## 1 Introduction  
Free-space optical communication (FSO) technology has been around for a long time. Perhaps the first kind of FSO system was the photophone that [PERSON] invented in 1880. Since then, FSO technology has progressed through extensive research efforts, and has even reached NASA's and ESA's deep-space communications projects [1]. FSO communication is based on data transmission in unguided propagation media through the use of optical carriers in the vicinity of the visible to near-infrared (NIR) spectrum. This fact allows for a much higher optical bandwidth comparing to their radio-frequency (RF) counterparts, thus providing an excellent solution for high rate communication within a range of several kilometers [2]. A wide variety of applications can be suitable for FSO communications, ranging from inter-chip connection to inter-satellite communication [2]. The main attractive features of FSO communications can be summarized to (i) the smaller size, weight and power (SWaP) requirements, (ii) the high-gain concentrated energy delivered due to the laser narrow beam and (iii) the absence of any license restrictions due to their operational wavelength [3]. In order to assess the performance of an FSO link, we usually make use of the ratio between the signal power received and the signal power required to achieve a specified data rate with certain error probability, namely the \"link margin\" [4]. Therefore, given a specified sensitivity threshold for an FSO system, we can assess its performance by measuring the received signal strength indicators (RSSI) in a certain location under different conditions. The RSSI parameter plays a key role to the functional behavior of a hybrid RF/FSO, since it determines the switch from one mode to another. Apparently, the FSO technology does not come without limitations for its performance. Since the optical signal travels through the atmosphere, it experiences losses due to several factors. The major cause of performance loss is molecular and aerosol absorption and scattering of the propagating beam. In order to quantify this attenuation, the optical depth (\(\tau\)) that relates the transmitted to received power is utilized as a suitable metric, and their ratio determines the atmospheric transmittance [5]. Several atmospheric \"windows\" exist where absorption effect is minimized, which is where most FSO systems operate. Moreover, the scattering effect is strongly related to the relative size of the particles to the optical signal wavelength and causes signal energy to be re-distributed in random directions. Secondary, but not negligible, losses are caused due to free-space propagation, divergence of the beam, weather conditions effects and pointing errors. All previously mentioned effects result in received power degradation that, in the worst-case scenario, can even cause link outages and increased bit error rate (BER), thus directly affecting the system's performance and reducing the effective operational range of the link [6]. Another significant limiting factor is the atmospheric turbulence, characterized by the random fluctuations of the refractive index (\(n\)) due to pressure and temperature variations along the beam path. Turbulence gives rise to intensity fluctuations of the received signal, the so-called scintillation effect, which in turn causes a loss of SNR and induces signal fades [5]. The characterization of the refractive index structure parameter within the surface layer has been a long lasting independent research effort for different kinds of surfaces and atmospheric conditions. Over the past 20 years, the US Naval Research Laboratory (NRL) has conducted extensive atmospheric turbulence measurements, focusing on the maritime environment. Their main goal has been atmospheric propagation research and photonic components development in order to characterize and overcome the limitations due to turbulence effects.