title: CesiumJS how to keep the zoom level of a Cesium.Viewer when the viewer.trackedEntity is changed
body: <p><del>I have 2 different Cesium.Viewer instances, I want sync the 2 viewers when user zoom either one of them.</del></p>
  
  <p><del>How to do that?</del></p>
  
  <p>update:
  In my app, I have 2 different Cesium.Viewer instances. But for this question here, I think it has nothing to do with the number of Cesium.Viewer. So I updated the question as below:</p>
  
  <p>I have 2 planes flying on a Cesium.Viewer as shown in the attached screenshot, one is on the red course (ref as red plane) and the other is on the red course (ref as red plane).</p>
  
  <p>Step-1: I tracked the yellow plane by double click it, then it looks like pic-1;</p>
  
  <p>Step-2: I zoom out it to pic-2;</p>
  
  <p>Step-3: I changed to track the red plane by double click it, and it looks like pic-3;</p>
  
  <p>Step-4: I zoom out it to pic-4;</p>
  
  <p>Whenever I changed the tracked entity (as step-3), I need to zoom out it manually again. </p>
  
  <p><strong>So, my question is how to keep the zoom level when changing the tracked entity?</strong></p>
  
  <p><a href="https://i.stack.imgur.com/LbYyy.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/LbYyy.jpg" alt="enter image description here"></a></p>