Optical downlink system

1. Optical downlink system ( 10 ) comprising:

a remote terminal ( 20 ) comprising a number of n optical communication terminals (OT 1 -OTn), where n is greater than or equal to two;

a ground terminal ( 30 ) comprising a cluster of n optical ground stations (OGS 1 -OGSn), and one of the n optical ground stations (OGS 1 -OGSn) being a master ground station that further comprises a sync unit that is configured to synchronise the n ground stations (OGS 1 -OGSn) so that at any moment in time, only one ground station (OGS 1 -OGSn) is transmitting, thus providing for temporal separation of the n optical uplink channels (UC 1 -UCn) and ensuring that only one of the n optical uplink channels (UC 1 -UCn) is active at a time;

each optical communication terminal (OT 1 -OTn) being connected with a corresponding optical ground station (OGS 1 -OGSn) with one of n optical downlink channels (DL 1 -DLn); and

each optical ground station (OGS 1 -OGSn) being connected with a corresponding optical communication terminal (OT 1 -OTn) with one of n optical uplink channels (UC 1 -UCn) that are pulse position modulated (PPM) signals;

wherein the optical downlink channels (DL 1 -DLn) are spatially separated from each other by means of the optical ground stations (OGS 1 -OGSn) being located a certain distance apart from each other; and

wherein the ground terminal ( 30 ) is configured such as to synchronise the n optical ground stations (OGS 1 -OGSn) such that a precise temporal separation is ensured between the n optical uplink channels (UC 1 -UCn) by a time division multiple access scheme, wherein pulses of each of the pulse position modulated PPM signals of any one of the n uplink channels (UC 1 -UCn) are emitted sequentially during phases of pulse silence (Ps) of the other uplink channels (UC 1 -UCn), by dividing the period (T) of the pulse position modulated (PPM) signals to the number n of optical uplink channels (UC 1 -UCn) and assigning each optical ground stations (OGS 1 -OGSn) a dedicated time slot (Ts) less than or equal to T/n, thereby avoiding overlap between the optical uplink channels (UC 1 -UCn).

2. Optical downlink system ( 10 ) according to claim 1 , characterised in that the n optical uplink channels (UC 1 -UCn) comprise n laser beacons directed towards the remote terminal ( 20 ) for pointing-acquisition and tracking by the optical communication terminals (OT 1 -OTn) in order to individually direct the optical communication terminals (OT 1 -OTn) towards the corresponding optical ground stations (OGS 1 -OGSn).

3. Optical downlink system ( 10 ) according to claim 2 , characterised in that one of the n optical communication terminals (OT 1 -OTn) of the remote terminal ( 20 ) is a master optical communication terminal configured to receive the laser beacon of the optical uplink channel (UC 1 -UCn) from the master ground station and distribute a clock synchronization signal to the other optical communication terminals (OT 1 -OTn) of the remote terminal ( 20 ), thereby initiating a gated viewing of the n optical communication terminal (OT 1 -OTn), where each optical communication terminals (OT 1 -OTn) sequentially receives signals from the corresponding optical ground station (OGS 1 -OGSn).

4. Optical downlink system ( 10 ) according to claim 1 , characterised in that the n optical communication terminals (OT 1 -OTn) of the remote terminal ( 20 ) are configured to simultaneously transmit payload data to the corresponding optical ground stations (OGS 1 -OGSn) of the ground terminal ( 30 ), and the optical ground stations (OGS 1 -OGSn) are configured to sequentially transmit control signals to the corresponding optical communication terminals (OT 1 -OTn), such as automated repeat request ARQ signals for requesting retransmission of lost/erroneously received data packets from the corresponding optical communication terminal (OT 1 -OTn).

5. Optical downlink system ( 10 ) according to claim 1 , characterised in that the optical ground stations (OGS 1 -OGSn) are located a distance (D) apart from each other, said distance (D) being sufficiently high in order to ensure that the optical receive power at any one of the optical ground stations (OGS 1 -OGSn) from its corresponding optical downlink channel (DL 1 -DLn) is at least one order of magnitude greater than the optical receive power from interference by any other optical downlink channels (DL 1 -DLn).

6. Optical downlink system ( 10 ) according to claim 1 , characterised in that the remote terminal ( 20 ) is a communication satellite or a communication platform flying in airspace.

7. Method of optical data transmission between a remote terminal ( 20 ) having a number of n optical communication terminals (OT 1 -OTn), where n is greater than or equal to two, and a ground terminal ( 30 ) comprising a cluster of n specially separated optical ground stations (OGS 1 -OGSn) comprising the steps:

establishing n optical downlink channels (DL 1 -DLn) connecting each optical communication terminals (OT 1 -OTn) with a corresponding optical ground station (OGS 1 -OGSn);

establishing n optical uplink channels (UC 1 -UCn) connecting each optical ground station (OGS 1 -OGSn) with a corresponding optical communication terminal (OT 1 - OTn);

transmitting payload data from the remote terminal ( 20 ) by the n optical communication terminals (OT 1 -OTn) to the corresponding optical ground stations (OGS 1 -OGSn) of the ground terminal ( 30 );

sequentially transmitting control data from the n individual optical ground stations (OGS 1 -OGSn) via the corresponding optical uplink channels (UC 1 -UCn) to the corresponding optical communication terminals (OT 1 -OTn) by means of a precise temporal separation by a time division multiple access scheme thereby avoiding overlap between the optical uplink channels (UC 1 -UCn)

wherein a sync unit synchronises the n ground stations (OGS 1 -OGSn) so that at any moment in time, only one ground station (OGS 1 -OGSn) is transmitting, thus providing for the temporal separation of the n optical uplink channels (UC 1 -UCn), ensuring that only one of the n optical uplink channels (UC 1 -UCn) is active at a time; and

wherein the n optical ground stations (OGS 1 -OGSn) modulate the n optical uplink channels (UC 1 -UCn) according to a pulse position modulated PPM scheme, wherein the sync unit controls the n optical ground stations (OGS 1 -OGSn) such that pulses of each of the pulse position modulated PPM signals of any one of the n uplink channels (UC 1 -UCn) are emitted sequentially during phases of pulse silence (Ps) of the other uplink channels (UC 1 -UCn), by dividing the period (T) of the pulse position modulated PPM signals to the number n of optical uplink channels (UC 1 -UCn) and assigning each optical ground stations (OGS 1 -OGSn) a dedicated time slot (Ts) less than or equal to T/n.

8. Method of optical data transmission according to claim 7 , characterised in that the n optical uplink channels (UC 1 -UCn) comprising n laser beacons are directed towards the remote terminal ( 20 ), the method further comprising the steps of pointing-acquisition and tracking by the optical communication terminals (OT 1 -OTn) by means of the laser beacons of the corresponding optical ground stations (OGS 1 -OGSn) thereby individuallydirecting the optical communication terminals (OT 1 -OTn) towards the corresponding optical ground stations (OGS 1 -OGSn).

9. Method of optical data transmission according to claim 8 , characterised in that one of the n optical communication terminals (OT 1 -OTn) of the remote terminal ( 20 ) is a master optical communication terminal, the method further comprises the following steps:

receive the laser beacon of the optical uplink channel (UC 1 -UCn) from the master ground station;

distribute a clock synchronization signal to the other optical communication terminals (OT 1 -OTn) of the remote terminal ( 20 ), thereby initiating a gated viewing of the n optical communication terminals (OT 1 -OTn), where each optical communication terminals (OT 1 -OTn) sequentially receiving signals from the corresponding optical ground station (OGS 1 -OGSn).

10. Method of optical data transmission according to claim 7 , characterised in that:

the n optical communication terminals (OT 1 -OTn) of the remote terminal ( 20 ) simultaneously transmit payload data to the corresponding optical ground stations (OGS 1 -OGSn) of the ground terminal ( 30 ); and

the optical ground stations (OGS 1 -OGSn) sequentially transmit control signals to the corresponding optical communication terminals (OT 1 -OTn), such as automated repeat request ARQ signals for requesting retransmission of lost/erroneously received data packets from the corresponding optical communication terminal (OT 1 -OTn).

11. Method of optical data transmission according to claim 7 , characterised in that the remote terminal ( 20 ) re-routes data to be transmitted by any of the n optical communication terminals (OT 1 -OTn) if the corresponding optical downlink channels (DL 1 -DLn) is unavailable and/or unreliable, thereby achieving redundancy and greater availability of the optical data transmission.