The communications subsystem is responsible for the communication between a ground station (GS) and the spacecraft. It can receive telecommands from the GS for setting different operating modes and requests to transmit data. There are two different types of downlink transmission modes:

• LPTM Low Power Transmission Mode (Beacon)
• HPTM High power Transmission Mode (Data)

The beacon is used for tracking the satellite and to get a simple overview of the satellite's status. The beacon data contains a small subset of telemetry data and it is transmitted periodically in Morse code. The HPTM is used for transmitting large amounts of mission data. This consists of telemetry data from each subsystem and the experiment data, for example a picture taken by the camera. HPTM is turned on only after receiving a certain telecommand. Because COM subsystem needs to be fault tolerant and reliable, it has autonomous modes for low-energy situations and different recovery operations in case errors occur.

Requirements
In low Earth orbit (LEO) the satellites movement is relatively rapid and therefore the Doppler effect causes a noticeable frequency shift as the satellite approaches the GS and moves away. The receiving GS needs to tune uplink and downlink frequencies according to this frequency shift. Faraday effect is a phenomenon which causes rotation of the polarization angle in a linearly polarized wave. Using a circularly polarized antenna in the GS the impact of the Faraday effect can be minimized. Requirements based on the Cubesat Design Specification:

• All deployables such as booms, antennas and solar panels shall wait to deploy a minimum of 30 minutes after the CubeSat's deployment switch(es) are activated from P-POD ejection.
• RF transmitters greater than 1 mW shall wait to transmit a minimum of 30 minutes after the CubeSat's deployment switch(es) are activated from P-POD ejection.
• Operators shall obtain and provide documentation of proper licenses for use of frequencies. For amateur frequency use, this requires proof of frequency coordination by the International Amateur Radio Union (IARU).

Deduced requirements from the mission:

• During the experiment the satellite will start spinning in one axis. The experiment will include an electron gun which will give the satellite a positive charge. Communication with the satellite must be possible under these experiment conditions.

Frequency bands
The satellite will operate in amateur radio frequency bands. Specific frequencies need to be agreed upon with IARU. Compared to using licensed frequencies there are no costs associated with using amateur radio frequencies. Radio amateurs will be included in the ESTCube mission to help with tracking the satellite and downloading data. Current ESTCube GS is capable of operating in 70 cm and 2 m bands, there is a possibility of upgrade, which will add 23 cm band operating capability. For full duplex connection 70 cm band is used for downlink and 2 m band for uplink. This enables to achieve higher data rate for downlink more efficiently.