Subsystem Description
Functional Architecture
The communications subsystem is responsible for telecommand reception and telemetry transmission. Being so that the functionality of the satellite's control from the Earth ground station completely relies on the COMMS subsystem. Therefore, it is imperative for the COMMS subsystem to fulfill specific requirements to facilitate accurate communication and seamless operation of the satellite.
It is opted for the utilization of LPWANs (Low Power Wide Area Networks), specifically employing the LoRa (Long Range) physical layer technology. COMMS hardware is located both in the OBC-COMMS board as well as in a lateral board (where the antenna is soldered or screwed). The subsystem can be divided into different blocks to identify their functionality.
The first one is the power circuit, tasked with ensuring the proper power supply and composed by passive components. The transciever is the key component of the SS, responsible of both the recepction as well as the transmission of information from and to the ground station. It communicates with the MCU through SPI, as a slave, and performs signal modulation and demodulation.
The transciever requires an stable reference signal in order to perform its tasks, this is provided by a crystal oscillator.
The RX Line transmits the received signal from the antenna to the transciever. The line is balanced by a balun and impendance matched. The TX Line transmits the transmitted signal from the transciever to the antenna and presents different filters and matchings. The line to be used at each instant is selected by the switch, controlled by the transciever, which is at the same time controlled by the MCU.
Finally, the antenna, connected to the switch through a line that boths filters and matches impedance, is responsible for physically receiving and transmitting electromagnetic waves.
Figure 1: COMMS Functional Architecture Block Diagram
Requirements
| Subsystem | ID | Requirement |
|---|---|---|
| COMMS | COMMS - 0000 | The Communications Subsystem (COMMS) shall work in the ISM band via radio links. |
| COMMS | COMMS - 0010 | The COMMS subsystem must transmit at a maximum power of 20 dBm. |
| COMMS | COMMS - 0020 | The COMMS subsystem must support half-duplex communication, enabling both transmission and reception of data. |
| COMMS | COMMS - 0030 | Be able to deploy the omnidirectional quarter wavelength antenna once the satellite is deployed in space. |
| COMMS | COMMS - 0040 | The COMMS shall periodically transmit the telemetry of the spacecraft. |
| COMMS | COMMS - 0050 | All packets shall be tagged with a timestamp. |
| COMMS | COMMS - 0060 | The COMMS must be able to receive Telecommands from the ground segment and send a reception acknowledgement. |
| COMMS | COMMS - 0070 | The COMMS shall have the capability to provide past telemetry housekeeping. |
| COMMS | COMMS - 0080 | The transmitted beacon shall contain a subset of information from the whole satellite housekeeping. |
| COMMS | COMMS - 0090 | OBC and COMMS subsystems must communicate through SPI. |
| COMMS | COMMS - 0100 | The S/C shall be capable of changing the operating frequency using a telecommand. |
| COMMS | COMMS - 0110 | The satellite must comply with European regulations. |
| COMMS | COMMS - 0120 | Be able to distinguish between wanted packets and unwanted packets. |