Designing an affordable RTLS

Discussion in 'The Projects Forum' started by GenericJoe, Aug 31, 2013.

  1. GenericJoe

    Thread Starter New Member

    Aug 31, 2013
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    Hello, I would appreciate you guys helping me figure out how to solve or at least get started with this “real-time location system” problem. I am quite new to electronics but am a fast learner.

    Imagine a small outside open area, like a field where physical obstruction is to a minimum. The size is about 200 meters squared. In this field are people running around, perhaps playing a sport like football, or racing bmx’s etc. I want to record / display in real time the location of every person on the field. The people I intend to track are willing to wear a small device to transmit/receive signals, and I can place static transceivers around the field, say one in every corner.

    At this moment I am not too concerned about the software needed to display the data - I am more interested in how this might work with affordable electronics, and unfortunately this area is not my expertise, hence me asking for advice.

    From what I have read, trilateration (not triangulation) is what I need. I should use four transceivers on the corner of the fields to collect the “info” from the people. The people on the field will use a trasmitter / transceiver / transponder to automatically emit their info, at as fast a rate as possible, maybe at least 30hz. This info will be a unique ID and the signal strength / channel power of the moving object, for which I can use trilateration to calculate the positions.

    I do not think GPS will be sufficient for me to due to lack of accuracy and high power consumption. I don't think that bluetooth will have sufficient range.

    Does this sound valid? Can anyone suggest improvements or critique this? How will I sync the transmissions?
     
  2. wayneh

    Expert

    Sep 9, 2010
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    I wouldn't be so quick to dismiss GPS. Anyone with a smartphone can "broadcast" their live position, log their tracks and so on. Al the heavy lifting of the software has been done already for you. There are apps, for instance, where you can watch where all your friends are in real time. You may need to use external GPS pucks to achieve better accuracy than the units in most phones.

    How much accuracy do you need? My gps (Sirfstar III based) is usually good to a meter or two if I'm out in the open.

    Steve Wozniak was working on something like you've described, for watching kids on a recess playground for instance, but it never hit the commercial market.
     
  3. GenericJoe

    Thread Starter New Member

    Aug 31, 2013
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    Hi, thanks for the reply. I am dismissing GPS because it is expensive but primarily because it is too inaccurate. With a local RF setup I am hoping for inch accuracy - GPS may sometimes be able to get within a couple of meters but that's not reliable or accurate enough.
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    You might look at Differential GPS. And if GPS receivers that are under $10 for the IC are too expensive, what kind of budget are you working with?

    The bigger concern I'd have with GPS might be getting a high enough update rate.

    Don't expect t get inch scale accuracy based on any kind of power measurement. These are notoriously innaccurate and wil vary as people turn or as one person passes in front of another person, or as someone lifts there arms. And that's without introducing metal bikes into the picture.

    Another thing to keep in mind is that one inch is about one wavelenth at 12GHz.
     
  5. wayneh

    Expert

    Sep 9, 2010
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    I think you've got it backwards. You can get higher accuracy from GPS, albeit at a cost. Do you have any example of getting inch accuracy by any other RF method?

    Video cameras and some clever (recognition) software could probably do it. That would free the participants from having to be broadcasting.
     
  6. GenericJoe

    Thread Starter New Member

    Aug 31, 2013
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    Thanks for the reply guys.

    I am slowly coming to think that perhaps radio frequencies are not the answer. I genuinely am surprised, I just presumed this sort of thing was a common problem that had been cracked by now. I read that RSSI or even RCPI is just not accurate or reliable enough.

    You are right - GPS receivers are not too expensive, but I do worry the update rate would unfortunately present a problem for fast real-time tracking at such an accuracy.

    Video based tracking is something I hadn't considered, but you make a very good point about the participants not needing to be broadcast, at least beyond the normal light they reflect. That could make it simpler, and certainly cheaper.

    Back to the drawing board...
     
  7. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Have a look at the "robot soccer" websites. Those guys have developed it to a fine art. And yep, they use an overhead camera.
     
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