My first job of the day on most days is dog-walking. Our preferred route runs behind a golf course along the edge of a cliff which overlooks the Rivelin valley. The cliff side is practically vertical, and once you're off the very narrow path that threads along the top, it's completely overgrown with bracken and brambles and trees.   So once the dog veers off the path and into the undergrowth, she's completely invisible.

Fortunately, she's pretty well-trained and will come straight back when she's called, so it's no problem - in fact it's a net positive because of the energy she burns off haring around through the bracken. But it's quite useful to know roughly how close she is, if only so I can moderate how loud I have to shout if I need to call her back for any reason.

I'm also intrigued by just how far off she wanders as I walk along, and by implication how much further she walks/runs than I do. So I got thinking about some kind of device that would provide an indication of how far off the dog (or any other moving thing, for that matter) was from me.

What I'm thinking of is something that provides a degree of ambient awareness - I don't want something that flashes or bleeps because that would be overly intrusive. One of the useful side effects of having to walk the dog is that it's an hour's peace and quiet to think about things, and a machine going "bing!" would spoil that. And I'm really not sure I want to sound like a submarine while trundling along.

So what I'm thinking of is something that could strap to the wrist like a watch, and provide a gentle pressure which varies with the distance of said animal from me. It could be a tightening band, like a watch strap - or something that pressed with varying force.  Perhaps some kind of eccentric cam that rotates to tighten or apply the pressure. The force would be proportional to the distance of the dog - a very light touch when she's within view or earshot, and increasing as she gets further away.

The "visualiser" (what is the word for something that provides feedback by means of pressure rather than sight or sound?) piece of the kit should be simple enough - it would basically be a very small stepper motor or something similar. But I'm not sure about the sensor side of things. Basically it's a question of measuring the time for a signal to reach and return from the dog - but because we're talking relatively small distances, I'd have thought that measuring the out-and-return of a radio pulse would be too short and error-prone to be reliable.

So then I wondered about measuring the difference between two GPS devices - but again, I'm not sure that they'd be accurate enough. Getting an accurate GPS fix in that sort of overgrown area could be tricky, so there's likely to be too much error involved.

Or maybe radar-esque ultrasound? The "person" device squawks at one frequency, and the "dog" device squawks at a different frequency in return when it hears the first signal. The time difference between the send and receive is a measure of the distance. But that might be affected by echoes, and I'm not sure what effect blasts of ultrasound would have on the surrounding wildlife - or the dog, for that matter.