  In satellite-based quantum key distribution (QKD), the number of secret bits
that can be generated in a single satellite pass over the ground station is
severely restricted by the pass duration and the free-space optical channel
loss. High channel loss may decrease the signal-to-noise ratio due to
background noise, reduce the number of generated raw key bits, and increase the
quantum bit error rate (QBER), all of which have detrimental effects on the
output secret key length. Under finite-size security analysis, higher QBER
increases the minimum raw key length necessary for non-zero secret key length
extraction due to less efficient reconciliation and post-processing overheads.
We show that recent developments in finite key analysis allow three different
small-satellite-based QKD projects CQT-Sat, UK-QUARC-ROKS, and QEYSSat to
produce secret keys even under very high loss conditions, improving on
estimates based on previous finite key bounds. This suggests that satellites in
low Earth orbit can satisfy finite-size security requirements, but remains
challenging for satellites further from Earth. We analyse the performance of
each mission to provide an informed route toward improving the performance of
small-satellite QKD missions. We highlight the short and long-term perspectives
on the challenges and potential future developments in small-satellite-based
QKD and quantum networks. In particular, we discuss some of the experimental
and theoretical bottlenecks, and improvements necessary to achieve QKD and
wider quantum networking capabilities in daylight and at different altitudes.
