How to attach arm to a static base using SDF

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I have been playing around with building my own robotic arm. It's very simple and I have all of the joints working. I was hoping work with the arm in a zero gravity environment so movement was easier. The problem I run into however is that the base will always float off or be affected by the arm rotation. I want to fix the base to the ground.

I have tried the following without success:

<?xml version='1.0'?>
<sdf version='1.4'>
  <model name="full_robot_arm">
    <include>
      <uri>model://robot_arm</uri>
      <pose>0.0 0 1.0 0 0 0</pose>
    </include>

    <include>
      <uri>model://arm_base</uri>
      <pose>0 0 0 0 0 0</pose>
    </include>

    <include>
      <uri>model://ground_plane</uri>
    </include>

    <joint name="fixed to ground" type="revolute">
      <parent>ground_plane::link</parent>
      <child>robot_arm_base::body</child>
       <axis>
        <limit>
          <lower>0</lower>
          <upper>0</upper>
        </limit>
        <xyz>0 0 1</xyz>
        </axis>
    </joint>

    <joint name="shoulder_joint" type="revolute">
      <parent>robot_arm_base::body</parent>
      <child>robot_arm::shoulder_rotator</child>
      <axis>
        <xyz>0 0 1</xyz>
      </axis>
    </joint>
  </model>
</sdf>

It does attach the arm to 'a' ground plane but this will rotation along with the rest of the body.