# Pico-Sat to Ground Control: Optimizing Download Link via Laser Communication
## 2 Physical Components and Methods
### FSO Communication--The Ground Station's Side
#### 2.3.1 Coarse Tuning  
Recall that the ground station includes a robotic telescope and a tracking system. Robotic telescopes have a 2-DoF (degree of freedom) aiming mechanism--defined by two angles: azimuth (\(Az\)) and elevation (\(Elv\)). Assuming the telescope is calibrated, the first angle, \(Az\), represents the horizontal polar angle from the north and the second angle, \(Elv\), represents the elevation angle. The orbit of each satellite is given by TLE (two-line element) parameters. The North American Aerospace Defense Command (NORAD) [23] detects, monitors, and updates the TLE of each satellite (and even space debris). Given the ground station's position, the time, and a TLE (of a satellite), a robotic telescope can aim itself and follow the satellite's TLE. The coarse tuning phase begins a few seconds before the  
Figure 4: Images and illustration of the SATLLA2.0 nano-satellite engineering model. The left side (**a**) shows the complete engineering model of SATLLA2.0, including a UHF antenna. The image in the middle (**b**) displays the FSO components: (A) laser diode with about 1 degree beam divergence (green 532 nm); (B) Sony’s IMX219 8 MP camera; and (C) an array of 10 LEDs emitting an equivalent to 20 flashes of smartphones. The satellite’s emitting light should be visible from Earth using a small 8\({}^{\prime\prime}\) telescope on a clear night. The right side (**c**) illustrates the main components of SATLLA2.0: (A) FSO communication unit and a calibrated laser light; (B) camera with an image processing unit; (C) LED—for ground tracking; (D) stabilization control (reaction wheels) unit; and (E) main controller with UHF radio transmitter-receiver.  
Figure 5: (**a**) Abstraction of a ground station–satellite communication link. (**b**) The ISS as captured from our ground station; shot taken by [PERSON] [22] on 3 June 2021.  
optical link is created. At this phase, the ground station calculates where the satellite is expected to rise and aims to reach that location. In order to aim a telescope to a satellite, the relative vector between the telescope and the satellite needs to be computed. The telescope's Cartesian coordinates representation is denoted as \(\{x_{t},y_{t},z_{t}\}\), and the satellite's expected Cartesian coordinates are represented as \(\{x_{s},y_{s},z_{s}\}\). The equations for computing the \(Az,Elv\) angles are presented next.  
Let \(\{x,y,z\}\)  
\[\Delta x =x_{s}-x_{t} \tag{1}\] \[\Delta y =y_{s}-y_{t}\] (2) \[\Delta z =z_{s}-z_{t}\] (3) \[Elv =\tan^{-1}\left[\frac{\Delta y}{\Delta x}\right]\] (4) \[Az =\tan^{-1}\left[\frac{\Delta z}{\sqrt{\Delta x^{2}+\Delta y^{2}}}\right] \tag{5}\]  
Computing the TLE of a satellite and converting GPS-like global angular coordinates (lat, lon, alt) to Cartesian coordinates \((x,y,z)\) are two technical topics which are out of this paper's scope; see [24] for a technical guide for satellite tracking by telescopes using TLE information. The above method only aims the telescope to the expected area of the satellite's orbit. Obtaining an accurate tracking of such a small object requires an accurate TLE and an accurate pre-phase calibration of the telescope. In this work, we suggest a hybrid model that will replace the need for an accurate TLE and a well-calibrated telescope system.