## 3 Real-Time Measurement Scheme for Light Intensity Flicker and Fiber Coupling Efficiency
During the descent of the airship at an altitude of 1000 m from the ground, a set of scintillation variance values is collected every 20 m. \(C_{n}^{2}(h)\) is calculated in real time using the formula \((C_{n}^{2}(h)=\sigma_{I}^{2}/1.23k^{7/6}L_{p}^{11/6})\). A schematic of the airship-airship experimental link is shown in Figure 3. The communication modes and related system parameters are shown in Table 1.  
The atmospheric refractive index structure constant, \(C_{n}^{2}(h)\), varies greatly with season, day and night, altitude, geographic location, and meteorological conditions. It is an  
Figure 3: (**a**) Schematic of atmospheric turbulence refractive index structure constant measurements. (**b**) Schematic of scintillation index and fiber coupling efficiency measurements (**c**) Airship-mounted laser communication system. (**d**) Ground station receiver. important representation parameter in laser atmosphere transmission. [PERSON] gave an empirical formula for the variation of \(C_{n}^{2}(h)\) with height, called the H-V model, which is applicable in the range of 3-24 Km, based on actual measurements, and can be expressed as follows:  
\[C_{n}^{2}(h)=0.00594(\frac{V_{RMS}}{27})^{2}(\frac{h}{105})^{2}\times exp\bigg{(} -\frac{h}{1000}\bigg{)}+2.7\times 10^{-16}exp\bigg{(}-\frac{h}{1500}\bigg{)}C_{n}^{2 }(0)exp\bigg{(}-\frac{h}{100}\bigg{)} \tag{8}\]  
where \(C_{n}^{2}(0)\) is the refractive index structure constant near the ground, and V\({}_{\text{RMS}}\) is the wind speed in the vertical transmission path. Seasonally and diurnally varying atmospheric conditions also affect \(C_{n}^{2}(h)\) at different altitudes, and the experiments were carried out under weak and medium turbulence weather from nighttime in winter to early morning the following day. The wind speed in the H-V model is set to a typical value of 21 m/s, and \(C_{n}^{2}(0)\) is set to a typical value of \(8.5\times 10^{-15}m^{-2/3}\) at night. Figure 4 shows the results of the \(C_{n}^{2}(h)\) actual measurement and the denoising measurement compared with the H-V model [21]. The measured \(C_{n}^{2}(h)\) value is obtained by the real-time \(\sigma_{l}^{2}\) of the laser link, which has a better real-time performance compared to the \(C_{n}^{2}(h)\) obtained by the weather forecast [22; 23] model and is able to more realistically reflect the turbulence change in the channel link. The measured value after denoising is close to the theoretical value of H-V. Real-time \(C_{n}^{2}(h)\) measurements help to evaluate the effect of light intensity flicker on fiber coupling efficiency. \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Parameters** & **Indicators** \\ \hline \multirow{2}{*}{Laser wavelength} & Beacon & 790/850 nm \\  & Communication & 1550/1530 nm \\ \hline \multirow{2}{*}{Divergence angle} & Beacon & 100 \(\upmu\)rad \\  & Communication & 2 mrad \\ \hline \multicolumn{2}{c}{Optical aperture} & 100 mm \\ \hline \multicolumn{2}{c}{Communication rate} & 1.25/2.5 Gbps \\ \hline \multicolumn{2}{c}{Tracking accuracy} & 5 \(\upmu\)rad \\ \hline \multicolumn{2}{c}{Receiving field of view} & 50 mrad \\ \hline \multicolumn{2}{c}{Focal length} & 300 mm \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Main parameters of laser communication system design. Figure 4: Measurement results of the refractive index structure constant of atmospheric turbulence before and after denoising. Since the tethered blimp is only suitable for working in the near-ground altitude range, in order to ensure the smooth progress of the on-board laser communication experiment, the blimp was lifted to a height of 1000 m. Six communication experiments between the airship and the ground were carried out under the weak and medium turbulence weather from 8:00 pm to 5:00 am the next morning in winter.