When updating the Debian based system, it may happens that the apt-get displays an error message like:
This is a new feature of the apt-get system that guarantee the authenticity of servers for updating Debian.W: GPG error: http://ppa.launchpad.net maverick Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY FA9C98D5DDA4DB69
W: GPG error: http://ppa.launchpad.net maverick Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A9BF3BB4E5E17B5
Solution
Simply type the following commands, taking care to replace the number of the key that displayed in the error message:
gpg --keyserver pgpkeys.mit.edu --recv-key 5A9BF3BB4E5E17B5
gpg -a --export 5A9BF3BB4E5E17B5 | sudo apt-key add -