RF Range Finder Doesn’t Need To See To Calculate Distance


Instructables user [Jones Electric] has been quite busy lately, building a radio-frequency range finder. Built as part of a German youth science competition, he and his partner built a pair of transmitter/receiver modules that can be used to measure distances of up to a mile (~1.5km). Their argument for radio-based rangefinders is that laser rangefinders are obviously limited to line of sight, whereas their range finders are not.

To determine the distance between the two stations, the base station is triggered, which starts a counter and sends a 433 MHz signal to the second station. When the second station receives the signal, it in turn broadcasts an 868 MHz signal, which is received by the base station. The total distance between the points is then calculated based upon the round trip time of the two radio signals.

[Jones Electric] claims that the range finder is relatively accurate, with a deviation of up to 5 meters per measurement, and that the accuracy could be increased by adding a higher frequency crystal to the timing circuit.

We are pretty sure using these two frequencies in the US without a license is not allowed, though we are unsure of the usage laws in Germany, where this was constructed.

35 thoughts on “RF Range Finder Doesn’t Need To See To Calculate Distance

  1. While it would probably take a much more powerful (and faster!) MCU, doing phase-comparison or rising/trailing edge detection on the signal can get you time-of-flight accuracy to a thousandth of a percent or better. That’s what GPS receivers are doing to measure distance to a few meters when they’re 20,000km away ;)

  2. Might be answered in the article (will take a look when a get more time), but how did they compensate for the “offset” or latency caused by the remote station (time to receive, detect, transmit). Given the speed of light, every nano second of additional delay will cause error (one feet (0.3m) per ns).
    I suppose that delay should be fairly constant, so some calibration can take care of it.
    Great idea!

  3. @macw
    Possibly I missed your point with detecting the phase detection, but not sure it’s applicable here.

    Remote “station” does not have a refference to compare the phase with, and if you do that on the main station’s side, it’s not te same signal anymore.

    GPS does it quite differently as they do not compare absolute dalays (what this device does), but relative delays between several (many) refferential points distributed arround the (visible) sky. Also all refferential points (satellites) are using high level of syncronization (interal clocks), that one can’t aford with a home brew :)

  4. Pointless. Laser range finders dont need to be in two places at once like this does. Thats really nice when you are trying to blow up the thing you are measuring the distance to. And there is a RF range finder, its called radar.

  5. ^^Dont be a delta bravo macona!
    Great project and congratulations on 1st prize. Your passive coordination sounds reminisce of LORAN, unlike that of an active RADAR.

  6. @macona I wouldn’t really call it useless. It says it’s part of a youth science competition, and I take this to mean they’re less than 18 years old (not professional engineers). While not very efficient as opposed to commercial alternatives, it is a very good exercise and project for young minds.

  7. They are using a 30Mhz counter and they are getting measurements that are +-5 meters?
    How is that possible (the light travels in a 30 millionth of a second 10 meters)?
    What about other delays, how are they compensating for temperature changes?
    I thing this is great work, I am just a little bit skeptical regarding the precision.

  8. 868MHz is used for alarms, RFID and similar low power devices in Europe. So the rangefinder is probably fine in Europe.

    OTOH, the various FM wireless microphones (bugs) transmitting in the 88-108MHz band that I have seen here are completely illegal in Europe. If someone complains of interference, you are in for a hefty fine.

    Check your local laws before tinkering with radio!


  9. If they took four measurements and calculated the average, they would in effect get one bit more information, which would mean that they would double their resolution.

    So, all they need is 16 times supersampling, and they’re down to 1 m accuracy, assuming that the deviation is random.

  10. Jan I think you will find European law not as simple as you make out.

    Low power FM transmitters are allowed. Infact my Nokia N97 handset has one built in which allows me to listen to MP3 files playing on the phone via my car stereo.

    The range is limited to a few meters, due to legal requirements.

  11. @Jan You are wrong. There’s a European guideline that had to be implemented in the whole Eurozone which allows you to run a FM rig on every frequency in 88 – 108MHz but only on low power (I thought it was 25 or 50mW EIRP).

  12. Can anyone tell me how to do this with Arduino or perhaps some other microprocessor(s)?

    The processor speed would be the limiting factor for precision. I really think this can be done WAY simpler.

    What if you use VxWorks or something

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