DESIGN AND IMPLEMENTATION OF A TELEMETRY PROGRAM FOR RECEPTION AND COMMAND OF THE SALLESAT-1
The project consists in the implementation of a software that will allow us the reception of the telemetry coming from the satellite, with the ability to analyze and monitor it in a display and checking so the internal system functionality and other variables, as for example the internal temperatures, the amount of solar radiation received by each panel, the power transmitted by the emitter and tensions everywhere.
This software will allow us as well sending orders from the terrestrial station in order to activate or deactivate every function of the satellite.
The main characteristics of the software are the following:
- Telemetry reception with a TNC modem.
The TNC Modem (Terminal Node Controller) is in charge of all basic functions (HDLC) of the AX.25 protocol and is the same used in the terrestrial net of radiopacket. The TNC can function in full-duplex and could transmit the information at different speeds, but the most frequently used is 9600 bps.
The Modem will be connected to the on-board computer through a serial port and will be one of the two sources that will pass data to the program.
- Reception of the telemetry with a sound card:
This is the second source of data coming from the software. If we don’t receive data coming from the TNC, we’ll receive them through a computer sound card, sending the audio to a computer audio board that will be used for decoding the radiopacket.
- AX-25 communication protocol
The AX.25 protocol is a direct application of the standard HDLC format from the CCITT, and it allows us sharing the frequency by several stations connected at the same time without interfering one to another. The messages are transmitted in a data packet frame that ends with several checking control characters that will allow us to know if the content has been correctly received. Technically speaking, AX.25 is a link level protocol oriented to connexion using numbered frames, that already has the possibility of using unnumbered fames (without connexion) that could be used for higher protocol levels. That means that any kind of net and any software could be easily adapted to a radio channel, through the use of AX-25 as the link level protocol.
So this link level protocol has 3 types of frames:
– The information frames (I frames) that we can consider as the message
– Control frames (S frames) that control the communication
– The unnumbered frames (U frames) with no bidirectional communication
The control frames will be useful for confirming the right content of data transmitted.
Packetradio is a simple and sure way that will allow us computer to computer communication through a radio link. The information interchange is made through the use of data burst encapsulated in packets, because the transmitted information will be divided in smaller parts that will be reassembled later in their final destination. This patron of transmissions is free of errors and permits the right administration of the channel that could be used by several users at the same time. Finally, the information transmitted is kept in the cue of a buffer until the final recipient is ready to receive the information.
So we can say that packetradio is the “transmission mode” we’ll use, and the TNC is the controller. Packetradio carries any data flux sent by a computer and transmits it to another receiver station similar equipped. The protocol used is AX.25
- Historical record of received data
The software will save in a data base all data received in each connexion, to keep all data saved in such a way they could be later read for statistical purposes. So, the software will be useful when the satellite is not on view and we have no link with it.
- Automatic connexion by estimation of orbital passes based on Kepler elements
For getting all this, we’ll use the work already made by the software Salletracker. The idea is that knowing our position and the satellite trajectory, we could follow the satellite all time we have direct vision to it, in such a way that during this time we could send orders and receive information coming from the satellite.
- The satellite orders module
This program do not finish with receiving all the information coming from the satellite as commented previously, but enables us to send information to the satellite, information that will consist in orders we could ask the satellite to comply. These orders will be sent with a confirmation control to assure us the right reception by the satellite.