  Quantum key distribution (QKD) is a pioneering quantum technology on the
brink of widespread deployment. Nevertheless, the distribution of secret keys
beyond a few 100 kilometers at practical rates remains a major challenge. One
approach to circumvent lossy terrestrial transmission of entangled photon pairs
is the deployment of optical satellite links. Optimizing these non-static
quantum links to yield the highest possible key rate is essential for their
successful operation. We therefore developed a high-brightness
polarization-entangled photon pair source and a receiver module with a fast
steering mirror capable of satellite tracking. We employed this
state-of-the-art hardware to distribute photons over a terrestrial free-space
link with a distance of 143 km, and extracted secure key rates up to 300 bits
per second. Contrary to fiber-based links, the channel loss in satellite
downlinks is time-varying and the link time is limited to a few minutes. We
therefore propose a model-based optimization of link parameters based on
current channel and receiver conditions. This model and our field test will
prove helpful in the design and operation of future satellite missions and
advance the distribution of secret keys at high rates on a global scale.
