Wether in geodesics, in security systems, production controls or just on the golf corse - the measuring of distances with lasers or laserscanners is well established nowadays. The time - namely the time, the light needs to pass the needed distance - is the value to be determined. This sounds so simple, but it isn't - because the velocity of light is very, very high - it's 2.99792*108 m/s. In consequence one has to deal with very small times. In only one microsecond the passes 300 meters. A high resolution in place therefore demands highest resolution in time.
For pratical reasons the reflection mode is used. Tranceiver and receiver are placed in one device. For the distance to be measured we find d=c*t/2 with c=velocity of light. So if we take a distance of 1 kilometer, the time of flight will be 6.7µs. If the resolution has to 1cm we already need a time resolution of 67ps. TDC-GPX driven in M-Mode shows a peak-to-peak resolution very close to this. The resolution can be further improved by averaging without seeing any systematic errors from the TDC.