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RF distance measurement

Discussion in 'Electronic Design' started by Joe G \(Home\), Aug 8, 2005.

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  1. HI All,

    I want to send data from transceiver A and receive it in transceiver B and
    retransmit back to A - I want to time the round trip.

    TRx(a) =>TRx(b)=>TRx(a).

    I want to time the round trip to calculate the distance between TRx(a) and

    Hence - I want to measure the distance between 2 transceivers between 5 and
    100 metres

    This method of distance measurement seems thwart with danger.

    Has this been done before?
    What problems (challenges) should I watch out for?
    Do you have any thoughts on this subject.

    Thanks in advance.
  2. Why have a re-transmitter?. Makes the system twice as complex as it needs
    to be, adds an extra delay, and requires use of two frequencies. Just use
    a retro-reflector. This is then radar...
    The biggest problem is just measuring time accurately.

    Best Wishes
  3. PeteS

    PeteS Guest

    1. This has been done before extensively for both the radar (from
    Roger) and re-transmitted. There are things called transponders
    (transmission responders) which may re-radiate the transmitted signal
    or radiate a different signal in response (This is how we get IFF,

    As noted, the simplest way is to just reflect the signal. If this is
    not possible (because you don't have a clear line of sight), then a
    transponder is appropriate.

    The pitfalls are ensuring a known and repeatable time between reception
    and retransmission / new transmission of the signal.

    Radar range is very simply
    Distance = C t / 2 where C = velocity of electromagnetic radiation [n
    free space], and t is the time between transmission and reception. The
    division by 2 simply shows that the pulse travels twice the distance
    (there and back :)

    Things to watch for.
    (Straight radar principles)
    Minimum range. This is set by the transmitted pulse (you can't receive
    while you are transmitting, but this may not be so for a separate
    receiver at perhaps a different frequency, such as a transponder).
    Distance (min) = C t / 2 where t in this case is the pulse width of the
    transmitted signal.

    Maximum range.
    Set by the pulse repetition rate. If there is some time t between
    pulses, then the maximum time to wait for a single pulse (with simple
    techniques, anyway) is the amount of time between the pulses. (Once
    another pulse starts, we reset the range - this can lead to aliasing).
    So Distance (max) becomes simply C t / 2 where t in this case is the
    time between pulses.


  4. choose time of flight pulse or continuous modulation/measure phase(easier)

    Your problem statement is too broad, narrow it down some more so real
    solutions can exist.
  5. Dave

    Dave Guest

    For measuring time accurately, you might consider buying an old HP 5370A
    or 5370B time interval counter. That has a resolution of 20ps, and
    absolute accuracy under 100ps can be achieved. A 5370B sold on eBay a
    couple of days back for $199. They cost >$20,000 in their day.

    There is a manual on it on my web site.
  6. quietguy

    quietguy Guest

    Well, you can buy the gadgets that do this for about $20 these days at the
    local hardware store - not sure if the cheapy ones go to 100 metres tho

  7. Mac

    Mac Guest

    Of course.
    Well, at 5 meters, I think you will need a short pulse and very fast RF
    switches unless you use a different frequency for the return. This is
    pretty difficult. You will need a lot of special equipment to get it up
    and running. I don't think you will succeed.
    Use ultrasound (see

    If you MUST use RF, I would suggest using a wide-bandwidth FM homodyne
    type radar. The distant target would be a retro-reflector or
    a phase-locking transponder. This still won't be trivial, and you would
    have a low probability of success, IMO.

  8. RF distance measurement can be greatly simplified by using linearly
    SWEPT frequencies that result in an audio difference.

    Originally used on the APN-1 radar altimeter.

    Many thanks,

    Don Lancaster
    Synergetics 3860 West First Street Box 809 Thatcher, AZ 85552
    voice: (928)428-4073 email:

    Please visit my GURU's LAIR web site at
  9. Mac

    Mac Guest

    I agree. This is, in fact, what I meant when I said FM homodyne. I just
    left out the "linear" part.

    A linear FM homodyne radar transmits and receives simultaneously, and
    mixes the TX and RX waveforms together. The mixer output is LPF'd to give
    the audio frequency you are talking about.

    I encountered a certain amount of flack in this newsgroup once when I
    suggested that downrange resolution is given by:

    R = C / (2 * BW)

    Where R is resolution, C is the speed of light in the relevant medium, and
    BW is bandwidth.

    This is really the definition of range, but there are radars, or
    radar-like devices (FM CW tank level indicators) out there which provide
    much more precise range estimation than this. I think this is only
    possible when it is known in advance that there is only a single
    scatterer, or a single scatterer in a certain area, and when the signal to
    noise ratio of the return signal is pretty high.

  10. Jim Backus

    Jim Backus Guest

    Yes - one common use is secondary radar (derived from IFF).

    Secondary radar uses a 3 microsecond delay in the transponder. The
    normal radar range calculation takes this 3 microsecond delay into
    The return time at 5 metres is ~ 30 ns; at 100 metres it is ~ 600 ns.
    Using radio transmission will give a real headache getting accurate
    results. Ultrasonics would give much longer transmission times and
    ease measurement.
    As above ;-)
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