Optimal Satellite Orbits for Delay-Tolerant Free-Space Optical Networks

NASA’s Terabyte Infrared Delivery (TBIRD) program demonstrates high-rate free-space optical (FSO) downlink from low-Earth orbit (LEO) to ground during short contact windows, serving as a hardware validation of store-carry-and forward data delivery over intermittent optical links. Motivated by such single-satellite architectures, which offer low computational complexity and reduced size, weight, and power (SWaP) requirements, this work presents a comprehensive framework for optimizing the orbit of a single satellite performing space-to ground FSO transmission to maximize ergodic capacity. Both circular and elliptical orbital configurations are considered. Numerical results reveal a unique altitude–beamwidth pair that maximizes the average ergodic capacity, and show that, under practical constraints, the elliptical orbit achieves its maximum capacity only when it degenerates to the circular case. The impact of key system parameters, including minimum elevation angle, pointing error variance, and turbulence strength, on the optimal altitude is also quantified. In addition, the Age of Information (AoI) is minimized under a constant data rate, showing that higher transmit power enables operation at lower optimal altitudes to maintain the same capacity, thereby reducing propagation delay and improving information freshness. The proposed framework provides design guidelines for high-throughput, low-latency FSO downlinks in single-satellite, intermittently connected scenarios without reliance on large constellations.