Radxa(Rock Pi) 更新時發生錯誤

使用硬體為ROCK 4 SE，作業系統為官方提供Ubuntu 20 Server(Linux 4.4)

執行apt update更新時發生下列警告與錯誤

W: GPG error: http://apt.radxa.com/focal-stable focal InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 9B98116C9AA302C7
E: The repository 'http://apt.radxa.com/focal-stable focal InRelease' is not signed.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

用以下兩行指令更新公鑰即可

export DISTRO=focal-stable
wget -O - apt.radxa.com/$DISTRO/public.key | sudo apt-key add -