var boundingSphere = new Cesium.BoundingSphere(Cesium.Cartesian3.fromDegrees(116.4, 39.9, 100), 15000);
// Override behavior of home button
viewer.homeButton.viewModel.command.beforeExecute.addEventListener(function(commandInfo) {
// Fly to custom position
viewer.camera.flyToBoundingSphere(boundingSphere);
// Tell the home button not to do anything
commandInfo.cancel = true;
});
// Set custom initial position
viewer.camera.flyToBoundingSphere(boundingSphere, {duration: 0});