sudo apt-get update

Hit:1 http://ph.archive.ubuntu.com/ubuntu xenial InRelease
Hit:2 http://ph.archive.ubuntu.com/ubuntu xenial-updates InRelease
Hit:3 http://ph.archive.ubuntu.com/ubuntu xenial-backports InRelease
Get:4 http://security.ubuntu.com/ubuntu xenial-security InRelease [94.5 kB]
Ign:5 http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04  InRelease
Get:6 http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04  Release [1,007 B]
Get:7 http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04  Release.gpg [481 B]
Ign:7 http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04  Release.gpg
Hit:8 http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04  Packages
Fetched 95.9 kB in 1s (56.7 kB/s)
Reading package lists... Done
W: GPG error: http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04  Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5A7D1D38BEB6D886
W: The repository 'http://download.opensuse.org/repositories/home:/Horst3180/xUbuntu_16.04  Release' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.