# Pico-Sat to Ground Control: Optimizing Download Link via Laser Communication
## 4 Results
### Performance Evaluation of SATLLA-2B  
We performed several experiments in order to validate the methods. Naturally, performing any experiment regarding nano-satellites requires significant resources. Thus, we employed the following methodology:  
* Performing an independent experiment on each component, e.g., ground station, satellite, swarm of satellites. * Ground station photon efficiency: A lab experiment was performed in order to validate the expected loss on the telescope. A known source of light was aimed towards the telescope (the laser beam was entirely within the telescope's field of view). On the telescope, a fiber optic was attached to the receiver adaptor. The received power at the fiber was measured to be at most 3 dB lower than the original laser power. We conclude that the overall power loss of the telescope can be assumed to be smaller than 3 dB (no more than 50%; in most cases, an efficiency of above 70% was achieved). * Ground station tracking: The tracking was performed using the ISS as a moving target (see Figure 5b). It is important to mention that two independent ISS tracking experiments were performed in suboptimal atmosphere conditions (central Israel in June and July 2021). In both experiments, a calibrated robotic telescope was able to track the ISS orbit using only time-based reference (without optical tracking). This allowed us to conclude that the expected aiming errors due to atmospheric turbulence are minor and have very limited effect on the performance of FSO from a ground station (the overall turbulence errors were lower than 0.01 degrees, whereas the telescope's field of view is 0.5-1.0 degrees). See [43] for detailed research regarding methods for adapting FSO links to atmospheric turbulence. Based on the ability to accurately track the satellite and assuming a clear night sky, the turbulence loss (\(Tur_{loss}\)) is expected to be bounded by 1 dB. * Performing ground experiments: We conducted a set of FSO ground experiments. These experiments were performed without a telescope in order to simulate a greater distance (from the satellite). Both the \(Att_{loss}\) and \(Tur_{loss}\) were equal to or greater than expected in the satellite FSO scenario (which assumes a clear night sky, as opposed to the \"noisy\" ground conditions). * Using existing FSO system analysis: As presented in the related works, satellite FSO has already been demonstrated several times. Therefore, we mainly focus on generalizing existing solutions, so as to make them applicable for swarms of picosatellites and a corresponding large set of optical ground stations. **Short-range FSO:** In this experiment [44], a low-power (1 mW) red laser was positioned about 1.8 km from the receiver, causing a \"spot\" of 10 m\({}^{2}\) (square meters). The receiver had an active area of 1 mm\({}^{2}\) (square millimeters), which formed a \(10^{-7}\) geometric ratio. The overall atmospheric path loss and Rx loss were also comparable with the expected losses from a satellite. During the experiment, a 42 kHz signal was transmitted and received usinga COTS APD detector, with an additional margin. Using a 500-times-larger optics (500 mm telescope), the range can be proportionally extended to 900 km. Moreover, by using a 1000-times stronger Tx power, the bit rate can be extended by a square-root factor, leading to an expected bit rate of above 1 Mbps. Note that this is a very conservative estimation, as the experiment setup can be improved dramatically with a better detector and upgraded optics and lasers. A longer-range experiment was repeated in a 9 km range scenario, as shown in Figure 7, resulting in similar performance evaluation under slightly harder atmospheric conditions (the 9 km range had an atmospheric loss of about 6 dB--twice the expected loss). In the short-range experiments, we modeled the expected satellite to ground station FSO parameters, in particular, \(G_{loss}\), \(Rx_{loss}\), \(Att_{loss}\), and \(Tur_{loss}\).