Re: Is it possible to build GPS "like" system for high-accuracy local position finding?

On Wed, 17 Aug 2005 19:24:41 -0700, blargg27 wrote:

> I am actually a software guy and only know a little about electrical
> engineering in general. (Since this crazy idea has come to mind, I am
> teaching myself as much as I can.) I do not yet know what I can
> realistically accomplish on my own.
>
> I want to build a scaled down GPS "like" system for an area about the
> size of
> my house. I would like it to be accurate to about 1 or 2 cm. This has
> nothing to do with mapping, surveying, or anything related to actual
> global coordinates. I'm only interested in the area in and around my
> house.
>

I have thought about this a great deal. In theory, it could work. In
practice, I don't think it would work. The problem is that inside a
typical house, in many cases a non-direct path will have a stronger signal
than the direct path, thus fouling up your distance calculation. In a big
room, like a warehouse, you still have the floor and ceiling to create
multiple paths, and the interference can change the amplitude and phase of
your carrier, which, ultimately, will also throw off your distance
calculation.

In a warehouse, if you could line the walls and ceiling with RF absorber,
and keep the transmitters and receivers near the floor, it could probably
work. But these are inconvenient/expensive/unrealistic restrictions.

If you envision this working in a large open area, you might consider
ultrasonics instead of RF.

> It would incorporate 4 GPS like transmitters and a number of receiver
> units. This would allow me to read the receiver unit's relative
> position in area surrounded by these transmitters. I only care about
> tracking within my house, and possibly right outside, but not any
> further.
>
> Based off my limited knowledge of the real GPS, building my own doesn't
> seem to be outside my technological capabilities. (At least with a
> decent amount of intense self-training.)

Ha ha! I think you are underestimating the sophistication of GPS.

>
> This is based off these facts:
> The real satellites use RF waves to send these psuedo-random data
> streams and the unit compares its local copy of psuedo-random stream
> with the "delayed" satellite streams. (Delayed due to distance RF
> waves have to travel.) This delay is used to calculate distance and
> eventually exact position of the unit. Using four satalites, it can
> triangulate position and calibrate it's own clock to be reasonably
> accurate.
>
> My transmitters would have to have very accurate clocks, but I could
> also calibrate these as often as needed.
>
> So, is this even possible? Is it too far fetched to even consider
> trying? Yes, I expect it to be hard, but would a seasoned EE
> profesional be able to pull this off?
>
> Real GPS solutions (DGPS) that have sub-meter (let alone 1-2 cm)
> accuracy are
> very expensive: $30k - $50k.
>
> One other note, this system would be incorporated into a potentially
> lucrative product--if it worked. It would be worth a lot
> of trouble to build this.
>
> Even further, does something like this exist allready? or would
> someone be interested in building this for me? This is a small step to
> my "real" idea, which has nothing to do with surveying, mapping, or any
> other global positioning problems.
>

Well, there is something called a GPS pseudolite. You should search using
that term. They are not cheap at all.

> Thanks in advance and I hope this is the most appropriate group to be
> posting in. I have posted to alt.engineering.electrical but fear this
> was a poor choice.
>
> Any suggestions/comments welcome!!

See if ultrasonics could work for you. Check out hexamite.com, intersense,
and others.

--Mac

.

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