Ubuntu: How to fix the apt-get update error: W: GPG error: http://ppa.launchpad.net intrepid Release: The following signatures couldn't be verified be

The problem is during apt-get update:

...
Reading package lists... Done
W: GPG error: http://ppa.launchpad.net intrepid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 8B9FBE5158B3AFA9
W: You may want to run apt-get update to correct these problems


Solution:

gpg --keyserver keyserver.ubuntu.com --recv 8B9FBE5158B3AFA9
gpg --export --armor 8B9FBE5158B3AFA9 | sudo apt-key add -


Update should work now:

sudo apt-get update