How to set new height in vertex shader

CesiumJS
1

Hi everyone!
Here is my code:

vec4 p = czm_computePosition();
vec4 fp = czm_modelViewProjectionRelativeToEye * p;
fp.y += time;
gl_Position = fp;

It working but the fly animation is not perpendicular to the surface
Anyone knows how to fix it?

2

use Cesium.CustomShader(options).setUniform(uniformName, value)