The microwave spectrum has become a highly limited resource in satellite communications owing to an ever increasing demand for bandwidth and capacity. Therefore, a shift to the exploitation of optical carrier frequencies is currently underway. Focusing on high-rate transmissions of payload data from remote sensing satellites, operational systems, like the well-known European Data Relay Satellite system, are based on optical inter-satellite links. Besides, direct-to-earth free-space optical communications from low Earth orbiting spacecraft hold high potential for upcoming space missions through lower complexity. In that regard, we study the viability of the ground-to-space beacon laser signal of optical ground stations to be additionally modulated with tele-command tokens. Such an optical return channel could be variously put into use, e.g. to trigger automatic repeat requests of payload data downlinks, for jamming-free control of the spacecraft or for high-rate software uploads to its on-board processor. A particular challenge is posed by the unequal fading behavior of the optical channel regarding the down-and uplinks, which cover asymmetric optical pathways through the atmosphere. We define the end-to-end architecture of the communication chain including the transmitter on ground and the spacebased receiver. Special attention is given to compatibility with established space data and system standards. Moreover, we examine the effects on the scheduling of satellite control, resulting from a constrained availability of the optical uplink due to cloud blockages. Our analysis aims at the employment of available space protocols for bidirectional optical communications with low earth orbiting spacecraft. Further on, we consider the adoption of upcoming standards to account for the optical fading channel. Certain applications like immediate automatic-repeatrequests for the downlink will require novel, optimized protocols.

In the early preparation phase for the upcoming robotic dual-satellite DLR mission several technical and operational challenges presented themselves. The mission itself shall produce advanced scientific findings for the on-orbit servicing missions. One of the satellites includes a sophisticated robotic arm with automated as well as manual operation modes. Very restrictive robotic payload requirements with respect to ground station visibilities as well as the quality of the data link became a main driver for the ground data system design. The real-time control and feedback of the robotic arm represents the particular challenge. Analysis of these requirements as well as technical and operational solutions will be presented, whereas some results are based on the successful ROKVISS mission. The usage of a dual-uplink antenna is discussed with regard to parallel operations of two satellites, here again, with already existing results provided by operations of the TanDEM-X mission. The design of the ground communication network as well as possible solutions allowing parallel robotic and housekeeping operations is shown. Results of this mission analysis and preparation are not only valuable for particular robotic, but for all dual-satellite, high data rate or realtime communication missions.

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The robotic on-orbit servicing technology promises an increase in life-time of operational satellites and the removal of space debris. Though such tasks are of high importance for future space exploration in general, many agencies and companies struggle with running such missions. One of the reasons is definitely to find the ultimate business case. Moreover, on-orbit servicing imposes very high risks on a mission due to its complexity which is almost as high as in human spaceflight. It is therefore essential to perform end-to-end hardwarein-the-loop simulations of a mission on ground before it is being carried out in space. For this purpose, the On-Orbit-Servicing End-to-End Simulation project (OOS E2E) has been established at the German Aerospace Center (DLR). The project uses expertise, resources and facilities from a couple of DLR institutes. Contributors are the German Space Operations Center (GSOC), the Institute of Robotics and Mechatronics (RM) and the Institute of System Dynamics and Control (SR). The aim of the On-Orbit-Servicing End-to-End Simulation project is to connect the different simulation facilities of these institutes and integrate them into a single end-to-end simulation of on-orbit servicing. One of the facilities is the European Proximity Operations Simulator (EPOS) to simulate the rendezvous maneuver between the client satellite and the chaser satellite. The other facility is the On-Orbit-Servicing Simulator (OOS-Sim) of the Institute of Robotics and Mechatronics to simulate the robotic telepresence operations.In this paper we focus on the implementation of data communication between all of the simulation facilities. Especially, the need for real-time robotic telepresence operations creates a new set of requirements for the communication chain. To account for a realworld scenario, it is therefore important to simulate the communication chain and the operational environment of an on-orbit servicing mission. The behavior of the space link, as well as the data transportation on ground, must be taken into account, including all communication parameters like possible loss, delay, jitter, corruption or duplication in the TM/TC data streams. As these parameters vary over time, the occurrence of bursts and the timely distribution of these parameters play a significant role. Furthermore, the beginning, the end and possible handovers of a satellite passage must be simulated. As the robotic telepresence operations are as important as the housekeeping operations, the setup must be optimized for processing both robotic real-time data and standard satellite TM/TC data in parallel. To do so, both data streams must be multiplexed into a single space link. This is done by specially developed FPGA devices that can be synchronized to a common master clock to multiplex/demultiplex both data streams into/from a single space link in a timely manner. Furthermore, Space Link Protocols have to be implemented between the space and ground components of the simulation. In the same way, the protocols of the ground seg...

In the early preparation phase for the upcoming robotic dual-satellite DLR mission several technical and operational challenges presented themselves. The mission itself shall produce advanced scientific findings for the on-orbit servicing missions. One of the satellites includes a sophisticated robotic arm with automated as well as manual operation modes. Very restrictive robotic payload requirements with respect to ground station visibilities as well as the quality of the data link became a main driver for the ground data system design. The real-time control and feedback of the robotic arm represents the particular challenge. Analysis of these requirements as well as technical and operational solutions will be presented, whereas some results are based on the successful ROKVISS mission. The usage of a dual-uplink antenna is discussed with regard to parallel operations of two satellites, here again, with already existing results provided by operations of the TanDEM-X mission. The design of the ground communication network as well as possible solutions allowing parallel robotic and housekeeping operations is shown. Results of this mission analysis and preparation are not only valuable for particular robotic, but for all dual-satellite, high data rate or realtime communication missions.