Ubuntu Launchpad GPG Error

If you've added testing repositories on Launchpad to your Ubuntu OS, you will encounter error messages like the following:

W: GPG error: http://ppa.launchpad.net jaunty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4D172E8E9371DCE6

Here's how to import the public key(s) to your system.

Using the error message above as an example, note the last eight letters/numbers (as highlighted above) and do the following in a command line window:

gpg --keyserver keyserver.ubuntu.com --recv 9371DCE6
gpg --export --armor 9371DCE6 | sudo apt-key add -