The following signatures couldn't be verified because the public key is not available

Ubuntu is nice to use, it works flawlessly on most systems. Especially the packet management is awesome and therefore I wanna say thanks to Debians developers for that.

What so ever, sometimes you have to add a Repository to the sources.list which creats a failure during fetching the package data. The failure most of the time is based on a not verified public key. To get rid of that there are a few steps to do:

GPG error: http://ppa.launchpad.net intrepid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 28A8205077558DD0

sudo gpg --keyserver x-hkp://pool.sks-keyservers.net --recv-keys 28A8205077558DD0
sudo gpg --export --armor 28A8205077558DD0 | sudo apt-key add -

Be aware that you should only add keys if you really trust the repository.

Update

A shorter version to add keys of a repository using one command: sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 28A8205077558DD0