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I am trying to understand if there is any way to measure the distance between two vehicles, possibly moving, without the aid of GPS. I want to measure a distance of more than 50 meters but less than 100, with an accuracy of at least 5 meters.

The purpose of this type of measurement would be so that vehicles could understand the proximity of each other if out of sight of each other in dense forest/bushland

I would like to read this measurement from both vehicles.

If anyone could give me a push in the right direction, or suggest a way of achieving this, I would greatly appreciate it.

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    \$\begingroup\$ Let's rule out other absolute position methods like Loran-C as 'GPS', especially as that one's been turned off! It sounds like you only need ranging, not bearing, so a simple distance measurement is adequate. 'Without line of sight' eliminates most optical methods, you need waves that can go round trees. Lower frequency radio, or audio ranging could be coerced into working. 5 metres resolution there and back is 30nS delay for radio, so you only need 30MHz or so bandwidth for timing resolution. \$\endgroup\$ – Neil_UK Nov 15 '16 at 10:43
  • \$\begingroup\$ I agree with Neil, probably a sonic transmitter in one vehicle and a receiver in the other with a bi-directional radio data link between them to share ranging info and also act as a sync reference for the sonic pulses. Don't ask me what frequency but i suspect below 1 kHz is going to give better results than above. \$\endgroup\$ – Andy aka Nov 15 '16 at 10:52
  • \$\begingroup\$ If using audio for ranging outside then you'll need to compensate for air movement if you want any level of accuracy. That includes both wind and relative airflow caused by the cars motion. \$\endgroup\$ – Andrew Nov 15 '16 at 12:27
  • \$\begingroup\$ A two way UWB ranging radio system would work well if you had line of sight. 100m is pushing it on the range but just about possible. But I have no idea what impact trees in the way would have on range and accuracy. \$\endgroup\$ – Andrew Nov 15 '16 at 12:29
  • \$\begingroup\$ I've seen some demo's recently (within the last 2 years) of UWB radio ranging & positioning (triangulation) which were fairly impressive. The setting was in a large exhibition hall though, so ymmv in a dense forest. \$\endgroup\$ – brhans Nov 15 '16 at 12:37

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