Communication Subsystem

Purpose

The main objective of Communication system is to provide a reliable communication between the satellite and the ground station, being responsible for transmitting and receiving signals from and to the satellite. In the Earth side the same role is played by the Ground Station system. An analogy could be made between the communication system and the human body, where the communication system is like the ears and the mouth of a person. The mouth is like the transmitter because it is used to transmit information in accordance with what his brain orders and the ears is like the receiver where it is used to listen what somebody has to say. Generally it is divided in this two categories: transmitter and receiver. A more detailed explanation you will read in the next section.
In this satellite we plan to install a receiver to receive commands from the Earth and three transmitters. One will be used to send the satellite's health in morse code, the second one will be used to send the Earth pictures from space and the last one will be used to send both satellite health and pictures.
It is important to remember that only the receiver and the transmitter is not possible to work perfectly requiring antenna, peripheral circuits and onboard computers.

Functions

The basic function of each component is explained bellow. As humans communicates each other using a common language, the satellite system imitates it too. The language in electronic and computing system is called protocol, so the satellite is going to use a well known protocol between amateur radios called AX.25 protocol. To use this communication language we plan to use amateur radio bands like CubeSat XI-IV and provide following functions.

Transmitter

• CW beacon
  It sends to the Earth signals in morse code (see Q&A column) containing satellite's heath status with 50 words per minutes.
  
• 1200bps FSK
  1200bps means 1200 bits per second, where 8 bits represents an alphanumerical caracterer. It will send Satellite's status information, much more fast than the CW beacon transmitter but it requires much more electrical power for that. It will send pictures too, but slower than the 9600bps transmitter. This transmitter has an important role to send satellite's history status to the Earth and is in that way that we are going to analyse in detail the satellite condition.
  
• 9600bps GMSK
  The main mission for that transmitter is to send Earth images from the space to the Earth. The idea is the same as 1200bps transmitter, but the modulation (i.e. how the transmitter talks) is GMSK, differing from the 1200bps's FSK.
  

Receiver

• 1200bps FSK
  As we have talked above, the receiver receives commands from the Earth and executes it or passes the command to the satellite's brain (C&DH system).

Works

• Design circuits for data en/decoding, transceiver control and modem system.
• Develop internal communication interface with the main onboard computer(C&DH) using CAN system.
• Develop transceivers collaborately with Nishi RF Laboratory.
• Evaluate and test transceivers.
• Antenna design and deployment structure development.
  

  Uplink

  1200bps FSK based on CubeSat1 system.
  RxTNC PIC16c716
  Modem FX614
  Receiver Nishi RF Lab. Custom made FM Receiver
  

  Downlink

  1200bps FSK based on CubeSat1 system.
  TxTNC H8/3048(shared with CW, 9600bps GMSK)
  Modem FX614
  Transmitter Nishi RF Lab. Custom made FM Transmitter
  1W output 3W use
  

  CW based on CubeSat1 system.

  TxTNC H8/3048(shared with 1200bps FSK, 9600bps GMSK)
  Transmitter Nishi RF Lab. Custom made FM Transmitter
  100mW output 475mW use
  
  

  9600bps GMSK new development.

  TxTNC H8/3048(shared with 1200bpsFSK, CW)
  Modem CMX589
  Transmitter Nishi RF Lab. Custom made FM Transmitter
  2W outputA7W use
  
  

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# The grade of each member was the status when the PRISM launched.