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21. August 2011; Charles Thibaut

As the FLP project advances, some of its systems need to be adapted due to new or modified constraints and boundary conditions.
The FLP's downlink was first based on several systems on different frequency bands. 
The aimed data rate was planed to be so high it would be possible to send all the data in a few seconds.
But this goal finally appeared to be oversized in comparison to what was actually needed. 
It was thus decided to re-evaluate the  requirements of this system to better fit to the FLP Project, its requirements but also ressources. The new concept has been defined, and is based on a single system.

The data downlink system has for objective to send the payload data to a ground station. 
In order to send the required data during the limited transmission time, the data rate has to be 10 Mbps. Furthermore, in order to limit the corruption of data, the downlink system must have a bit error rate lower as 1 for 10e9. A Reed Solomon encoder will be used in the communication protocol for error correction and detection and thus reduce the gain needed at the satellite's antenna interface.
These values are among the most important to build a link budget for the downlink.

The Data downlink system is yet a S-Band only system, which will send data on a 2,4 GHz QPSK modulated carrier wave. Two transmitters will be used, one will be nominal as the other one will be redundant. They will be connected to a single antenna system thanks to a coaxial switch which will be switched in case of failure on the nominal transmitter.
The antenna system has also changed. A corrugated horn antenna will be used to reach a gain greater as 13dB. Furthermore, a polarizer will be used in order to generate a circular polarisation which is important to LEO satellites as horizontal and vertical polarisations would need more complex ground station antennas systems.

Geometry of the S-Band axial corrugated horn A model for the polarizer EM analysis