Free space optical, FSO, space data transmission system comprising: a first ground or aeronautical free space optical FSO data transmission terminal; a second free space optical FSO data transmission terminal housed on board a satellite; a relay platform, located between the first transmission terminal and the second transmission terminal, and configured so as to serve as a regenerator relay for transmitting data: - between a first uplink transmission link from the first terminal to the relay platform, and a second uplink data transmission link from the relay platform to the second transmission terminal, and/or - between a third downlink transmission link from the second terminal to the relay platform, and a fourth downlink transmission link from the relay platform to the first terminal; the free space optical FSO space data transmission system being characterised in that the relay platform is configured to move at an altitude higher than that of clouds and atmospheric turbulence, and the relay platform is configured so as to: - receive the data transmitted by the first terminal, carried by a first optical signal of the first uplink that uses a first wavelength in the MWIR/LWIR, Medium Wave Infrared/Long Wave Infrared, domain of long wave infrared LWIR wavelengths, between 8 and 12 μm and/or medium wave MWIR wavelengths, between 3 and 5 μm, and transmit the received data on a second optical signal carrying the second uplink that uses a second wavelength in the domain of SWIR, Short Wave Infrared, wavelengths; and/or - receive the data transmitted by the second terminal, carried by a third optical signal of the third downlink that uses a third wavelength in the domain of short wave infrared SWIR wavelengths, and transmit the received data on a fourth optical signal carrying the fourth downlink that uses a fourth wavelength in the MWIR/LWIR domain of long wave infrared LWIR wavelengths and/or medium wave MWIR wavelengths. a first ground or aeronautical free space optical FSO data transmission terminal; a second free space optical FSO data transmission terminal housed on board a satellite; a relay platform, located between the first transmission terminal and the second transmission terminal, and configured so as to serve as a regenerator relay for transmitting data: - between a first uplink transmission link from the first terminal to the relay platform, and a second uplink data transmission link from the relay platform to the second transmission terminal, and/or - between a third downlink transmission link from the second terminal to the relay platform, and a fourth downlink transmission link from the relay platform to the first terminal; - between a first uplink transmission link from the first terminal to the relay platform, and a second uplink data transmission link from the relay platform to the second transmission terminal, and/or - between a third downlink transmission link from the second terminal to the relay platform, and a fourth downlink transmission link from the relay platform to the first terminal; the free space optical FSO space data transmission system being characterised in that the relay platform is configured to move at an altitude higher than that of clouds and atmospheric turbulence, and the relay platform is configured so as to: - receive the data transmitted by the first terminal, carried by a first optical signal of the first uplink that uses a first wavelength in the MWIR/LWIR, Medium Wave Infrared/Long Wave Infrared, domain of long wave infrared LWIR wavelengths, between 8 and 12 μm and/or medium wave MWIR wavelengths, between 3 and 5 μm, and transmit the received data on a second optical signal carrying the second uplink that uses a second wavelength in the domain of SWIR, Short Wave Infrared, wavelengths; and/or - receive the data transmitted by the second terminal, carried by a third optical signal of the third downlink that uses a third wavelength in the domain of short wave infrared SWIR wavelengths, and transmit the received data on a fourth optical signal carrying the fourth downlink that uses a fourth wavelength in the MWIR/LWIR domain of long wave infrared LWIR wavelengths and/or medium wave MWIR wavelengths. - receive the data transmitted by the first terminal, carried by a first optical signal of the first uplink that uses a first wavelength in the MWIR/LWIR, Medium Wave Infrared/Long Wave Infrared, domain of long wave infrared LWIR wavelengths, between 8 and 12 μm and/or medium wave MWIR wavelengths, between 3 and 5 μm, and transmit the received data on a second optical signal carrying the second uplink that uses a second wavelength in the domain of SWIR, Short Wave Infrared, wavelengths; and/or - receive the data transmitted by the second terminal, carried by a third optical signal of the third downlink that uses a third wavelength in the domain of short wave infrared SWIR wavelengths, and transmit the received data on a fourth optical signal carrying the fourth downlink that uses a fourth wavelength in the MWIR/LWIR domain of long wave infrared LWIR wavelengths and/or medium wave MWIR wavelengths.