Recently I started to get some errors when trying to update my system. I received the error
W: 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
So, I wrote a script to download and apply the keys. Just highlight and copy the number following the "NO_PUBKEY".
Download my script and make it executable.
chmod +x update-gpg-keys
Then run my script and pass the key number to the script like so
./update-gpg-keys 28A8205077558DD0
Now you can run apt-get update or run synaptic without errors.