The following apt-get error is quite common while updating.
W: GPG error: http://ppa.launchpad.net karmic Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 991E6CF92D9A3C5B
Generally, we just try searching for a public key and and adding the key to the apt key-list, which sometimes doesn't work.
There is a better option. Try any one of the following links.
All you have to do is:
1. Copy-paste the key from the error on the terminal to the search-box, preceded by 0x.
2. Click Submit Query.
3. Click on the link next to pub.
4. Add the key to the key-list by typing in the terminal,
varsha@varsha-laptop:~$ wget -q "http://keyserver.ubuntu.com:11371/pks/lookup?op=get&search=0x991E6CF92D9A3C5B" -O- | sudo apt-key add -
Note that the server address may vary. So, make changes accordingly in your command.
An OK in the next line tells you that the key has been successfully fetched from the key-server.
You can repeat the above steps if you get the error for more than one key.