  Global-scale quantum communication networks will require efficient
long-distance distribution of quantum signals. Optical fibre communication
channels have range constraints due to exponential losses in the absence of
quantum memories and repeaters. Satellites enable intercontinental quantum
communication by exploiting more benign inverse square free-space attenuation
and long sight lines. However, the design and engineering of satellite quantum
key distribution (QKD) systems are difficult and characteristic differences to
terrestrial QKD networks and operations pose additional challenges. The typical
approach to modelling satellite QKD (SatQKD) has been to estimate performances
with a fully optimised protocol parameter space and with few payload and
platform resource limitations. Here, we analyse how practical constraints
affect the performance of SatQKD for the Bennett-Brassard 1984 (BB84) weak
coherent pulse decoy state protocol with finite key size effects. We consider
engineering limitations and trade-offs in mission design including limited
in-orbit tunability, quantum random number generation rates and storage, and
source intensity uncertainty. We quantify practical SatQKD performance limits
to determine the long-term key generation capacity and provide important
performance benchmarks to support the design of upcoming missions.
