Abstract

The line-averaged refractive-index structure parameter $C_n^2$ can be measured conveniently by observing the irradiance fluctuations of an incoherent light source with a receiver, where both the transmitter and receiver apertures are the same diameter and large compared to a Fresnel zone.¹The measurement is most sensitive to $C_n^2$ in the center of the path, tapering to zero weight at the ends of the path. The incoherent apertures form broad spatial filters that have a peak response to refractive-index irregularities having a spatial wavelength slightly larger than the aperture diameter. The weighting function may be shifted from the center of the path by using different diameter transmitters and receivers and by taking the difference of two transmitting or two receiving aperture signals. To avoid the effects of scintillation saturation, however, each individual aperture should be sufficiently large,¹which restricts the filter complexity for an instrument of practical size. With these factors in mind, we developed a compact profiling instrument that measures $C_n^2$on three segments of an optical path between three-element transmitters and receivers.

© 1988 Optical Society of America