Re: [mowbot] Cheap accurate location

Monta Elkins (monta nospam at
Mon, 19 May 1997 10:19:51 -0400

Mike Ross wrote:
> On Fri, 16 May 1997, Monta Elkins wrote:

> > Delorme makes Tripmate a $150 GPS receiver
> > (12 channel parallel?) with a "standard" PC
> > serial connection.
> >
> > It might be possible to use two of these
> > to do differential GPS for positioning.
> > I'm far from an expert on the subject and
> > was looking for other comments....
> >
> I'm no expert either, but I tried just what you're suggesting... It
> didn't work at all. To do it right, each GPS unit needs to be figuring
> off the same satellites, but there's no guarantee that the same satellites
> will be visible to both of them. The GPS reading jumps when a satellite
> switch occurs, so each unit would have to be synchronized. I'm not saying
> it's impossible, but real DGPS with mm accuracy is what, $20K? Doing it
> the cheapo way is *amazingly* cheaper, hence that fact that it isn't
> being done already tells me there's a barrier. After all, it would be the
> perfect way for a mowbot.
> I was using units on loan from a Trimble rep, so I only had about 2 weeks
> to write and test code. I plotted the computed position of one of the
> units, and it would wander around within a 20 ft area, then jump up to 100
> ft when a satellite was swapped out.
> I also tried averaging. I think if you averaged for a week, you'd get
> something pretty close, but you can't do it for moving objects.
> * Michael L. Ross/C33 | Lockheed Martin Eng & Sci Svcs Co. ****
> * Robotics Department | 2400 Nasa Rd. 1, Houston, TX 77058*****
> * (281)333-7094; (281)333-6071 fax* #include <disclaimer.h>*
> * Finger me for my public key. mross nospam at ****

I understand what you're saying about the ability to
track error for a given satellite;

As I understand it the TripMate can theoretically track
12 satellites simultaneously (vs. 3? for the trimble models).

What I don't know is if there is a communications protocol
(command in NMEA) to get information from _specific_ satellites
with the TripMate. Or tell it to use a specific set of
satellites. If there are no NMEA commands there very
well could be proprietary commands to do it.

BTW the problem of visibile satellites is a rather small
one given the relative proximal location of the two
receivers for typical mowbot applications.

-Monta Elkins

| "All that is necessary for evil to triumph is |
| for good men to do nothing." -- Edmund Burke |
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