pacman -Syu : key could not be looked up remotely.

# sudo pacman -Syu
...
error: key "5F702428F70E0903" could not be looked up remotely
error: required key missing from keyring
error: failed to commit transaction (unexpected error)
Errors occurred, no packages were upgraded.
...
#sudo pacman -S archlinux-keyring && sudo pacman -Syu