Calculating Quaternion for czml Cone

I’m trying to figure out how to calculate the quaternion value for a cone so that the cone is pointed directly at the earth. I’m guessing it needs to be based on the current lat/lon but I am not sure how to do that.

This is the czml that I am currently using.

[

{

“id”: “cone”,

“position”: {

“cartographicDegrees”: [0, 10, 200000]

},

“orientation” : {

“unitQuaternion” : [0.0, 0.0, 0.0, 0.0]

},

“cone”: {

“innerHalfAngle”: 0.0,

“outerHalfAngle”: 0.5,

“radius”: 5e5,

“show”: true,

“showIntersection”: true,

“intersectionWidth”: 4.0

}

}

]