title: How to draw a line in Cesium Sandcastle between two points when one of them is moving?
body: <p>I am using Cesium Sandcastle to visualize my scenario of a satellite moving on his orbit.
  I want to create a line between the satellite and the ground station to visualize the accesses.
  The satellite is create as a variable called entity and his position is calculate with an SGP4 function, that report the variable "posizione".
  The problem is that I'm not able to draw a polyline between a point that is moving and another point fixed on the earth.</p>
  
  <p>I tried a few things and it didn't work, for example, following is my code:</p>
  
  <pre><code>function Accessi() {
  var access = new Cesium.Polyline()
        show: true
        width : 1.0
        material:{
          solidColor:{
            color:{
              rgba:[
                0,255,255,255
              ]
            }
          }
        }
        positions:{
          references:[entity,observerPos
          ]
        }
  };
  
  if (lookAngles.El &gt; 0) {
       Accessi()
  };
  </code></pre>
  
  <p>the error is the following: "Expected transform to be typeof object, actual typeof was undefined"</p>