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I have a static transmitter and a mobile receiver. Is there any wireless technology to find the distance between the two in real time. Apart from using gps!

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    \$\begingroup\$ Welcome to EE.SE. More details needed: frequency, modulation, other aspects of the signal, environment, distances involved, desired accuracy, purpose. Otherwise, the question is kind of broad. Or, well, use GPS. \$\endgroup\$ – Nick Alexeev Jul 10 '13 at 5:56
  • \$\begingroup\$ Thanks Nick.. I think I need to do some research.. I will get back soon.. Sorry for posting this bluntly.. \$\endgroup\$ – Mullaly Jul 10 '13 at 6:03
  • \$\begingroup\$ Radar would work \$\endgroup\$ – Andy aka Jul 10 '13 at 8:45
  • \$\begingroup\$ If you can measure the direction from two receivers of a known location, you can calculate the position of the transmitter by triangulation. Search for "radio direction finding". \$\endgroup\$ – Phil Frost Jul 10 '13 at 13:10
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Depending on the distances involved, sensing / timing precision available at master transceiver, response speed at slave transceiver, and computational power available on the master device, Time of Flight mechanisms may be usable for distance estimation.

ToF measurement involves transmitting an identifiable, unique bitstream from the master transceiver, echoing it back either passively or actively from the slave transceiver, and measuring either the time taken for the round trip, or for shorter distances, phase differences between outgoing and incoming signals.

A useful paper that details this approach is this one by Steven Lanzisera et al, UC Berkley, June 2006.

It is noteworthy that ToF mechanisms are far more reliable than any signal strength measurement mechanism, because the impact of environmental conditions on the speed of light (or, to be precise, the speed in air of the RF frequency chosen) is marginal compared to the impact of environmental conditions on signal strength.

A point of key significance is that this sensing mechanism is severely impacted by reflection paths for the radio signal, which result in multiple longer round-trip radio paths, and thus multiple invalid distance values for the one minimal-path value. In other words, such ranging mechanisms provide poor precision indoors, compared to outdoor.

Also, the presence of (in effect) a long conductive path that would work as a sympathetic antenna would reduce the perceived time-of-flight, thus generating a shorter detected distance than in reality. For instance, measuring ToF while both transceivers are close to a long metal pipeline would significantly invalidate results.

Can RF ToF ranging be done with typical DIY electronics? The paper referenced above shows that it is feasible. That is not to say that it is simple, or that it is computationally feasible using a low-cost microcontroller development board such as used by hobbyists.

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You can use the RSSI level (Received signal strength indication) of the signal to detect the approximate the distance between transmitter and receiver in the line of sight situation. But this method is not of much use when it comes to non-line of sight environment since RSSI level can fluctuate by a large amount in the non-line of sight environment environment. of course this requires RSSI calibration.

Caution:

  • Use this method only when you dont have any other option.
  • This method will not provide accurate distance.
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    \$\begingroup\$ Also, Line of sight RSSI will vary with sunspot activity, (and partly related to this) air ionization, (and partly related to this) ambient humidity, time of day, ambient temperature, and other signals present in the band of interest. In short, it's not really useful for distance measurement. \$\endgroup\$ – Anindo Ghosh Jul 10 '13 at 10:36
  • \$\begingroup\$ RSSI is a terrible measure of distance for all the reasons above, plus the numerous much better methods available. \$\endgroup\$ – John U Jul 10 '13 at 12:33

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