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

Tue Jan 1 23:59:03 CST 2008

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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