12.04 PPA GPG Error

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>

Reference

http://askubuntu.com/questions/127326/how-to-fix-missing-gpg-keys