How to retrieve the pitch and roll of a added entity.

CesiumJS
1

I’m trying to update the heading of a entity.But i don’t want to change the pitch and roll.I want to keep the original values for pitch and roll.

adding entity:

var entity = viewer.entities.add({

position :Cesium.Cartesian3.fromDegrees(longitudeString,latitudeString,1000),

model : {

       uri:'10.gltf'

}

});