Using the Local Time Server
Many embedded computers (such as the ones on drones or mobile robots) do not have a real-time clock that maintains the correct time between power cycles. For some applications, such as visualizing sensor data from multiple robots using ROS, the clocks for the robots need to be synchronized in order to properly visualize the data in RVIZ.
Typically, computers also use the internet to periodically adjust the clock. However, the Flight Room network does not have general internet access, in accordance with Stanford IT rules. Therefore, the relay computer in the Flight Room (see Lab Computers) has been configured so that devices can receive the correct (internet) time.
Network Time Protocol (NTP) Setup
The Network Time Protocol (NTP) allows for time synchronization to within a few milliseconds of Coordinated Universal Time (UTC) using a client-server model. You will need administrator privileges to install the service and modify a configuration file.
Ubuntu 16.04 / 18.04
Install the service by running sudo apt-get install ntp in a terminal. After installation, open the configuration file etc/ntp.conf with administrator privileges using your favorite text editor.
To add the computer relay as an NTP resource using .local hostnames (see Local Hostnames), add server relay.local iburst to the configuration file,
...
# Specify one or more NTP servers.
server relay.local iburst
...
Running ntpq -p shows the servers being used for time synchronization and correction.
$ ntpq -p
remote refid st t when poll reach delay offset jitter
==============================================================================
*relay.local 45.33.48.4 3 u 40 64 1 6.670 -3.943 1.491
0.ubuntu.pool.n .POOL. 16 p - 64 0 0.000 0.000 0.000
1.ubuntu.pool.n .POOL. 16 p - 64 0 0.000 0.000 0.000
2.ubuntu.pool.n .POOL. 16 p - 64 0 0.000 0.000 0.000
3.ubuntu.pool.n .POOL. 16 p - 64 0 0.000 0.000 0.000
ntp.ubuntu.com .POOL. 16 p - 64 0 0.000 0.000 0.000
Issues with NTP are usually resolved by restarting the service with sudo service ntp restart (or simply rebooting). Also note that there is an initialization period for syncing with a server through NTP so there may be a delay before ntpq indicates that relay is being used for synchronization.