The telemetry module consists in a TNC modem that uses the AX.25 protocol. The modem communicates with the satellite CPU that supplies all the information frames coming from sensors, batteries, etc… and modulates a signal to be sent to the transmitter. In the same way, when we send to the satellite any telemetry order coming from the Earth station, the modem has to decode it in a digital form in order to communicate with the CPU.
Further to the modem, the project requires the design of a controller inserted between the modem and the CPU that will serve for replacing this one when there is a problem.
If the CPU hangs, it is very important we still could receive telemetry. That’s why this controller must be able to get the information coming from the sensors and code it in frames to be passed to the modem. Further to this it must be able to get a system reset (and communicate with the batteries) and inform the terrestrial station that there is a CPU malfunction.
Finally, a receiving software is needed in order to display all the information received from the telemetry frames, of course requiring the development of a protocol that is going to be encapsulated in AX.25 frames.
At the present time, I am studying the AX.25 protocol, although it has not to be implemented on the modem, but is quite important for me to know it at the byte level. At the same time, I am studying Visual Basic to develop two programs that are going to simulate the satellite and the terrestrial station , in order to be able to test the coding and decoding of the telemetry frames, and the concerning protocol that is going to be encapsulated too in AX.25 frames.