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I am looking to build a a device that can transmit/receive its distance from another device, with the other device doing the same up to 50 meters. Will using a fm transmitter and receiver with a microprocessor be able to do this?

I just started in Engineering at university so I don't have a broad background in electronics.

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  • \$\begingroup\$ No, it would not. Calculate the time taken for a round trip at the speed of light and you'll start to see why it is challenging - it starts to be in the range that a fast microcontroller could measure, but the modulation bandwidth of your typical "fm radio" is too low, and it's not bidirectional. \$\endgroup\$ – Chris Stratton Nov 4 '14 at 19:43
  • \$\begingroup\$ (Ultra) sound travels slower, hence it is easier, but requires a line-of-'sight', and I think 50 meters might be a challenge because the reflected signal will be very weak. \$\endgroup\$ – Wouter van Ooijen Nov 4 '14 at 20:12
  • \$\begingroup\$ What about measuring signal strength? Does your receiver produce an RSSI value? I've never done it, but it might be worth looking into. \$\endgroup\$ – curtis Nov 5 '14 at 5:38
  • \$\begingroup\$ @ Leon Heller, Michael Karas, PeterJ, Chetan Bhargava, Nick Alexeev♦: The newly edited title: "How would I measure distance with a transmitter/receiver pair?" puts the device (the T/R pair) squarely into the unique class of devices known as "transponders", so the reason used to put the post on hold - being "too broad" - is hardly accurate. In addition, claims that: "there are too many possible answers" or that: "good answers would be too long for this format" are attempts to predict the future or stifle subjects and should not be used to subjectively censor either questions or answers. \$\endgroup\$ – EM Fields Nov 6 '14 at 7:14
  • \$\begingroup\$ Echo as Radar, Interferometry techniques, and/or deferential reception techniques. \$\endgroup\$ – Optionparty Nov 6 '14 at 14:57
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If both devices can transmit and receive, then you could arrange a simplex system where when one unit receives a ping from the other it immediately answers, and so on.

Knowing the receive-to-transmit delays in each unit and subtracting them from the total time per transmit-receive cycle would allow you to get the time of flight of a pair of pulses and, knowing the velocity of propagation of RF, the distance between the two devices.

There's obviously a lower limit to the measurable distance, but depending on what you need to do, the scheme might work for you and is sketched out, roughly, below:

enter image description here

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    \$\begingroup\$ I feel this is really non-trivial and errors on rx-to-tx will dominate the time of flight for the specified distance. \$\endgroup\$ – clabacchio Nov 5 '14 at 10:01
  • \$\begingroup\$ @clabacchio: Definitely non-trivial, but the point is that if the "errors" are constant and can be quantified as chunks of time,"DT" say, then they can be subtracted from the the TOF1+DT+TOF2 cycle and leave the difference as 2TOF since the distance will, ostensibly, be the same for both paths. \$\endgroup\$ – EM Fields Nov 5 '14 at 21:32
  • \$\begingroup\$ @clabacchio: Please see my edited answer. \$\endgroup\$ – EM Fields Nov 6 '14 at 0:51
  • \$\begingroup\$ I see. Maybe it could also be done with ultrasounds, that should be more accurate for such short distances. Provided that he has line of sight of course. Or maybe GPS? \$\endgroup\$ – clabacchio Nov 6 '14 at 7:47
  • \$\begingroup\$ @clabacchio: The problem with ultrasound is the horrible attenuation hit you take with frequency; something like 1.3dB/m at 40kHz with 50% relative humidity, data from kayelaby.npl.co.uk/general_physics/2_4/2_4_1.html GPS would be good if the locations of the units could resolved with the required accuracy, and of course there would be the added RF burden needed to send their coordinates to ??? \$\endgroup\$ – EM Fields Nov 6 '14 at 12:53
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If you sent either a radio signal, or a flash of light (perhaps at a given wavelength that could be filtered and more easily recognized), and then simultaneously sent a loud sound at a specific frequency (again so it could be filtered and recognized above any ambient sounds), then you could calculate the distance much the same way one calculates the distance from a bolt of lightning, by starting to count when you see the flash, and stopping when you hear the thunder.

If you don't want anyone to be aware of your activity, you could use infrared light and ultrasonic sound frequencies. If you don't have line of sight for the light signal, use a radio signal for the initial synchronization.

To be precise, you would need to adjust the speed of sound by temperature (which can be handled via a cheap 30ȼ temperature sensor and a table lookup).

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