From LedHed's Wiki
If you get an error when doing apt-get update along the lines of:
GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY <GPG_KEY>
Then re-download and apply the GPG key as follows: sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys <GPG_KEY>