## 2 Methods and Materials
### AlfaCrux Spacecraft
#### 2.2.3 User Interface with OSI  
A general overview of the user interface with the AlfaCrux satellite open systems interconnection (OSI) layers is presented, specifically for the telemetry downlink. It is worth mentioning that the AlfaCrux downlink signal is not encrypted, in accordance with the amateur satellite service regulation, specifically article 25.2A 1A of the International Telecommunication Union (ITU) radio regulation RR25-1.  
The TTC physical layer is as follows. TTC frequency of 437.100 MHz, effective isotropic radiated power of 30 dBm, Turnstile antenna, and mixed circular polarization (right and left hand).  
The data link layer is summarized in Table 3. The use of Gaussian Minimum Shift Keying (GMSK) modulation provides a good compromise between speed and bandwidth. This type of modulation has no phase discontinuities thus the side lobes of the signal spectrum are reduced. Apart from minimizing channel interference, it provides efficient use of spectrum and enables high efficiency.  
All the packets transmitted by the AlfaCrux use the Cubesat Space Protocol (CSP) ([[https://github.com/libcsp/libcsp](https://github.com/libcsp/libcsp)]([https://github.com/libcsp/libcsp](https://github.com/libcsp/libcsp)), accessed on 20 September 2022). Inside the CSP data field, telecommand transfer frames are used to encapsulate upper layer data. The TM  
Figure 5: AlfaCrux operational modes schematic.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Description** \\ \hline Modulation & Gaussian Minimum Shift Keying. \\ \hline Bitrate & 4800/9600 bps. \\ \hline Sync word & 0x930B51 DE. \\ \hline Frame format & ASM+Golay (AX100 mode 5). \\ \hline Bit encoding & NRZ, most significant bit first. \\ \hline Data randomization & Consultative Committee for Space Data Systems (CCSDS) randomization. \\ \hline Error-correcting code & Reed-Solomon (255, 223). \\ \hline \hline \end{tabular}
\end{table}
Table 3: AlfaCrux data link layer.  
transfer frames are an adaptation of CCSDS TM transfer standards. When the satellite is in communication with the ground station, a reliable channel is established between the station and the satellite using CSP Reliable Data Protocol.  
The TM transfer frames transport standard CCSDS Space Packets implementing ECSS Packet Utilization Standard (PUS) services. A detailed description of PUS packets can be found in the ECSS, Telemetry and telecommand packet utilization, ECSS-E-ST-70-41C of 15 April 2016.  
The satellite transmits a set of 5 packets every 30 s. Each packet contains a TM frame with a single space packet inside. Standard PUS service 3 (Housekeeping) is used to format these space packets. Five different beacons are transmitted, each one with a different ID, as follows:  
* B1-OBC: telemetry from the main on-board computer;
* B2-EPS: telemetry from the electric power subsystem;
* B3-TTC: telemetry from the telemetry and telecommand subsystem;
* B4-UHF: telemetry from the antenna deployment subsystem;
* B5-Temps: temperature telemetry of different subsystems of the satellite.  
Apart from the beacons, the satellite can generate other telemetry packets, the majority of them only under specific telecommand from the ground station, that is related with command and control of the satellite, with some packets out of the PUS standard. More information can be found at the official website of the mission at [[https://lodestara.aerospace.unb.br/projects/alfacrux](https://lodestara.aerospace.unb.br/projects/alfacrux)]([https://lodestara.aerospace.unb.br/projects/alfacrux](https://lodestara.aerospace.unb.br/projects/alfacrux)) (accessed on 20 September 2022).