注意：安装PX4的时候一定要自己 git clone ！！！👇
Ubuntu20.04默认没有python2 和 pip2 所以需要先安装这两个：
1. python2 & python3
2. pip2 for python2.7
sudo curl https://bootstrap.pypa.io/pip/2.7/get-pip.py --output get-pip.py
sudo python2 get-pip.py
3. pip insall -i
清华源
-i https://pypi.tuna.tsinghua.edu.cn/simple
4. ubuntu sources.list 替换为阿里云
sources.list更换为清华源会提示不安全的引用，所以博主这里选择用阿里云，没什么问题。
sudo gedit /etc/apt/sources.list
# Ubuntu20.04
deb https://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ focal main restricted universe multiverse
deb https://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ focal-security main restricted universe multiverse
deb https://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ focal-updates main restricted universe multiverse
# deb https://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
# deb-src https://mirrors.aliyun.com/ubuntu/ focal-proposed main restricted universe multiverse
deb https://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
deb-src https://mirrors.aliyun.com/ubuntu/ focal-backports main restricted universe multiverse
5.鱼香ROS
wget http://fishros.com/install -O fishros && . fishros
6. 安装Gazebo11
安装之前，先把这个包的版本换一下，不然会出错
pip3 uninstall empy
pip3 install empy==3.3.4