[UBUNTU_LINUX] How to fix missing GPG Key error

While i install ROS indigo followed by "http://docs.erlerobotics.com/simulation/configuring_your_environment", I encountered the Error like this.

W: GPG error: http://packages.ros.org trusty InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY F42ED6FBAB17C654

so, i started to find the solution of this error and it was very helpful and solved my problem.

"https://askubuntu.com/questions/127326/how-to-fix-missing-gpg-keys"

check the sixth answer.

The Most Easiest Way Is With Y PPA Manager