Method for optical data transmission from low earth orbit to earth and corresponding communication system

A method for optical transmission of payload data (PD) from a low earth orbit satellite (20) to an optical ground terminal (30) comprising the steps:

• buffering, fetching and grouping of said payload data (PD) into frames by a communication subsystem (CS) of the low earth orbit satellite (20);
• the optical ground terminal (30) activating an uplink channel (UC) connecting the optical ground terminal (30) with the low earth orbit satellite (20), the uplink channel (UC) being an acquisition and tracking beacon channel comprising a wide angle beam (W) for acquisition and a guidance beam (G) for tracking;
• said communication subsystem (CS) acquiring said wide angle beam (W) followed by a continuous tracking of the guidance beam (G) of the uplink channel (UC);
• transmission of said frames of payload data (PD) by the communication subsystem (CS) when a downlink channel (DL) is available connecting the low earth orbit satellite (20) with said optical ground terminal (30);
• the optical ground terminal (30) requesting retransmission via the uplink channel (UC) of unsuccessfully/erroneously received frame(s);
• said communication subsystem (CS) retransmitting each frame for which a request for retransmission has been received;
• wherein the uplink channel (UC) is pulse position modulated PPM.

A method for optical transmission of payload data (PD) according to claim, characterized in that a sliding window mechanism is implemented, wherein the sliding window contains:

• the current window boundaries;
• a list of received frames;
• a list of received frames not yet acknowledged;
• a list of frames not yet received;
• wherein the information of the sliding window is stored at the optical ground terminal (30) and wherein frames are retransmitted by the communication subsystem (CS) on request.