Critical Subsystem Analysis for a 1000 AU Interstellar Pathfinder Mission

Recent studies described at COSPAR18, conducted in a JHU graduate course, initiated an ongoing focus on an interstellar pathfinder mission to 1000 AU. We addressed several system requirements that must be resolved to jointly optimize sensors, data volume, communications, and guidance, navigation and control (GNC) requirements, while accounting for specific technology feasibility and attributes. Concept design and interfaces cannot be properly constructed without considering all reasonable technology options. Our study goal for communications is to determine what parameters support effective and reliable data transmission to earth, by comparing two frequencies (RF and optical). We are taking a fresh look at the need, definition and quantification of sensor output and data types and how they can be traded-off versus optimally achievable data rates and complexity, given only RF and RF-with-optical channels. Thus, we must look seriously at data compression versus source encoding to drastically reduce data volume. Our link analysis addresses transmitter output power, data rate, bit error rate, efficiency, antenna area and gain, receiver antenna area and pointing loss, as well as path attenuation and turbulence effects, link availability, and longevity. Comparisons will be made on the basis of link range and transmit frequency, adjusted for implementation losses, especially pointing loss for the optics. Unlike the optical channel, the RF subsystem has no significant pointing loss even at these great ranges, but its link budget will result in substantially lower data rates (by a factor of 10-3.5), but spacecraft recovery cannot do without it. For the optical channel, two other key variables influence trades: high downlink optical amplification, if tolerable, and very large aperture size for the earth receiver, if not too costly. The most critical GNC issue is pointing, acquisition, tracking and stabilization (PATS), which is essential to the optical downlink. An on-board inertial measurement unit (IMU) containing a gyro and accelerometer triad must be co-located and precisely aligned with the star trackers and downlink laser to support attitude estimation and fine laser steering with minimal

error.