Jetson orin (Arm64架构) 上降级Ubuntu22.04到20.04，并安装ROS-noetic

降级Ubuntu22.04到20.04

查看当前版本号。

lsb_release -a

ubuntu不同版本的简称：

备份你的系统软件源

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

打开配置文件

sudo gedit /etc/apt/sources.list

只保留两行：

deb http://archive.ubuntu.com/ubuntu saucy main restricted universe multiversedeb http://archive.ubuntu.com/ubuntu trusty main restricted universe multiverse

因为我们是arm架构，使用清华源，20.04是focal，22.04是jammy。所以需要做一点修改：

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

前面是目标旧版本，后面是现有新版本。
然后修改或增加文件/etc/apt/preferences，只填入以下内容：

Package: *Pin: release a=focalPin-Priority: 1001Package: *Pin: release a=jammyPin-Priority: 60

接着重新

sudo apt updatesudo apt upgrade

一路y和ok回车下去，漫长等待后：
终于回到了20.04。血泪的教训，更换软件源的时候看好版本号，别随便复制就用了。。。

orin上安装ros-noetic

重新软件源并更新那些就不在这里赘述了，附上一个我的源：

# 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 multiversedeb 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 multiversedeb 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 multiversedeb http://ports.ubuntu.com/ubuntu-ports/ focal-security multiverse restricted main universe# 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 multiversedeb http://ports.ubuntu.com/ubuntu-ports/ focal main universe restricted multiversedeb http://ports.ubuntu.com/ubuntu-ports/ focal-updates multiverse restricted main universedeb http://ports.ubuntu.com/ubuntu-ports/ focal-backports multiverse restricted main universe

再运行

sudo apt install ros-noetic-desktop-full

时，会提示有些依赖无法被安装，细看就是关于仿真的东西。我理解的是arm上面不支持ros做这个事，反正作为机载电脑也用不上，所以直接

sudo apt install ros-noetic-desktop

然后按照我前面的ros安装教程操作，跑起来小乌龟就没问题了！