Wireless communication has long been essential for transmitting mission-critical information across both military and civil domains. From ancient fire signals, smoke beacons, ship flags, and semaphore telegraph messages to modern satellite networks, wireless communication technologies have continuously evolved to overcome limitations in propagation range, transmission latency, and spectral efficiency. The growing demand for high-volume data transmission and the increasing congestion of the radio frequency spectrum have accelerated the development of free-space optical (FSO) communication technologies. FSO communication has the potential to provide high-speed, high-throughput, and spectrum-unregulated channels that can serve as the backbone of future integrated terrestrial, airborne, and space networks. However, despite their advantages, the performance of FSO systems remains critically sensitive to background noise—most notably solar radiation—as well as atmospheric fluctuations, which collectively compromise their operational robustness. This susceptibility is exacerbated in satellite-to-ground links wherein intensity modulation techniques are widely adopted, as signal strength is directly exposed to fluctuations in background light.

To address these challenges, we sought to quantify and analyze the impact of background solar noise and to develop ground terminal architectures that mitigate its effect on the signal-to-noise ratio, bit error rate (BER), and beacon detection accuracy. To this end, we designed the Agency for Defense Development optical ground station (ADD-OGS), an all-free-space ground terminal capable of supporting 2.5 Gbps in ground-satellite links while operating during the daytime (Figure 1). Designed in compliance with Space Development Agency (SDA) standards, the system uses spatial and spectral filtering with optimized wavelength selection, to effectively suppress solar-induced background noise. To validate its performance, we conducted a terrestrial experiment simulating a low-earth orbit (LEO) satellite downlink by establishing a 7-km FSO link between the ADD-OGS and a mobile terminal that was approximately equal to the atmospheric path length encountered in LEO satellite-to-ground scenarios. Despite the high solar noise environment, the ADD-OGS achieved a BER of 5.85 × 10⁻⁸ at a data rate of 2.5 Gbps. This result agrees closely with the theoretical performance predicted through the analytical modeling of multiple noise sources, including shot, thermal, dark current, signal-background beat, and background-background beat noises. The relevant research results are published in Photonics Research, Volume 13, Issue 9 (2025). [Heesuk Jang, Hajun Song, Hansol Jang, "Evaluation of daylight background noise for satellite-to-ground free-space optical communication during daytime operation," Photonics Res. 13(9), 2630 (2025)]

Fig. 1 ADD-OGS illustrations: (a) Schematic of daytime ADD-OGS operation in horizontal coordinate system; (b) Photograph; (c) Optical configuration; (d) Optical configuration on a piggyback; (e) Optical configuration on a Coudé table.

A distinctive contribution of this study is the quantitative spatiotemporal mapping of solar background noise encountered during satellite-to-ground FSO communication. Through the tracking of an actual LEO satellite, solar noise variations over sample satellite orbital passes were systematically characterized with respect to relative right ascension and declination by using a quadrant photodiode-based detection system (Figure 2). These findings establish a comprehensive empirical framework for assessing solar noise exposure along realistic satellite trajectories. Figure 3 illustrates the temporal evolution of solar background noise across a full daytime observation window, revealing pronounced asymmetries caused by localized, terrain-dependent reflections. Most notable are the noise power spikes observed in the afternoon due to reflective structures near the ground terminal. Moreover, by integrating 3D background noise maps with the satellite trajectory, the analysis visualizes how elevated background noise during satellite tracking correlates with surrounding topographical reflection sources. This observed coupling between local topographic features and background noise fluctuations underscores the critical importance of incorporating terrain-aware solar noise distribution modeling into satellite trajectory optimization. Furthermore, the optical design and strategic placement of high-performance FSO ground terminals during the early phases of space mission architecture also need to be considered.

Fig. 2. Satellite tracking and solar noise measurement tests: (a) Evening satellite-tracking test; (b) Daytime solar noise measurement test while the ADD-OGS was following the satellite trajectory; (c) Image-based tracking of satellite using an SWIR camera; (d) Altitude angles; (e) Right ascension angles (RAs); (f) Declination angles (DECs); (g) Solar noise incident on ADD-OGS and angular changes; (h) Altitude angles; (i) RAs; (j) DECs

Through both modeling and experimental validation, this study demonstrates that robust high-speed optical links are feasible even under challenging daytime conditions. This study further contributes a solar noise database linked directly to system-level link performance, bridging the gap between theoretical link budget models and a practical FSO system implementation. The results serve as a catalyst for advances in next-generation FSO terminals supporting terrestrial, airborne, satellite, and even underwater communication platforms. Additionally, this study lays the groundwork for future applications in quantum communication, light detection and ranging (LiDAR), and green energy technologies. Building on the validated performance of ADD-OGS, a next-generation transportable version (ADD-OGS v2) is under development to enable diversity of weather and site operations. The systems will undergo real-world validation via an upcoming mission involving optical communication with a LEO satellite scheduled for launch soon, providing critical insights for the deployment of robust and scalable FSO infrastructure across ground, air, and space.

Fig. 3. Solar noise incident daytime measurements of ADD-OGS: (a) Measurement setup for 12 h incident on ADD-OGS; (b) Measurement setup for incident on ADD-OGS in different spatial positions; (c) Temporal dependence; (d) Altitude angles; (e) RAs; (f) Daytime DECs of sun and ADD-OGS; (g) Angular dependence; (h) Midday 3D spatial map at different RAs and DECs of ADD-OGS relative to sun; (i) 2D contour map overlaid with satellite trajectory from Fig. 2(g).