# US 2013/0156439 A1 · Method for optical data transmission from low earth orbit to earth and corresponding communication system > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 2013/0156439 A1 - Document ID: US20130156439A1 - Status: Published application - Kind: A1 - Application number: 13/675,015 - Filed: 2012-11-13 - Published: 2013-06-20 - Earliest claimed: 2011-12-20 - Assignees: RUAG Schweiz AG - Inventors: Felix Andreas Arnold; Thomas Dreischer - Web page: https://patentgrep.com/patent/US20130156439A1 ## CPC - H04B 10/118: specially adapted for satellite communication - H04B 7/185: Space-based or airborne stations; {Stations for satellite systems}(H04B7/204 takes precedence) - H04B: TRANSMISSION ## Abstract Optical communications system (10) and method for transmission of payload data (PD) from a low earth orbit satellite (20) to an optical ground terminal (30), the low earth orbit satellite (20) being connectable with the optical ground terminal (30) via an optical downlink channel (DL), and the optical ground terminal (30) being connectable with the low earth orbit satellite (20) via an uplink channel (UC); wherein said uplink channel (UC) is an acquisition and tracking beacon channel by means of a ground beacon (GB) controlled by a point-acquisition-track subsystem (PAT), the ground beacon (GB) comprising a wide angle beam (W) for acquisition and a guidance beam (G) for tracking; and wherein the ground beacon (GB) for the uplink channel (UC) is a pulse position modulated PPM channel. ## Claims ### Claim 1 Optical communications system (10) for transmission of payload data (PD) from low earth orbit to earth, the optical communications system (10) comprising: a low earth orbit satellite (20) comprising: a communication subsystem (CS) with a satellite transceiver (STR), a payload data source (PDS) making payload data (PD) available to the communication subsystem (CS); an optical ground terminal (30) comprising: a ground transceiver (GTR); a point-acquisition-track subsystem (PAT), the low earth orbit satellite (20) being connectable with the optical ground terminal (30) via an optical downlink channel (DL), and the optical ground terminal (30) being connectable with the low earth orbit satellite (20) via an uplink channel (UC); wherein: said uplink channel (UC) is an acquisition and tracking beacon channel by means of a ground beacon (GB) controlled by said point-acquisition-track subsystem (PAT), the ground beacon (GB) comprising a wide angle beam (W) for acquisition and a guidance beam (G) for tracking; said satellite transceiver (STR) is adjustable so that it can be adjusted with respect to the spatial direction of the ground beacon (GB); said ground beacon (GB) for the uplink channel (UC) is a pulse position modulated PPM channel. ### Claim 2 (depends on 1) Optical communications system (10) according to claim 1, characterized in that the low earth orbit satellite (20) further comprises a buffer (B) for temporarily storing said payload data (PD) until a line-of-sight between the low earth orbit satellite (20) and the optical ground terminal (30) becomes available, thereby making the payload data (PD) available for fast downlink transmission via said optical downlink channel (DL). ### Claim 3 (depends on 1) Optical communications system (10) according to claim 1 characterized in that said satellite transceiver (STR) is configured to scan and acquire said wide angle beam (W), and once acquired, track the uplink channel (UC) of the optical ground terminal (30) with the aid of said guidance beam. ### Claim 4 (depends on 1) Optical communications system (10) according to claim 1, characterized in that the communication subsystem (CS) is configured to convert said payload data (PD) into a modulated electrical signal, preferably into an on-off keying modulated signal, for transmitting in data frames by the satellite transceiver (STR) to the ground transceiver (GTR) via the optical downlink channel (DL). ### Claim 5 (depends on 1) Optical communications system (10) according to claim 1, characterized in that said communication subsystem (CS) comprises: a user data interface adapter (UDIA), comprising: a payload interface (PI) for interfacing with said payload data source (PDS); and a buffer control (BC) for controlling said buffer (B); and a communication controller (COMCO) comprising: a communications control (CC) for automatic repeat request of unsuccessfully/erroneously received frame(s); an encoder (ENC); a detector (DET); ### Claim 6 (depends on 1) Optical communications system (10) according to claim 1, characterized in that the low earth orbit satellite (20) is further connected with the optical ground terminal (30) via a second optical downlink channel (DL2) for increased downlink speed and for redundancy. ### Claim 7 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. ### Claim 8 (depends on 7) A method for optical transmission of payload data (PD) according to claim 7, characterized in that the transmission; request for retransmission and retransmission of frames is implemented in a selective repeat mechanism, preferably an automatic repeat request ARQ mechanism. ### Claim 9 (depends on 8) A method for optical transmission of payload data (PD) according to claim 8, characterized in that said request for retransmission is implemented by negative acknowledge NACK signals, the optical ground terminal (30) transmitting a negative acknowledge NACK signal for requesting retransmission of the unsuccessfully/erroneously received frame(s), preferably by transmitting negative acknowledge NACK signals identifying boundaries of the unsuccessfully/SUBSTITUTE erroneously received range of frame(s) thus requesting the retransmission of the entire range of frames delimited by said boundaries. ### Claim 10 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. ### Claim 11 (depends on 10) A method for optical transmission of payload data (PD) according to claim 10, characterized in that: data frames requested to be retransmitted are re-fetched from the buffer (B) and frames before the window are received, acknowledged and are then deleted from the buffer (B) to free memory space for new payload data (PD). ### Claim 12 (depends on 7) A method for optical transmission of payload data (PD) according to claim 7, characterized in that before transmission, the communication subsystem (CS) converts said payload data (PD) into a modulated electrical signal, for transmitting in data frames via the optical downlink channel (DL), wherein preferably forward error correction scheme is implemented in the encoder (ENC) and/or detector (DET). ### Claim 13 (depends on 11) A method for optical transmission of payload data (PD) according to claim 11, characterized in that the communication subsystem (CS) converts said payload data (PD) into an on-off keying modulated signal, preferably DC balancing added. ### Claim 14 (depends on 7) A method for optical transmission of payload data (PD) according to claim 7, characterized in that, the downlink channel (DL) is available when line-of sight is established between the low earth orbit satellite (20) and the optical ground terminal (30). ### Claim 15 (depends on 7) A method for optical transmission of payload data (PD) according to claim 7, characterized in that when informed of a pass-over of the low earth orbit satellite (20), a point-acquisition-track subsystem (PAT) of the optical ground terminal (30) wakes the optical ground terminal (30) from a standby mode; activates the uplink channel (UC) and points; acquires and tracks the low earth orbit satellite (20) in order to maintain said downlink channel (DL) and said uplink channel (UC); said point-acquisition-track subsystem (PAT) of the optical ground terminal (30) de-activates said uplink channel (UC) once the pass-over of the low earth orbit satellite (20) is finished and line-of sight between the low earth orbit satellite (20) and the optical ground terminal (30) can no longer be maintained; and the optical ground terminal (30) is switched back into said a standby mode.