observerCoordsEcf, velocityEcf not defined

[bdefore]

Example in README references:

dopplerFactor = satellite.dopplerFactor(observerCoordsEcf, positionEcf, velocityEcf);

But neither of observerCoordsEcf or velocityEcf appear to be available.

[shashwatak]

I apologize for the extreme delay in response.
You will have to define those yourself using the functions described in the README.

[nhamer]

The readme code doesn't quite run. velocityEcf and observerCoordsEcf just aren't defined.

velocityEcf = satellite.eciToEcf(velocityEci, gmst);
dopplerFactor = satellite.dopplerFactor(observerEcf, positionEcf, velocityEcf);

[thkruz]

Recommend using Eci for the dopplerFactor example in the readme. There is no available function to convert velocityEci to velocityEcf and the readme already explains you can use ecf if you have those values.

[nhamer]

satellite.eciToEcf converts it

[thkruz]

It converts ECI Position but it doesn't convert ECI Velocity because it doesn't account for the rotation of the earth.

There is a reference at http://www.dtic.mil/dtic/tr/fulltext/u2/a015815.pdf on page 29 and 30 that explains the math to convert ECI Velocity to ECF Velocity.

[ericsvendsen]

I realize this issue thread is really old, but thought I would post my solution here for calculating ECF velocity in case others have the same question. If accuracy is vital and you're able to track down and decipher the math mentioned above, then great! I was not though, and this seemed to worked out pretty well for me. :)

Basically, you can determine ecf velocity using the eci data that satellite.js provides via the propagate or sgp4 methods (Note: I'm using dayjs elsewhere in my project, so that's why I'm using it here in this example. It is certainly not required)

const now = dayjs(); // date/time right now
const pv = propagate(satrec, now.toDate()); // eci position and velocity for a given satrec
if (pv.position && pv.velocity) {
    const ecfPosition = eciToEcf(pv.position, gstime(now.toDate())); // convert eci position to ecf
    const ecfMoved = eciToEcf( // add eci velocity to eci position, and convert the result to ecf
        {
            x: pv.position.x + pv.velocity.x,
            y: pv.position.y + pv.velocity.y,
            z: pv.position.z + pv.velocity.z,
        },
        gstime(now.add(1, 'second').toDate()) // be sure to use a time 1 second later, since velocity is km/s
    );
    const ecfVelocity = { // subtract original ecf from ecfMoved to obtain velocity in ecf
        x: ecfMoved.x - ecfPosition.x,
        y: ecfMoved.y - ecfPosition.y,
        z: ecfMoved.z - ecfPosition.z,
    };
}

[thkruz]

Slick idea. That should actually account for the rotation of the earth perfectly. Feels super obvious now that you post it - thanks for helping out!