# Pico-Sat to Ground Control: Optimizing Download Link via Laser Communication
## 4 Results
### SATLLA-2B's Flight Result  
In this subsection, we present the preliminary results of SATLLA-2B from its low Earth orbit. The satellite was launched on January 2022 and was operational for 5 months. SATLLA-2B's angular speed is about 6-7 degrees per seconds, allowing a balanced thermal state of the satellite. Figure 9: The LED array of SATLLA-2 (Eng-model) as seen from 1.1 km. The image was taken with a standard 70-degree smartphone camera—shooting at 30 fps in 1080p video format. It should be noted that in low-light-pollution conditions (on clear nights), SATLLA2 can be detected from more than 10 km using a standard Raspberry Pi V3 camera (FoV = 80, Sony IMX519). Figure 9 presents the angular velocity of SATLLA-2B without any ADCS--as it rotates naturally from its initial launch (inertia). Such a low angular velocity implies that the required force needed to overcome the pico-satellite's momentum is minimal, and in fact can be constructed using a set of miniature reaction wheels (see Figure 7). Figure 9 presents the temperature of the satellite both on the outer and in the inner parts of the satellite. Figure 10 presents some sensory data as sensed by the satellite. Several preliminary experiments are performed on this pico-satellite in order to test the FSO demonstration payload. It should be noted that the general concept of global RF communication (via LoRa protocol) was already demonstrated successfully, using the wonderful initiative of \"TinyGS\" [45]. Figure 11 displays a real RF beacon packet that was transmitted by SATLLA-2B and was captured by 15 amateur (RF) ground stations in a distance exceeding 1500 km. In the first few weeks after its launch, the TLE of SATLLA-2B was inaccurate. Nonetheless, about two months after launch, the TLE became stable and well defined. On average, about 30-60 messages transmitted from SATLLA-2B are received daily by various ground stations and about 4-10 messages transmitted by ground stations are received by SATLLA-2B. Due to the narrow bandwidth (62.5 kHz) and the high speed of the satellite (\(\sim\)28,000 km/h), a large portion of the messages from the satellite are received with errors, as a result of significant Doppler shift errors. It should be noted that using 125 or 250 kHz bandwidth should eliminate most of these CRC errors. The RF uplink performance of the satellite is somewhat better than was expected according to simulation results, as the actual uplink to the satellite is \(\sim\)2-3 dB better than expected; contrary to that, the download link is \(\sim\)1-2 dB lower than simulated. We argue that both gaps between the real and the simulated results are due to the RF noise level (in space it is very low, while on the ground it is relatively high). The overall cooperation with the TinyGS community is outstanding; they even allowed us to incorporate several relay experiments. Figure 10: (a) The angular speed of SATLLA-2B in each axis, as captured by the onboard MEMS-gyro. (b) The temperature (in Celsius) of the satellite as sensed in the batteries (\(rtc_{1}\), \(rtc_{2}\)) and in the satellite’s IMU (\(bmp\)). Negative Results of SATLLA-2B  
While the performance of SATLLA-2B in terms of RF communication and energy optimization can be considered as a success (it truly exceeded our expectations), several key components are not functioning properly, in particular the onboard GPS of SATLLA-2B is not working (most probably due to a faulty SD card); thus, we are relying on the satellite's TLE which is not updated on a recent basis. Moreover, due to reliability concerns, SATLLA-2B was not equipped with an ADCS, making the detection of the LED array a tricky task. We were able to operate the LED flashing mechanism successfully (based on the current reading of the satellite), yet until this time we were not able to detect SATLLA-2B's LEDs. Therefore, we are currently constructing an improved ground station (18\({}^{\prime\prime}\) telescope with the rapid tracking of LEO satellite capabilities).