Communications considerations for satellite servicing in or near-geosynchronous Earth orbit

Developing a robotic servicing vehicle (RSV) capable of providing high-value, reliable services to satellites in geosynchronous Earth orbit (GEO) require solving a myriad of technical challenges and answering numerous regulatory and policy questions. One such challenge is space/ground communications. Space/ground communications present a significant challenge to the RSV designer: establishing the means by which the RSV would receive commands from and send telemetry, including video, to its ground controllers during critical servicing operations. RSV operators would require good situational awareness for safety of operations, which would require large-telemetry bandwidth supporting multiple high-resolution, high-frame-rate, near-simultaneous, low-latency, low-jitter video feeds. Constraints on potential RSV communications subsystem design include: a) the ability to obtain frequency allocations in the appropriate location for a given servicing event; b) ground station access over all GEO longitudes; c) the need to avoid electromagnetic and radio frequency (RF) interference between the RSV and the client satellite (client-sat); d) communications subsystem design impacts on the RSV; and e) communications redundancy approaches, at least for critical command functions and recovery following outages. The impact of these constraints on a number of options for communications system designs are discussed, and some ambiguities in the regulatory environment are identified. RSV system design would be highly dependent upon the desired set of customer spacecraft to be serviced, as foreign-flagged spacecraft have different spectrum allocations than U.S. spacecraft, and military spectrum allocations are different from commercial spectrum allocations. A number of communications system options are presented. Communications system hardware requirements are discussed for each option, as are the advantages and disadvantages of each approach. Proposed system concepts include the use of novel communications approaches not in widespread use today - the W band and laser (free space optical) communications. Electromagnetic and RF interference between the RSV and client-sat would present a significant challenge. In addition to the transmitters/receivers on each vehicle, there is the potential for interference from unintentional sources (e.g., spurs, harmonics, broadband noise). There is also the possibility raised in some servicing scenarios that the same grounding path used to mitigate charging differential between the RSV and the client-sat when docked would also provide a path for interfering signals to travel between the servicer and client-sat. Steps to mitigate these sources of potential interference would be unique to each space system, and only preliminary approaches will be discussed.