## 1 Introduction  
Avalanche photodiode (APD) detectors, with indium-gallium-arsenide semiconductor technology (InGaAs), together with low-noise transimpedance amplifiers (TIA) operating at 15 xx nm wavelength, are becoming the standard receiver frontend (RFE) technology for optical inter-satellite links (as with mega- constellation satellite networks), as well as in space-to-ground communication [1, 2, 3, 4, 5, 6, 7, 8]. APDs have been in use in low-cost fiber communication links (for up to 10 Gbps data rates) since their internal photo-electron gain allows for extended distance reach when compared to less sensitive -10 dB (and more conventional) positive intrinsic negative photodiode (PIN) receiver technology [9]. While a constraint-linked budget in fiber links can be compensated for through intermediate optical amplifiers, this is not possible in long-range free-space optical communications (FSO). Therefore, APD-RFEs are the preferred opto-electric frontend for optical space links with intensity modulation and direct detection (IM/DD). Most relevant is the simple On/Off-Keying (OOK) modulation, i.e., a digital 'one' is coded as a signal pulse. The foundations for such downlink technology were laid by the GEO-downlinks from OPALE, onboard ESA's Artemis satellite in the SILEX demonstration [10] and the OICETS experiments from 2005-2009, with silicon-based APDs operating at 8 xx nm wavelength at that time [10, 11, 12, 13]. In current implementations, the wavelength range of around 1550 nm has evolved to become the most useful due to the re-use of terrestrial fiber communication components (foremost, the efficient erbium-doped fiber amplifier, but also the range of wavelength channels, data-modulators, and detectors). Also, the high achievable data rates and other methods for increasing the sensitivity (like optical pre-amplification or coherent modulation) are desirable. Here, optical space communications follow terrestrial systems [14], but these often require a higher implementation effort, such as with adaptive optics for the single mode fiber coupling of an atmospherically distorted optical field [15]. Bulk AP-detectors do not require such sophisticated techniques and allow focal illumination with an atmospherically distorted light field. This typically works for up to 10 Gbps and slightly more, whereas, for higher data rates, only the coupling of this into a single-modefiber is required. Besides pure communication, optical data transmission allows for the application of other photon-based techniques, like the exploitation of quantum effects for key-distribution transmission [16]. Current implementations of space-ground links employ component technology from dense wavelength division multiplexing (DWDM) with bulk optical detectors at optical ground stations (OGS) [17; 18; 19]. A variety of environmental and orbital parameters influence the short- and long-term received optical power in long-range mobile FSO systems, such as satellite up- and down-links [20]. The reasons for changes in the mean received power-besides being blocked by cloud [21]-are the intermittent increase in atmospheric attenuation (haze, precipitation) [22], varying free-space losses (caused by changes in link-range), beam pointing errors [23], and scintillations caused by the atmosphere's index of refraction turbulence (IRT) [24; 25; 26]. Altogether, the mean received optical power can change by orders of magnitude over very short time frames. Sophisticated data-based electronic techniques, like adaptive coding and rate adaptation, might be able to cover these steep and fast variations. However, physical APD-RFE parameters, like diode bias voltage, limiter decider threshold, and receiver bandwidth, must also be constantly adopted to ensure optimum receiver performance. Specifically, the bias voltage is not only a function of temperature but also of instantaneous received optical power, dark current, and foremost, the background light [27; 28; 29; 30; 31; 32].