European Space Agency

Optical Communications Transceiver for Fade Tolerant Data Relay Links


The overall objectives of the OCTAL project can be stated as follows:

  • To design an optical communications system for reliable and efficient data relay links through the atmosphere (e.g. UAV to GEO satellite) based on pulse-position-modulation (PPM) and a pulsed beacon laser.
  • To design the GEO optical terminal and UAV optical terminal.
  • To derive specifications for the Optical Core Unit (OCU) for the UAV-LCT.
  • To develop and test the engineering model (EM) of the OCU of the UAV terminal (comprising the pulsed laser transmitter (PLT EM) being developed by LusoSpace and fade tolerant acquisition and tracking sensor (FT-ATS EM) being developed by SensL).
  • To design a bread-board of a high-rate multi-pulse PPM communications electronics for extension of the current raw data rate of 25 Mbit/s by at least one order of magnitude
  • To verify the PLT and FT-ATS performance in a high-rate optical link under worst-case atmospheric conditions in an inter-island test campaign.


An optical communications system for reliable and efficient data relay links through the atmosphere for an UAV – GEO satellite link based on PPM and M-PPM and a pulsed beacon laser has been designed on system level including a GEO and UAV optical terminal. Based on this system design, specifications for the OCU, the UAV-LCT and the subsystems PLT and FT-ATS have been derived. The EM of the OCU of the UAV terminal was developed and built comprising the PLT EM (Figure 3) being developed by LusoSpace and FT-ATS EM (Figure 4) being developed by SensL.
Figure 3: PLT EM final configuration
Figure 4 – Fade-Tolerant Acquisition and Tracking sensor (FT-ATS)
Key critical items regarding space heritage of the PLT and the FT-ATS detector were successfully subjected to a space assessment test programme comprising also radiation and thermal-vacuum testing followed by the manufacturing design. The EM of PLT and FT-ATS went through a verification test campaign at unit level and were also tested on system level in the OCU. Under a CCN tests have been performed with M-PPM in worst case atmospheric environment in a link experiment between the Canary Islands La Palma and Tenerife. The performance of the tracking loop has been demonstrated in laboratory under presence of micro-vibrations and scintillations disturbances modulated on the beacon beam.
The objectives of the project OCTAL have been fulfilled and the requirements have been verified to be compliant with the intended system performance. It therefore can be stated that the PLT and FT ATS have been validated on component level in relevant environment therefore reached TRL 5 status. Furthermore, verification of the link performance on OCU level has been demonstrated under worst case atmospheric conditions up to user data rates of 372 Mbps with M-PPM modulation scheme.

The PLT and the FT-ATS are key units for optical communications through the atmosphere which can be either the OCTAL UAV-GEO link scenario or the units can also be used as key components for e.g. LEO-Ground link scenarios. Both units are ready for further development to flight status. The detailed development roadmap has been elaborated.


Development of key technologies for optical links through the atmosphere.


Figure 1 is the block diagram for the OCU. The OCU comprises the optics, mechanisms and sensors needed to point, acquire and track a laser communications link between the UAV / aircraft and a partner GEO optical terminal.

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Figure 1: Block Diagram for the UAV OCU


The OCTAL project is divided into two parts – Part I and Part II. Part I comprises three tasks, namely:

  • Task 1 System level design of the optical communications link.
  • Task 2 Preliminary design of the optical core unit (OCU).
  • Task 3 Space assessment for the critical components.

Part II comprises four tasks associated with the development and testing an engineering model for the OCU. The tasks are:

  • Task 4 Manufacturing design of the OCU
  • Task 5 Manufacturing, sub-system assembly and integration
  • Task 6 Performance verification testing of the OCU
  • Task 7 Assessment, recommendations and development roadmap
In addition to the baseline contract the communications system has been improved under a CCN to provide data rates of about a factor 10 higher than initially planned and the classical PPM modulation has been accompanied by a multi-pulse PPM format. The system has been verified in an inter-island link experiment on the Canary Islands between the islands La Palma and Tenerife (Figure 2). All PPM and M-PPM modulation and coding schemes could be successfully demonstrated under worst case atmospheric conditions in a 142 km atmospheric path for user data rates up to 372 Mbps.
Figure 2: OCU during Inter-Island link experiment

Current status


Status date

Tuesday, March 21, 2017 - 08:22