  The sodium laser guide star (sLGS) adaptive optics (AO) system has become an
essential component for large ground-based optical/infrared telescope. The
performance of the AO system can be significantly hindered if the on-sky spot
profile of its sLGS is degraded by the turbulence along the laser launching
uplink path. One effective method to overcome this problem is to perform
real-time pre-correction on the laser before it is launched onto the sky to
counter this turbulence effect especially for the lower altitude where
turbulence is stronger. The by-product Rayleigh backscatter generated by large
molecule and aerosols at low altitude when projecting sLGS is a perfect
candidate for real time detection of low altitude turbulence. It is therefore
important to evaluate whether this by-product of a sodium laser guide star
could achieve suitable performance for wavefront sensing. In this paper, we
attempt to answer the question regarding to the achievable photon return of
such Rayleigh backscattered plume. A comparison between results from our field
test and theoretical model of MSISE-90 was presented. We showed that the
greatest differences is less than 20% which hints the applicability of using
the defocused Rayleigh plume as wavefront sensing signal for different
turbulence strengths. The result also showed that in case of strong (Fried
parameter (r0) of 5 cm) or moderate (r0 of 10 cm) turbulence, the optimum
center for range gating is 9 km, and the corresponding value of range gate
depths are 3 km, 1.1 km, 0.56 km and 0.28 km when subaperture sizes are 3 cm, 5
cm, 7 cm and 10 cm, with the maximum pulse repetition frequency at 1500 Hz.
