Ignore the range rings, they are temporary to help with calibration. I'm having trouble with this, for various technical reasons, one being the pixel shader is single pass and I can't store any information at all.
After some early success hooking the radar range readouts up to the engine RPM gauges (which was a bit strange), Here's the state of play at the moment:
The game engine only sends the info in meters, but then again the radar range is in meters, even if you choose imperial. The range is correct up to 9,999 meters, after which the first digit refuses to move. I haven't been able to find a send range to TDC button - does anyone know if there is one for the U-Boat campaign on the German set?