I have tried to clone ardupilot submodules with the following command for a hundred times, I always got the following same errors:
john@DESKTOP-0P475SS:~/ardupilot/ArduCopter$ sudo git submodule update --init --recursive
[sudo] password for john:
Cloning into '/home/john/ardupilot/modules/PX4Firmware/mavlink/include/mavlink/v1.0'...
fatal: unable to connect to github.com:
github.com[0: 20.205.243.166]: errno=Connection timed out
fatal: clone of 'git://github.com/mavlink/c_library.git' into submodule path '/home/john/ardupilot/modules/PX4Firmware/mavlink/include/mavlink/v1.0' failed
Failed to clone 'mavlink/include/mavlink/v1.0'. Retry scheduled
Cloning into '/home/john/ardupilot/modules/PX4Firmware/src/modules/uavcan/libuavcan'...
fatal: unable to connect to github.com:
github.com[0: 20.205.243.166]: errno=Connection timed out
fatal: clone of 'git://github.com/UAVCAN/libuavcan.git' into submodule path '/home/john/ardupilot/modules/PX4Firmware/src/modules/uavcan/libuavcan' failed
Failed to clone 'src/modules/uavcan/libuavcan'. Retry scheduled
Cloning into '/home/john/ardupilot/modules/PX4Firmware/mavlink/include/mavlink/v1.0'...
fatal: unable to connect to github.com:
github.com[0: 20.205.243.166]: errno=Connection timed out
fatal: clone of 'git://github.com/mavlink/c_library.git' into submodule path '/home/john/ardupilot/modules/PX4Firmware/mavlink/include/mavlink/v1.0' failed
Failed to clone 'mavlink/include/mavlink/v1.0' a second time, aborting
Failed to recurse into submodule path '../modules/PX4Firmware'
john@DESKTOP-0P475SS:~/ardupilot/ArduCopter$
My internet connection was ok. What was the reason for the failures? Thanks!