Our CubeSat’s radiocommunictaion system is needed to exchange both downlink and uplink data. Downlink is the satellite’s transmitted data received by the GS (Ground Station), with uplink being the exact opposite. We will be most likely using frequencies: 435 MHz for Downlink and 144 MHz for Uplink.
Our ground station has to both transmit commands to our CubeSat and receive telemetry and payload data consisting of sensors and Earth photos from camera(s). The ground station is being built entirely by us and it consist of directional Yagi antenna(s), a radio and a rotator. The rotator is needed for the ground station to be able to follow our satellite according to TLEs (Two Line Elements) provided by NORAD organization. TLEs are pieces of data which include information about our CubeSat’s trajectory.
Our onboard transceiver will transmit data to the ground station with the AX.25 link layer protocol. However, not all data can be transmitted al the time. For example, photos are too big to be transmitted frequently and they drain too much power. Therefore, we our transceiver has to be able to receive uplink data. A command sent from the ground station can tell our CubeSat to start transmittin the photo in a particular moment. In addition, uplink can be also very helpful in emergency situations.
As one of the deployables, antennas shall wait to deploy a minimum of 30 minutes after CubeSat’s kill switches are activated, and then another 45 minutes before transmitting any signal.
Usually CubeSats have deployable antennas optimalized for two frequencies: 435 MHz and 144 MHz. Even though these antennas are omnidirectional, they still need the CubeSat to be stabilized, because it is tumbling after being deployed. A spinning Cubesat wouldn’t provide us with a good, continuous transmission.