When updating Ubuntu machines I often get this error:

W: GPG error: http://archive.debian.org etch/updates Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 16126d3a3e5c1192

Solution:

Receive publickey for the source from the keyserver with the following command:

sudo apt-key adv –recv-keys –keyserver keyserver.ubuntu.com 16126d3a3e5c1192