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I'm trying to build a smart frisbee for use in Ultimate. I'd like to be able to track the position of the frisbee in a playing field with pretty good accuracy (within ~2ft). GPS is a possibility, but I'm not sure if it is accurate enough. Are there any other technologies that would help, such as some system where I set up transmitters on the four corners of the field and place a receiver on a frisbee?

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  • \$\begingroup\$ You need to provide much more information. You should specify the size of the field; the area over which you intend to locate the frisbee. Do you just want the frisbee to record its location for later analysis or do you want to be able to somehow find the frisbee if it gets lost on the field? How long (how many hours) does it need to run before recharging or getting fresh batteries? Do you have a weight limit? \$\endgroup\$ – Joe Hass May 25 '14 at 21:11
  • \$\begingroup\$ I think you need to specify also how long a foot is. Mine are quite big, I beet you want to be "two tiny girl's feet" precise at least. \$\endgroup\$ – Vladimir Cravero May 25 '14 at 21:14
  • \$\begingroup\$ The field is approximately 50yards by 30yards. I want to track the position of the frisbee in real time and wirelessly feed data back to a laptop to visualize the flight path as it is flying. If the frisbee flies "out of bounds", it will perhaps trigger some LEDs to indicate out of bounds. \$\endgroup\$ – Ryan Norton May 25 '14 at 21:31
  • \$\begingroup\$ look into the cricket system from MIT. It's an utlrasound-based indoor positioning system. \$\endgroup\$ – kolosy Jan 6 '15 at 20:39
  • \$\begingroup\$ You should investigate some of the things the disc golf people are doing with this very same issue. pdga.com/chips-discs-now-what \$\endgroup\$ – philbrooksjazz Dec 30 '15 at 16:50
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You may be able to do this with a TrackR like low energy bluetooth tag attached to the center of the frisbee, and then four readers or more readers, along the side lines. The readers would then use the power level and triangulation to determine the position of the frisbee.

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GPS has a relatively low accuracy for every sample (in the order of 5-10 meters 2sigma [95% of values]) at 10Hz. A frisbee is not slow, so averaging (which reduces the position fix rate) to reduce the confidence interval is not going to make it suitable.

Inertial measurement units are much faster and much more accurate but drift over time (also called "random walk", due to the double integration of noise on accelerations). It may be acceptable for you, especially if you reset them regularly using an absolute position sensor or by putting the frisbee at a location which position is precisely known. Data fusion is doing this automatically on-the-fly, that's what GPS navigators do on top of shifting the position to the nearest known road.

For accurate absolute measurement, you can use differential GPS (also known as d-GPS) which will require a fixed beacon somewhere on the field and a wireless connection. This can give sub-meter accuracy down to 15cm.

Though d-GPS is perfect for you, it's usually complex and/or expensive, so I'd recommend turning yourself to IMUs with at least 6 degrees of freedom. You may want to decouple the system from the frisbee spin (i.e. bearing), in which case you'll need an IMU which integrates a magnetometer to take care of the angular motion of the accelerometers frame of reference while decoupled.

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  • \$\begingroup\$ The problem with an IMU on a frisbee is that you'll effectively lose 1 degree of freedom to its spin. I'm not sure how that would affect your ability to do the remainder of the necessary integration work. \$\endgroup\$ – kolosy Jan 6 '15 at 21:08
  • \$\begingroup\$ Well spotted. IMUs can be found in 6DoF versions though; and/or the system can also be decoupled from the frisbee rotation (good bearing). \$\endgroup\$ – Mister Mystère Jan 6 '15 at 21:13
  • \$\begingroup\$ But if you decouple it from its spin, then you still lose that degree of freedom - you're just ignoring all angular motion in that plane. \$\endgroup\$ – kolosy Jan 6 '15 at 21:25
  • \$\begingroup\$ The OP is solely interested in the position of the frisbee, why would he need any information on angular motion? I thought you were referring to the change in spin velocity that would give erroneous acceleration readings, greater as the sensor is further away from the center. \$\endgroup\$ – Mister Mystère Jan 6 '15 at 22:03
  • \$\begingroup\$ To get accurate 3D position data you have to make use of all 6 degrees of freedom. The accelerometer always measures in terms of its own frame of reference, and you need the gyro data to understand how that frame is changing over time, and how to exclude the gravity vector. \$\endgroup\$ – kolosy Jan 7 '15 at 5:30
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You can use ultra wide-band (UWB), but I think if you're asking the question it's not something you actually want to get into.

GPS is a good bet, both in ability to implement and low cost to experiment with. Plus a robust frisbee system there could be marketed to disc golf players. It will suffer the precision issues you mentioned.

Likely the best solution is computer vision. It won't be easy. If you're willing to add electronics to the disc you may as well place UV LEDs on it and track it with a Wii type system in addition to standard video tracking.

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  • \$\begingroup\$ To expand on the computer vision, using a few visible cameras to track based on frisbee color, you can also use IR cameras and rather than add anything to the disc, just have it painted with a highly IR reflective material. Using multiple spectra will make the system more versatile in varying lighting conditions. \$\endgroup\$ – rjp May 30 '14 at 16:42
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A microcomputer with an accelerometer and wireless transmitter on the frisbee can calculate and send its 3d position. I think it is possible to construct them sufficient in weight, except the battery, I'm not sure about it.

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