Terrain with models issue

hello,

i am trying adding a model(ground vehicle and aircraft) with enabled terrain.
but the problem is models does not shown above the terrain.

here is my code...please help.
Thanks
  
var terrainProvider = new Cesium.CesiumTerrainProvider({
    url : ‘https://assets.agi.com/stk-terrain/world
});
var positions = [
    Cesium.Cartographic.fromDegrees(86.925145, 27.988257),
    Cesium.Cartographic.fromDegrees(87.0, 28.0)
];
var promise = Cesium.sampleTerrain(terrainProvider, 11, positions);
Cesium.when(promise, function(updatedPositions) {
    // positions[0].height and positions[1].height have been updated.
    // updatedPositions is just a reference to positions.
});

viewer.terrainProvider = terrainProvider;
//Provide terrain.

viewer.scene.globe.depthTestAgainstTerrain = true;

var scene = viewer.scene;
var clock = viewer.clock;

var lon = 79.06666666;
var lat = 30.73388888;

function kedarnath() {
    Sandcastle.declare(kedarnath);
    viewer.camera.flyTo({
        destination : Cesium.Cartesian3.fromDegrees(lon, lat, 5000.0)
    });
}

Sandcastle.addToolbarMenu([{
    text : 'Fly Locations'
}, {
    text : 'Kedarnath',
    onselect : function() {
        kedarnath();
        Sandcastle.highlight(kedarnath);
    }

}]);

Sandcastle.reset = function() {
    viewer.entities.removeAll();
    scene.primitives.removeAll();
    scene.tweens.removeAll();

   viewer.camera.lookAtTransform(Cesium.Matrix4.IDENTITY);

    clock.multiplier = 1.0;
    scene.globe.enableLighting = false;
};

scene.morphComplete.addEventListener(function() {
    Sandcastle.reset();
});

function createModel(url, height) {
    viewer.entities.removeAll();
    
  // var heading = Cesium.Math.toRadians(135);
    //var pitch = 0;
    //var roll = 0;
    //var orientation = Cesium.Transforms.headingPitchRollQuaternion( heading, pitch, roll);

    var modelMatrix = Cesium.Transforms.eastNorthUpToFixedFrame(
    Cesium.Cartesian3.fromDegrees(lon, lat, height));
var model = scene.primitives.add(Cesium.Model.fromGltf({
    url : url,
    modelMatrix : modelMatrix,
  minimumPixelSize : 128,
            maximumScale : 20000,
    model : {
            uri : url,
            minimumPixelSize : 128,
            maximumScale : 20000
        }
}));
    
    Cesium.when(model.readyPromise).then(function(model) {
    model.activeAnimations.addAll({
        loop : Cesium.ModelAnimationLoop.REPEAT
    });
});
   
// viewer.trackedEntity = entity;
}

var options = [{
    text : 'Select Model'
},{
    text : 'Ground vehicle',
    onselect : function() {
        createModel('../../SampleData/models/CesiumGround/Cesium_Ground.gltf', 500);
    }
},{
    text : 'Aircraft',
    onselect : function() {
        createModel('../../SampleData/models/CesiumAir/Cesium_Air.gltf', 1500.0);
    }
}];

Sandcastle.addToolbarMenu(options);

Hello,

We don’t currently have support for entities on terrain. Tom has been working on implementing it and we hope to have it finished soon. Keep an eye on the progress in this pull request: https://github.com/AnalyticalGraphicsInc/cesium/pull/3903

I think the last hold-up are some performance issues, but we’re hoping to get that figured out soon.

Best,

Hannah