: Wednesday, January 02, 2008 9:55 AM
To: dprglist at dprg.org
Subject: [DPRG] Further Quandries on GPS...

Howdy,

Here's a plot of 40 minutes worth of wandering GPS locations recorded with
a handheld Garmin Etrex sitting in a fixed location:

<http://www.geology.smu.edu/~dpa-www//gps/gps_40min.png>

I need to convert the lat/lons to something that makes more sense
(i.e. feet or meters) but you can see that it doesn't drift anything
like what Kenneth is getting.

Kip wrote:
> The first thing I would try is a bigger ground plane under the patch
> antenna. You should have a minimum of 40mm x 40mm, but anything bigger
> is better and it needs to be grounded to vehicle ground. Bigger should
> help reduce ground reflections.

I found I had to do this on jBot to get reasonable GPS data, and also
mount the GPS rather high above the rest of the robot platform.  Here's
a picture:

<http://geology.heroy.smu.edu/~dpa-www/robo/Encoder/imu_odo/jbot_imu_odo_02.
jpg>

that shows the GPS (yellow and black device) mounted in a grounded aluminium
channel which shields it on 3 sides, which is what I eventually had to do
to get a reasonable performance.

FYI, jBot continously calculates its position from odometry as well as GPS
and
then calculates the difference between the two locations.  In a typical 15
minute
run the two never drift more that 96 inches from each other, and are
typically
in the 17 to 54 inch range.  (I think 17 inches is the quanta of the
smallest
measureable GPS increment with the handheld garmin).

HNY,
dpa

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