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

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.

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.)

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.

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!!

Reply to
blargg27
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Dependent on the wavelength you are using? And the wavelength you can use depends on what is legal wherever you are (?)

You could feed all the transmitting antennas from a single point and calibrate for the exact antenna cable lengths. (The satellite folk don't have this advantage! :-) I believe back in the days of Loran they did it this way (a single transmitter/exciter and long land-lines to the transmitting sites.)

Perhaps irrelevant to this specific question, but very tiny GPS receiver modules and cell-phone modules and similar cool stuff is available at

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One of my favorite places to windowshop and make imaginary designs.

Reply to
Richard Crowley

Look up the value of c. Compare this to 1-2 cm. Analyze just HOW accurate your clock needs to be.

Numerous vendors have fielded or are working on fielding COTS systems with this capability (but not working off these principles) for applications such as tracking the position of personnel within a building.

Bosch is one such, for instance, as is my employer.

Reply to
larwe

If you are working within a restricted area, some of this stuff might do what you want:

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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Reply to
Paul Hovnanian P.E.

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.

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

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

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

--Mac

Reply to
Mac

Just hire a guy to look for it. There is no technology that will find everything/anything to within an inch inside a house.

Reply to
nospam

Theory of Too Many Items. If you have too many items, you can't find anything.

Reply to
nospam

What you are looking for is a :

Pseudolite Applications in Positioning and Navigation.

google found lots of hits, the first one looks good.

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Reply to
Donald

did u consider Sound source Localization techniques?

CMOS

Reply to
manusha

Does it have to work through the walls of the house? If so what are the walls made of?

[...]

Your transmitters would be easy to run a cable to, wouldn't they? The clocks could all be synced to a 10MHz or so signal on the cable. This would reduce the need to buy expensive atomic clocks down to buying VCXOs.

Does this have to work for moving objects?

How long can we average for before giving an answer?

Do you need accuracy or repeatability? Making something repeat well is a lot easer than making it absolutely accurate.

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Reply to
Ken Smith

A couple of $50 GPS modules and some smart programming should see you get to the 10cm mark (be prepared for some horrible calibration issues due to signal reflectance inside the rooms). Some sort of RADAR may work I suppose, but if someone asked me to do 1cm level surveying in a (say) 20m cube, I would be inclined to do it with stereo photogrammetry instead. Scanning with a semiconductor laser ($5 plus cost of stepper motor system) may assist.

Either that or buy a 30cm ruler and a 25m tape measure.

Mike

Reply to
MSC

You would think she would finally give up, wouldn't you? ;-)

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Reply to
Michael A. Terrell

1 cm at speed of light is about 30ps. Not impossible, but quite a challenge. Good Luck. Go ultrasonic, on a per-room basis. Much higher likelyhood of success.

It's known as GPS pseudolites. As others pointed out, the multipath situation is going to be really bad. Forget 1cm. If you must do it, use 4 pseudolites *per room*.

Speaking as someone who did it. Design cost around $15M, a good sized team, big company, 5 years. I don't want to stop you. :^)

90% of the effort is to get at the last 10% of performance. If you know everything you need for GPS, you could probably get your first position fix within a few months. Assuming you buy a RF frontend and learn first how to do digital designs in FPGAs.

That's correct. At about the same level as "a car uses an engine to go faster and breaks to go slower" is correct. I'm sorry for being so condescending.

The main problem is that a simple compare of the PRN sequences is not possible.

First, the signal is weaker than the noise, by about a factor of 100. GPS is a CDMA system, you need to know what the delay is in order to demodulate the signal. This is done by searching and later tracking the code and carrier phases.

Second, if you want to measure a delay directly, the resolution depends on your clock speed. For 1cm, that's 30GHz. For GPS, most receiver do all the signal processing at less than 10MHz clock speed for power saving. The resolution is achieved by having fractional PRN generators and integrating over a relatively long period of time.

As someone else suggested, clock accuracy is not a problem. Simply run all pseudolites from the same clock via cables.

The problem is clock stability. The exact frequency doesn't matter, but it has to stay reasonably constant within your integration period. The more accuracy or sensitivity you want, the better the clock. Standard GPS accuracy and sensitivity can be done with TCXOs, if you want much better, the other end of the scale are atomic time standards.

Here is a wrist-sized model. ;^)

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There are at least 50 independent receiver designs out there. It's most definitely possible, but you'll have to dedicate a significant part of your life to do it alone. :^)

Put a price tag on it. For a few megadollars worth of training, effort and infrastructure, you can design yet another GPS receiver. Double that, and you may be able to also come up with the pseudolite design and get everything down to 1cm accuracy, in good conditions.

You can buy indoor positioning from people who have done it already. It all comes down to how lucrative. Companies design a GPS receiver from scratch because they hope to sell millions of them.

Google for "indoor positioning". Quite a few products, the cheap ones seem to use ultrasonics.

Kind regards,

Iwo

Reply to
Iwo Mergler

'splains everything...

[snip]

That makes quite a lot of sense, you don't know squat so therefore you can do it. Finding your butt would be outside your "technological capabilities"- go take a hike, file head.

Reply to
Fred Bloggs

Thank you for the responses.

I answered the most relavant questions:

easer than making it absolutely accurate.

I need repeatability over absolute accuracy. The tracking movements will be traced as a path and they should appear relatively continuous.

Yes, it must work through walls. The walls can be any lightweight matierial (wood,sheet rock). The house is really an analagy for the real area. Their will be no wires or insulation in the real walls.

Can be 2.4GHZ, 900MHZ, and I'm sure others. I'm in the USA.

It needs to be wireless so they can move freely. (I am exploring a wired system, but won't go that direction until I exhause the wireless solution first.)

I've done a few rough calculations and a nanosecond clock would suffice. I am limited to how close I can be to my four "transmitters"--anything less then 3 meters is too short a distance.

I am aware of clock stability, and don't know how to address this yet.

Well, their is no technology yet! DGPS is accurate to 1 - 2 mm . It costs $30K - $50K and I cannot afford this. I am still researching and half the battle is knowing what to look for.

I'll look into ultrasonic and pseudolite, but would this work for multiple clients roaming around simultaneously? I'll soon find out.

Last question: I'm desperate now... :) Is their a publication on GPS technology? Is it an open standard?

-Ben

Reply to
blarggstar

I thought more about this!

How about:

No walls, large wharehouse, all transmitters hardwired together (to sync clocks).

Direct line of site from transmitter to receiver.

Would it be possible then?

Reply to
blarggstar

Pretend I was a seasoned EE professional. Could I build this on my own? Without huge budget and lots of expensive equipment!

The one thing I have an excess of is....time!!

Reply to
blarggstar

My missus complains that I can't find anything she sends me to look for.

Ken

Reply to
Ken Taylor

Explore beyond the GPS model. Who says the mobile devices need to be the receivers (which requires some hefty horsepower to decode & triangulate the signals).

  • Use a simple transmitter fob on the mobile object.
  • Have the object transmit occasionally (as frequently as you desire).
  • Have N towers listen for the signal (however many it takes to cover the site)
  • Towers timestamp the signal with very high (nS?) accuracy. Distance from each tower is calculated based on propagation delay.
  • Calculate (x,y) from the 3 closest towers, or (x,y,z) from the closest 4

Sounds a lot like cell tower triangulation, eh?

Options:

  • Device transmits very infrequently when stationary
  • More frequently when accelerometer detects movement

Using such a system, the trick would be keeping the time sync'd very accurately between the towers. (Separate GPS receivers, or calibrated inter-tower cabling.) To really nail the (x,y) accuracy, the field could be calibrated by walking around with a transmitter to correct for multipath effects.

This approach lets you keep the mobile units very cheap and put the power-sucking number-crunching in the towers. If the mobile fobs need to know their location data, address it to them on a return channel from one of the bases.

Incidentally, at least one asset tracking product works very similarly, but the (x,y) resolution is something like 10m. Their fobs run for years on a coin cell. (And no, I can't find their card anywhere here...)

Cheers, Richard

Reply to
Richard H.

I don't know. I don't have high hopes. I would still want to use ultrasound for this. If you correct for air-temperature effects, the possible precision is pretty good. I thought of another ultrasound website: massa.com.

Now, if you were to get rid of the roof on the warehouse, or make it out of an RF transparent material, then you could just use differential GPS. Honestly, this might be cheaper. ;-)

If you must use RF, and design it yourself, then I think you will have to keep it simple. Designing a GPS-like system will be tricky. Maybe you can use something simpler, like some kind of UWB ranging system. Take a look at time domain's website

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for ideas. Maybe even give them a call?

If you do use RF, then anything you can do to avoid reflections from the walls, ceiling, and floor will help tremendously. But covering the entire warehouse with RF absorber might get expensive and ridiculous.

Good luck.

--Mac

Reply to
Mac

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