Ping from one device to another.

I would like to use a ping from one device to another at a specific frequency (so that multiple devices can be in the same vicinity and paired).

My idea is the initial ping causes the second device to ping back, giving a time lapse (i.e. distance).

The trick is, I don’t know where the devices are relative to each other, so I can’t just use an SRF05 or similar, I just need to know the distance between them (direction is another issue).

Is there an Ultrasonic sender module and separate receiver module that is frequency controllable and Omni-directional?

The purpose of this is to keep one device at a fixed distance from the other as a pair. One device is the master, the other a slave that will move to keep the distance between them fixed.

Any other methods or ideas would be appreciated.

Thanks in advance for any help.

Why not simply fit ultrasonic module to one section and make it keep fixed distance from other?

Weedpharma

The problem is I don't know the direction to look. Could be anywhere: up, down, 360 degrees on all axis...

I also won't know if what I detect is the correct object...