anyone aware of options to build a proximity sensor grid?

For example, to determine the 3 dimensional location of an object relative to the sensors.

I'm trying to look into the viability of building a tool for teaching climbing technique in indoor rock gyms. I assume I would need a sensor in each hold that constantly measured the distance to 5 targets in real time - one on each wrist, one on each foot, and one on the waist.

The problem is that in order to work, the sensors need to differentiate between the five targets. Maybe sonic sensors in each hold, with five emitters, each operating at a different frequency?