# Pico-Sat to Ground Control: Optimizing Download Link via Laser Communication
## 2 Physical Components and Methods
### FSO Communication--The Satellite's Side  
In order to establish an FSO link, both communicating sides need to be accurately aligned towards each other. The motion of the satellite requires both the ground station and the satellite to perform accurate tracking. The handshake process between the satellite (\(S\)) and the ground station (\(GS\)) consists of the following stages:  
1. \(S\) performs a **coarse alignment** to \(GS\) and turns on the LED.
2. \(GS\)**detects and tracks \(S\)** and turns on its laser beacon.
3. \(S\) detects the laser (of \(GS\)) and **accurately aims** itself towards \(GS\).
4. \(S\) turns on its FSO laser and starts the **data transmission**.
5. \(GS\) signals to \(S\) an **FSO feedback** with its laser beacon, using a very low data rate.  
Commonly, satellites have an onboard GPS. Moreover, their orbits are determined and monitored by the Space Surveillance Network [29] and the TLEs are publicly available. Both the GPS and the TLE allow global positioning and velocity. The satellite orients itself towards Earth, searches for the telescope (the visual beacon), and aims its laser to the visual beacon (the center of the camera's FOV).  
We now elaborate on each stage of the FSO link establishment process:  
* **Coarse alignment**. The satellite aims itself roughly at the region of the ground station. For performing an Earth-pointing rough alignment, see [30].
* **Detection and tracking**. Figure 7 demonstrates a laser beacon ground experiment. The detection process (in this case) is rather straightforward. Yet, when the beacon is relatively weak, we use both the blinking rate and the laser's frequency (color) to increase the signal-to-noise ratio (SNR) of the laser beacon.
* **Accurate aiming**. Figure 8 shows the attitude control determination of the satellite, which is composed of three reaction wheels, an IMU, and a beacon-detection sensor (based on a wide-angle camera).
* **Data transmission**. After the satellite aligns itself with respect to the laser beacon of the ground station, it starts transmitting the data using a simple on-off keying (OOK) protocol; see [31; 32].
* **FSO feedback**. The ground station may signal back to the satellite (using the laser beacon) low bandwidth information regarding the quality of the FSO link. In particular, the following types of information are passed: packet drop rate, suggested optimal modulation, and time to expected non-line-of-sight (NLOS).  
Most satellites in orbit have an attitude determination and control system (ADCS), which allows the satellite to orient itself in order to aim its antenna or camera towards the Earth. It should be noted that while FSO is a means of communication, the required orientation accuracy is higher than a directional RF antenna and closer to aiming a camera--commonly better than 1 milliradian. We propose the use of micro reaction wheels to aim the FSO pico-satellite to the ground station (see Figure 8a). The main challenge of adjusting the stabilization system to FSO usage is that the FSO transceiver should point to the ground station with high accuracy and follow it while orbiting. In order to achieve such accurate aiming, this model uses the laser of the ground station as orientation feedback, instead of the self-orientation sensor (such as star-tracker). By doing so, the satellite is able to track the ground station in a close loop with the help of the  
Figure 7: Terrestrial laser-beacon detection and tracking. A 0.002 Watt green laser as captured from \(\sim\)9 km by a basic Raspberry Pi camera in VGA mode.  
IMU. The satellite angular accuracy requirement is defined by its laser divergence. Thus, assuming a 1 milliradian laser beam, a sub 0.5 milliradian angular accuracy is needed. Using a 2 megapixel camera (1080p) with 30 deg field of view, the pixel angle is about 0.25 milliradian, which is sufficient for continuous tracking, even without super-resolution; see [33] for an arcminute (about 0.3 miliradian) attitude stability on a nano-satellite.