Linux:Como corrigir erro de GPG do apt-get no Ubuntu

Se ocorrer erro de GPG durante um update no apt-get, como abaixo:

... ... Reading package lists... Done 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 8C8B989CC5202CFB
 * 1) apt-get -y update

Para resolver faça o seguinte:

gpg: requesting key C5202CFB from hkp server pgpkeys.mit.edu gpg: /root/.gnupg/trustdb.gpg: trustdb created gpg: key C5202CFB: public key "Launchpad PPA for Alex Eftimie" imported gpg: Total number processed: 1 gpg:              imported: 1  (RSA: 1) OK
 * 1) gpg --keyserver pgpkeys.mit.edu  --recv-keys 8C8B989CC5202CFB
 * 1) gpg --armor --export 8C8B989CC5202CFB | apt-key add -

Outros servidores de PGP:


 * wwwkeys.eu.pgp.net