modelMatrix

// Adjust a tileset's height from the globe's surface.
const heightOffset = 20.0;
const boundingSphere = tileset.boundingSphere;
const cartographic = Cartographic.fromCartesian(boundingSphere.center);
const surface = Cartesian3.fromRadians(cartographic.longitude, cartographic.latitude, 0.0);
const offset = Cartesian3.fromRadians(cartographic.longitude, cartographic.latitude, heightOffset);
const translation = Cartesian3.subtract(offset, surface, new Cartesian3());
tileset.modelMatrix = Matrix4.fromTranslation(translation);