Flywheel Calibration of Coherent Doppler Wind Lidar

Within the field of lidar measurement it is often debated why it is necessary to calibrate lidars given that they are ‘absolute’ instruments. By this is meant that, given two parameters; the laser wavelength and the frequency at which we sample the backscattered light, we are able to calculate the measured radial speed through the well-known equation 푉푉푟푟 = 1 2 휆휆 ⋅ ∆푓푓. Unlike for example, a cup anemometer or even an LDA, there are no empirical constants that have to be found through a calibration.
Why then do we claim that lidar calibration is necessary anyhow? Probably the most direct answer is that without a calibration (comparison to a reference with known uncertainty) we cannot know that the lidar is getting it right. There could be wrong constants or some (maybe subtle) errors in the algorithm (frequency analysis is not trivial). Only by comparing to a known ‘truth’ can we be completely sure that the lidar gives the correct speed. More formally, since the uncertainty of the reference is known and, by
implication, the reference is traceable to international measurement prototypes, we can assign an uncertainty to the lidar radial speed and claim traceability. In commercial measurements where the outcome can have financial consequences, it will usually be a requirement that the measurements are traceable.