  Global quantum communications will enable long-distance secure data transfer,
networked distributed quantum information processing, and other
entanglement-enabled technologies. Satellite quantum communication overcomes
optical fibre range limitations, with the first realisations of satellite
quantum key distribution (SatQKD) being rapidly developed. However, limited
transmission times between satellite and ground station severely constrains the
amount of secret key due to finite-block size effects. Here, we analyse these
effects and the implications for system design and operation, utilising
published results from the Micius satellite to construct an empirically-derived
channel and system model for a trusted-node downlink employing efficient BB84
weak coherent pulse decoy states with optimised parameters. We quantify
practical SatQKD performance limits and examine the effects of link efficiency,
background light, source quality, and overpass geometries to estimate long-term
key generation capacity. Our results may guide design and analysis of future
missions, and establish performance benchmarks for both sources and detectors.
