Comm Link
In our kick-off meeting we identified two layers in our communication system with the satellite:
- physical layer
- protocol layer
The first layer is a physical and link layer, the coding and modulation and accordingly the demodulation and decoding. The link should be in both directions, with
- telemetry downlink,
- data downlink,
- command uplink and
- data uplink.
Easiest and most important is the telemetry downlink, a ‘beacon‘, to know that the satellite is alive and to be able to read the most important paramters. Second, a command uplink should allow us to shut the satellite down, reboot it, switch different comm or operation modes. This two links provide our basic communication to fully run the satellite. Third is the data downlink. We want the payload to tell us new things. With only telemetry and data downlink we would be satisfied The third, least important is the data uplink. Maybe we want to upload new data to transmit or to feed to our payload.
Protocol
The second layer is a protocol, that establishes a virtual channel between the satellite and the control center in Graz. This protocol should involve everything needed to collect all received transmissions from the satellite and to send transmissions to the satellite. This will likely include software for distribution to decode the transmissions, and to send it to the control center, like with mail or a direct connection to some collecting server.
Parameters
Talking to the satellite is no trivial thing. A lot of factors and parameters have to be considered: Frequency range, bandwidth of the signal, desired bitrate, reliability, available processing power, available electric power, size and weight, available equipment for transmission and reception, targeted audience, analog or digital signals, fading, antenna design, costs, interfaces. We are sure that we missed some more.
Electric power
For mur.sat, we have a very stringent power regime. The solar cells are expected after a quick calculation to provide not more than 600mW. We will test that and calculate the average and maximum power available. Some power will be needed for the IHU (internal house-holding unit), some for the payloads. Given that the comm system is running in parallel with other systems, not more than 100mW can be spent on communications, be it receiving or transmitting. This gives an order of magnitude for the electric power constraint.
Antenna
The Tubesats are designed with a dipole antenna with a length of up to approximately 30cm. That would be resonant in the range of 500MHz upwards without tuning network. The satellite will be unpredictable tumbling, which will cause through rotation of polarisation and orientation of the antenna – the dipole is not radiating in the direction of the rods, and be covered by the satellite body. Fading caused by rotation of the polarisation can by compensated by turning of the receiving antenna in case of a handheld transceivers, or with a circular polarized antenna. The circular polarized antenna will receive linear polarized signals of any rotational angle, with a constant loss of 3dB in signal strength.
More on this in one of the next post. Stay tuned.