Ubuntu GPG error: http://ppa.launchpad.net

During the "apt-get update" you can receive following warnings:

W: GPG error: http://ppa.launchpad.net natty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY EC7305B5ECF7E0B3 W: GPG error: http://ppa.launchpad.net natty Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3B22AB97AF1CDFA9 W: GPG error: http://ftp.sk.debian.org testing InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY AED4B06F473041FA

The reason is missing GPG key. You can repair it with following commands:

gpg --keyserver subkeys.pgp.net --recv EC7305B5ECF7E0B3 gpg --export --armor EC7305B5ECF7E0B3 | sudo apt-key add - gpg --keyserver subkeys.pgp.net --recv 3B22AB97AF1CDFA9 gpg --export --armor 3B22AB97AF1CDFA9 | sudo apt-key add - gpg --keyserver subkeys.pgp.net --recv-keys 473041FA gpg -a --export 473041FA | apt-key add -

After that steps, keys will be added to a list and the error will not reappear. You can again run "apt-get update".