先备份系统原下载源：

sudo cp /etc/apt/sources.list /etc/apt/sources_back.list

更新新的下载源：

sudo vi /etc/apt/sources.list

将原有内容删除后添加如下内容：

# 默认注释了源码镜像以提高 apt update 速度，如有需要可自行取消注释

deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal main restricted universe multiverse

# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal main restricted universe multiverse

deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-updates main restricted universe multiverse

# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-updates main restricted universe multiverse

deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-backports main restricted universe multiverse

# deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-backports main restricted universe multiverse

# deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-security main restricted universe multiverse

# # deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-security main restricted universe multiverse

deb http://ports.ubuntu.com/ubuntu-ports/ focal-security main restricted universe multiverse

# deb-src http://ports.ubuntu.com/ubuntu-ports/ focal-security main restricted universe multiverse

# 预发布软件源，不建议启用

# deb https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-proposed main restricted universe multiverse

# # deb-src https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/ focal-proposed main restricted universe multiverse

添加ros下载源：

sudo sh -c '. /etc/lsb-release && echo "deb http://mirrors.tuna.tsinghua.edu.cn/ros/ubuntu/ `lsb_release -cs` main" > /etc/apt/sources.list.d/ros-latest.list'

添加密匙：

sudo apt-key adv --keyserver 'hkp://keyserver.ubuntu.com:80' --recv-key C1CF6E31E6BADE8868B172B4F42ED6FBAB17C654

更新并下载ros，再安装依赖：

sudo apt update

sudo apt install ros-noetic-desktop-full

sudo apt install python3-rosdep python3-rosinstall python3-rosinstall-generator python3-wstool build-essential

ros使用需要配置环境变量，可以放到.bashrc中自动执行：

echo "source /opt/ros/noetic/setup.bash" >> ~/.bashrc

source ~/.bashrc

官方使用ros:

sudo rosdep init

rosdep update

外网可能下载失败，可以使用：

sudo apt install python3-pip -y

sudo pip3 install -i https://pypi.tuna.tsinghua.edu.cn/** rosdepc

sudo rosdepc init

rosdepc update

所有东西都下载好后，可以改回默认的下载源：

sudo cp /etc/apt/sources_back.list /etc/apt/sources.list