A CubeSat communication subsystem needs to provide a method of transferring data between a control team on the earth and an orbiting satellite. The system can be divided into two overlapping blocks, the ground station subsystem, and the on-satellite subsystem. This article is from the perspective of the ground station subsystem.

The ground station team for SEDSAT II currently consists of engineering and science students from India, Canada and the United Kingdom; all working together to design and build at least one installation that will provide daily contact with the SEDSAT II cubesat. The installation will use a high gain antenna to receive payload data from the orbiting cubesat, and also to transmit commands that maintain the health and update the task list of the satellite.

The first installation is planned to be installed above the Electronic Engineering department at the University of Bath, UK (shown above) to be ready in time for testing with a SEDSAT II prototype communication subsystem. When both designs have been declared spaceworthy, other installations on different continents can be quickly built to increase the coverage before and after launch day.

The first difficulty with orbital communication is that a satellite can only be ’seen’ when above the horizon, and only then for a limited time known as a ‘window’. These short windows (approximately 20 minutes, as given by the comms team) require a fast tracking antenna that can follow the cubesat from horizon to horizon, which requires the use of either an electrically guided Yagi antenna or a mechanically steered parabolic dish.

The second difficulty is in picking up the weak signal from the cubesat, which is currently being designed to have a transmitter with a power less than 1 watt. The comms team are working on methods of error detection which is caused by signal noise, this however can only be implemented once the signal is received, which will then need to be cleaned. To do this, the highest possible signal to noise ratio (SNR) needs to be achieved by the ground antenna, to ease the pressure on the requirements of the tiny cubesat transmitter. The simplest way to get a high SNR is to build a bigger antenna; however there are subtler ways like carefully selecting impedance matched equipment, or more accurate pointing at the cubesat.

On the other hand, acting in our favor, the transmission power of the ground antenna (up link) can be anything up to 100 Watts with a full amateur radio license, and the antenna will be as large as wind loading and the university will allow. An advantage of the proposed location is that Bath University is located on top of a hill, making more of the horizon visible than would be possible in most urban areas.

During the mission phase of SEDSAT II the ground station will operate in a semi-autonomous mode, requiring it to be able to track the cubesat independently by use of orbital prediction software and data from previous orbits. Adjustments to the known orbit of the cubesat can be sent to the ground station via a web interface from anywhere around the world, and this same interface can be used to distribute payload data and reports on the cubesat health, as well as commands to be transmitted back up to the cubesat.

Right now we are still in the design stage, which makes it a perfect time to join before all of the building starts! We are looking for any computer/engineering/science students with basic knowledge in anything described above, or at least the willingness to learn the basic knowledge of anything described above. The intention is to create a ground station that will last the length of the SEDSAT II mission, and still be around for use by its inevitable successors.

To join SEDSAT II visit the application page:

http://wiki.seds.org/index.php/SEDSAT-2_Team_Application

To learn more about the ground system visit the wiki page:

http://wiki.seds.org/index.php/SEDSAT-2_Ground