javascript - Cesium Earth: Show satellites in ECI coordinate system

Question

I'm using Cesium Earth to develop an application for satellite tracking.

Now, the satellite coordinates are in Earth Fixed system and it works OK.

However, I need to show them also in ECI coordinate frame and for that I have to make the Earth rotate.

How to do that?

Answer 1

I'll start by mentioning that Cesium often uses the name ICRF as a synonym or replacement for ECI, so if you're searching the documentation you'll have better luck looking for ICRF.

The CZML Sandcastle Demo shows some satellites orbiting the Earth with paths shown in the Inertial frame. This is done in the CZML file by doing two things:

• Set the value "referenceFrame":"INERTIAL" in the position section
• All of the actual position values must themselves be expressed in Inertial, not Fixed frame.

You can tell the path is in Inertial because it is an ellipse. If it were being shown in Earth-fixed, it would look like a spiral, looping crazily around the Earth. As time passes the orbit ellipse should of course remain in the Inertial frame with the stars, not remaining fixed above any one landmass on the Earth.

However, I need to show them also in ECI coordinate frame and for that I have to make the Earth rotate.

Those are two separate problems. In Cesium, the Earth's fixed frame is already rotating (internally) with respect to the ICRF frame. But the camera stays in Earth-fixed (ECF) by default. So the user sees the Earth appear stationary, and the stars and satellite orbits appear to rotate around the Earth. This is actually a valid way to view the system, as if the camera were just stuck on a very tall pole that was attached to the Earth, sweeping through different orbits.

To make the Earth visually rotate on-screen as time passes, you have to update the Camera's position to keep it stationary in the ICRF frame, as opposed to the default fixed frame.

The Camera Sandcastle Demo has a live example of this. Click the dropdown and select View in ICRF from the list. The code for this begins around line 119 in the live-edit window on the left side:

function icrf(scene, time) {
    if (scene.mode !== Cesium.SceneMode.SCENE3D) {
        return;
    }

    var icrfToFixed = Cesium.Transforms.computeIcrfToFixedMatrix(time);
    if (Cesium.defined(icrfToFixed)) {
        var camera = viewer.camera;
        var offset = Cesium.Cartesian3.clone(camera.position);
        var transform = Cesium.Matrix4.fromRotationTranslation(icrfToFixed);
        camera.lookAtTransform(transform, offset);
    }
}

viewer.scene.postUpdate.addEventListener(icrf);

This code just updates the camera's position as time passes, such that the camera appears to be stationary in the ICRF frame with the stars and the satellite orbits, and the Earth itself is shown to rotate.