GPG error: the public key is not available

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

I have had it happen on several occasions where I try to update software and I get a GPG error like this:

$ sudo apt-get update
W: GPG error: http://inverse.ca lucid Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 06FD0B51C1FF59BB

If you ever run into an issue similar to this when running apt-get update, take the GPG key fingerprint (in the above example the fingerprint is 06FD0B51C1FF59BB) and attempt to retrieve it from the Ubuntu keyserver, then add it to your  aptitude keys:

$ gpg --keyserver keyserver.ubuntu.com --recv 06FD0B51C1FF59BB
gpg: requesting key C1FF59BB from hkp server keyserver.ubuntu.com
gpg: key C1FF59BB: public key "Inverse Support (package signing) <support@inverse.ca>" imported
gpg: Total number processed: 1
gpg:               imported: 1

So, the key was found, now add it to our aptitude keys:

$ gpg --export --armor 06FD0B51C1FF59BB | sudo apt-key add -

Now, when you run apt-get update you should have no errors.