Before you begin building the mobile manipulator in Gazebo and ROS, you need to keep the units of measurement and coordinate conventions ROS follows in mind. Information like this is defined in design documents called ROS Enhancement Proposals (REPs). They act as standard references to the community members who use ROS while building their projects. Any new feature that's introduced or is planned to be introduced in ROS would be available as a proposal document for the community. The standard units of measurement and coordinate conventions are defined in REP-0103 (http://www.ros.org/reps/rep-0103.html). You can find all the available lists of REPS in the REP index here: http://www.ros.org/reps/rep-0000.html.

As far as we're concerned, the following information is sufficient enough that we can go ahead with building our mobile manipulator.

For units of measurement, we have the following:

•  The base units: Length is in meters; mass is in kilograms; time is in seconds
•  The derived units: Angle is in radians; frequency is in hertz; force is in newtons
•  The kinematic-derived units: Linear velocity is in meters per second; angular velocity is in radians per second

For coordinate system conventions, we have the following:

• The right-hand thumb rule is followed, where the thumb is the z axis, the middle finger is the y axis, and the index finger is the x axis. Also, positive rotation of the z axis is anti-clockwise and the negative rotation of z is clockwise.

We'll look at Gazebo and ROS assumptions in the next section.