Measuring atmospheric turbulence along folded paths using a laser-illuminated differential image motion monitor

A laser-illuminated differential image motion monitor (DIMM) is presented that is able to measure the atmospheric coherence length r ₀ along horizontal ground paths. This is accomplished by implementing a mono-static setup in which the transmitter and receiver are co-located and transmit to a retroreflective target. The impact of propagating along a folded path through the same volume of turbulent atmosphere is investigated in detail and the overall impact to angle-of-arrival measurements described. In addition, an outdoor test campaign was conducted to validate the findings by testing two commercial scintillometers and the laser DIMM side by side in both bi-static and mono-static configurations. Both analytical and experimental results show that under certain conditions, folded-path propagation can be treated identically to traditional single-path propagation.